ACI 336.3R-14: Report on Design and Construction of Drilled ......ACI 336.3R-14 Report on Design and...

Report on Design and Construction of Drilled PiersReported by ACI Committee 336

AC

I 336

.3R

-14

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Not for Resale, 01/26/2015 01:43:11 MSTNo reproduction or networking permitted without license from IHS

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First PrintingAugust 2014

ISBN: 978-0-87031-914-3

Report on Design and Construction of Drilled Piers

Copyright by the American Concrete Institute, Farmington Hills, MI. All rights reserved. This material may not be reproduced or copied, in whole or part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of ACI.

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This report covers design and construction of 30 in. (760 mm) diameter or larger foundation piers constructed by excavation of a hole in a subgrade that is later filled with concrete. The 30 in. (760 mm) diameter boundary is an arbitrary size; smaller-diameter drilled piers can be designed and installed in accordance with ACI 543R. Although determination of overall pier size and concrete section design are two basic drilled pier design procedures, emphasis is focused on the determination of overall pier size, which is affected by the interaction between subgrade and pier. Because pier capacity is significantly affected by construction means and methods, the licensed design professional should understand these limitations. Construction methods described include excavation, casing, reinforcing steel installation, and concrete placement. Acceptance criteria and recommended procedures for construc-tion, engineering, and evaluation are presented.

Keywords: bearing capacity; caisson; casing; excavation; foundation; geotechnical engineering; lateral pressure; lining; slurry; tremie.

CONTENTS

CHAPTER 1—INTRODUCTION AND SCOPE, p. 21.1—Introduction, p. 21.2—Scope, p. 2

CHAPTER 2—NOTATION AND DEFINITIONS, p. 22.1—Notation, p. 22.2—Definitions, p. 2

CHAPTER 3—GENERAL CONSIDERATIONS, p. 33.1—General, p. 33.2—The structural and geotechnical teams, p. 33.3—Factors to consider, p. 43.4—Pier types, p. 43.5—Geotechnical considerations, p. 5

CHAPTER 4—DESIGN, p. 74.1—Loads, p. 74.2—Loading conditions, p. 84.3—Vertically loaded piers, p. 94.4—Laterally loaded piers, p. 94.5—Piers socketed in rock, p. 154.6—Strength design of piers, p. 164.7—Pier configuration, p. 16

CHAPTER 5—CONSTRUCTION MEANS AND METHODS, p. 17

5.1—Excavation and casing, p. 175.2—Placing reinforcement, p. 185.3—Dewatering, concreting, and removal of casing, p.

185.4—Slurry displacement method, p. 195.5—Safety, p. 21

CHAPTER 6—CONSTRUCTION ENGINEERING AND TESTING, p. 22

6.1—Scope, p. 226.2—Geotechnical field representative, p. 226.3—Preconstruction activities, p. 226.4—Construction geotechnical engineering procedures,

p. 236.5—Concrete placement, p. 25

William H. Oliver Jr., Chair Bernard H. Hertlein, Secretary

ACI 336.3R-14

Report on Design and Construction of Drilled Piers

Reported by ACI Committee 336

Clyde N. Baker Jr.William D. Brant

Diane M. CampioneJoseph P. ColacoConrad W. Felice

Rudolph P. FrizziShraddhakar HarshMatthew JohnsonJohn W. JohnstonJohnny H. Kwok

Hugh S. LacyAdam C. RammeSatish K. SachdevRodrigo Salgado

Harold R. Sandberg

Edward J. Ulrich

Consulting membersRonald W. Harris

John F. Seidensticker

ACI Committee Reports, Guides, and Commentaries are intended for guidance in planning, designing, executing, and inspecting construction. This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom.

Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer.

ACI 336.3R-14 supersedes ACI 336.3R-93 and was adopted and published August 2014.

Copyright © 2014, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by any

means, including the making of copies by any photo process, or by electronic or mechanical device, printed, written, or oral, or recording for sound or visual reproduc-tion or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors.

1Copyright American Concrete Institute Provided by IHS under license with ACI Licensee=University of Texas Revised Sub Account/5620001114, User=qrtr, tety4


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6.6—Post-construction assessment, p. 276.7—Reports, p. 276.8—Criteria for acceptance, p. 276.9—Corrective measures, p. 28

CHAPTER 7—REFERENCES, p. 28Authored documents, p. 28

CHAPTER 1—INTRODUCTION AND SCOPE

1.1—IntroductionThis report addresses design and construction of drilled

pier foundations constructed by digging, drilling, or other-wise excavating a hole in the subgrade that is subsequently filled with plain or reinforced concrete. Although structural design and construction of drilled pier foundations are the primary objectives of this report, relevant aspects of geotech-nical engineering are also discussed, as variations in subgrade properties have a critical influence on design, construction, and subsequent performance. Successful drilled pier design, construction, and performance requires reliable data on the applied loads and supporting subgrade. Because construc-tion limitations often govern design, combined cooperation among the geotechnical engineer, structural engineer, and drilled pier contractor are essential.

1.2—ScopeThis report is generally limited to piers of 30 in. (760

mm) or larger diameter, made by open or slurry stabilized construction methods. A 30 in. (760 mm) diameter boundary is an arbitrary size. Although smaller-diameter drilled piers have been designed and installed in accordance with this report, it is difficult to detect sidewall collapse. Refer to ACI 543R for concrete piles having diameters smaller than 30 in. (760 mm), piles installed by the use of hollow stem augers, or other pile types. Also beyond the scope of this report are rectangular columns on spread footings in deep excava-tions and foundations constructed without excavations by methods such as mortar intrusion or mixed-in-place.

Piers installed by tapping or ramming concrete or aggre-gate into an excavated shaft are beyond the scope of this report. Engineers and contractors have used the terms “cais-sons,” “foundation piers,” “bored piles,” “drilled shafts,” and “drilled piers” interchangeably. The term “drilled pier” is used in this report. A drilled pier with an enlarged base can be called a belled caisson, belled pier, or drilled and under-reamed footing. Drilled pier foundations excavated and concreted with water or slurry in the hole have been called slurry shafts, piers installed by wet-hole methods, or piers installed by slurry displacement methods.

CHAPTER 2—NOTATION AND DEFINITIONS

2.1—NotationAb = pier base area, in.2 (mm2)Ap = pier shaft surface area, in.2 (mm2)D = dead load, lb (N)

Dg = dead loads from the supported structure and weight of the pier (gross weight of the pier), lb (N)

d = diameter of pier, in. (mm)E = earthquake load, lb (N)Ec = modulus of elasticity, psi (MPa)F = vertical load, lb (N)FS = allowable strength design safety factorFS1 = allowable strength design safety factor for bearing

resistanceFS2 = allowable strength design safety factor for side

resistancefp = average side resistance, ton/ft2 (kPa)fz = unit load transfer from shaft to ground at depth z,

ton/ft2 (kPa)Hg = horizontal shear at ground surface, lb (N)I = moment of inertia, in.4 (mm4)ks = modulus of horizontal soil beam reaction, psi

(MPa)L = live load, lb (N)Mg = moment at ground surface, usually applied to pier

by superstructure, in.-lb (N-mm)N = number of blows in a standard penetration testPan = anchorage resistance, lb (N)Pq = ultimate end bearing acting at the base, lb (N)Pup = uplift force, lb (N)p = subgrade resistance per unit length along the pier,

lb/in. (N/mm)p-y = lateral load deflection curve at an element of pier,

lb/in., in. (N/mm, mm)Q = ultimate axial capacity, tons (kN)qb = unit end-bearing pressure, ton/ft2 (kPa)Sn = force from side friction, lb (N)Sp = positive side resistance, lb (N)Sp1 = positive side resistance, acting upward on the pier;

normally caused by downward movement of the pier relative to surrounding soil, lb (N)

T = relative stiffness factorW = wind load, lb (N)wb = movement of the pier at depth z, in. (mm)wz = unit deflection corresponding to fz, in. (mm)y = lateral deflection of pier, in. (mm)

2.2—DefinitionsACI provides a comprehensive list of definitions through

an online resource, “ACI Concrete Terminology,” http://concrete.org/Tools/ConcreteTerminology.aspx. Definitions provided here complement that source.

bearing-type pier—a pier that receives its principal vertical capacity from a subgrade layer at the bottom of the pier.

bell—an enlargement at the pier bottom to spread the load over a larger area, or to engage additional subgrade mass for uplift loading conditions; also called under-ream.

cap—an upper pier termination, usually placed sepa-rately, to correct deviations from desired location, facilitate anchor bolt or dowel setting within acceptable tolerances, or combine two or more piers into a unit supporting a column.

casing—a protective temporary or permanent steel tube, usually cylindrical in shape, lowered into the excavated hole

American Concrete Institute – Copyrighted © Material – www.concrete.org

2 REPORT ON DESIGN AND CONSTRUCTION OF DRILLED PIERS (ACI 336.3R-14)



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to protect personnel from sidewall collapse or cave-in, or to exclude soil, water, or both from the excavation.

combination bearing- and side-resistance pier—a pier that receives its vertical capacity from bearing at the bottom and resistance developed along the shaft sides.

construction geotechnical engineer—a geotechnical engi-neer with experience in drilled pier construction who is desig-nated by the owner to carry out the responsibilities defined in this report; also known as a soils engineer, soils and founda-tion engineer, or earthwork and foundation engineer.

construction geotechnical engineering—the interpre-tation and assessment of drilled pier construction observa-tions, equipment and materials used therein, and the field testing and results necessary to permit the construction geotechnical engineer to render a professional opinion as to conformance with the contract documents and founda-tion design. Construction geotechnical engineering does not include drilled pier contractor direction or supervision.

drilled pier—concrete cast-in-place foundation element with or without casing that may have an enlarged bearing area and that extends downward through weaker soils, water, or both, to a subgrade stratum capable of supporting the loads imposed on or within it.

flexible pier—a pier with a length-to-diameter ratio that will allow flexural deformations from lateral loads; the theo-retical point of fixity is within the pier shaft.

geotechnical engineer—an engineer with experience in geotechnical investigations for, and the design of, drilled piers who is designated by the owner to prepare the design geotechnical report and work with the structural engineer on the drilled pier design.

head—top of pier.Kelly bar—drill stem used to advance pier excavation

tools.obstruction—underground material that prevents a drill

rig from advancing the pier excavation to the desired final bearing level; may be concrete, concrete pipe, rubble, wood, or a boulder that is not part of the parent bedrock.

pig—a disposable device inserted into a tremie or pump pipe to separate the concrete from the pier excavation fluid inside the pipe. Also called rabbit, rabbit plug.

project specifications—the specifications stipulated by contract for a project that should employ ACI 336.1 by refer-ence and that serve as the instrument for defining the manda-tory and optional selections available under the specification.

rigid pier—a pier with a small depth-to-diameter ratio that will have negligible flexural deformations under lateral load. Lateral movements will be rotational type involving the entire length of the pier.

rock—a naturally formed mineral formation underlying the site including intact rock and partially weathered and weathered rock that should be defined by the geotechnical engineer for each project.

rock socketed pier—a pier that derives its capacity from both side resistance and end bearing within rock for some portion of its length.

shaft—portion of drilled pier above bearing surface exclu-sive of the base or bell, if any.

side resistance—subgrade friction or adhesion developed along the side surface of the pier. This resistance may resist force in either the positive or the negative vertical direction (uplift or compression).

slurry—drilling fluid that consists of water or water mixed with one or more of various fine solids or polymers that is primarily used to help prevent sloughing, collapse, or a borehole or drilled pier excavation, or both.

slurry displacement method—method of drilling and concreting, where drilling fluid is used to maintain the stability of the uncased drilled pier hold, to allow acceptable concrete placement, or both, when water seepage in a drilled pier hold is too severe to permit concreting in the dry.

socket—portion of pier within the bearing stratum.underream—see bell.wet method—construction procedure used when a pier

extends through a caving stratum; typically includes inserting a casing past unstable subgrade layer then cleaning out the casing; the pier may continue to advance by drilling dry.

CHAPTER 3—GENERAL CONSIDERATIONS

3.1—GeneralThe drilled pier’s function is twofold: 1) to transfer axial

loads, lateral loads, torsion loads, and bending moments to the surrounding subgrade; and 2) to support the subgrade surrounding. To perform these functions, the pier interacts with the subgrade around and below it, as well as with the superstructure above. The pier’s relationship to the subgrade is one of the most important variables in pier design. The subgrade can act as both the driving force, such as in earth pressure in a failing slope, and as resistance. In the absence of a theory that can encompass all the factors involved, simplifying assumptions should be made. However, subtle aspects of construction often govern the design.

3.2—The structural and geotechnical teamsDrilled pier design is the joint responsibility of the struc-

tural and geotechnical engineers. Many variables are consid-ered in successful drilled pier design, including the inter-dependence of the structure and soil interaction, structural materials, and construction means and methods. A structural and geotechnical engineering team approach provides a more rational design than an independent one. The structure, drilled pier, and subgrade stiffness can induce composite system loading variations relative to the loads calculated on the assumption that all drilled piers are sized for the same subgrade-bearing pressure and assumed settlement. A reasonable approximation of the state of compatibility can greatly improve design, and allow concrete and rein-forcement to be placed where they will function efficiently. Achieving this goal usually requires an iterative design process with an examination after each step to decide the required adjustments necessary to accomplish compatibility at the next stage of the process. The proposed approach will lead to a rational understanding of the design requirements and reduced risk of unacceptable performance.


REPORT ON DESIGN AND CONSTRUCTION OF DRILLED PIERS (ACI 336.3R-14) 3



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3.2.1 Continuity from design through construction—The importance of design continuity through construction by the structural and geotechnical engineers is emphasized because the design’s success relies on the underground conditions along with construction means and methods. The geotech-nical engineer should have a thorough understanding of the limitations of construction means and methods and their influence on design, and should provide the construction geotechnical engineering with assistance during assessment of the as-installed pier compatibility with design require-ments (Ulrich 2007).

The experience of geotechnical engineers varies; one geotechnical engineer might assess observed field condi-tions differently than another who performed the design. It is not unusual for them to report differing evaluations for design soil resistance and loading conditions taken from the same site using similar tests.

The presence of the construction geotechnical engi-neer should be required during pier installation (Ulrich and Ehlers 1995; Ulrich 2007). The geotechnical engi-neer should develop the specifications, which should clearly define requirements for testing agency services and construction engineering because the construction geotech-nical engineer will form the opinion on each pier’s accept-ability. The geotechnical engineer may collaborate with the structural engineer as needed to complete the specification for installation. Because field decisions and interpretations of contractor conformance with the design specification can affect design, the geotechnical engineer or construction geotechnical engineer could assume responsibility for both.

3.3—Factors to considerComputational results and expected behavior should be

evaluated based on 3.3.1 through 3.3.5.3.3.1 Subsurface conditions—Soil stratification; ground

water conditions; and the depth, thickness, and nature of the subgrade constituting the bearing stratum influence the choice of construction method and drilled pier design. Specifically, design-bearing pressure and side shear deter-mine the required pier bottom area and socket length in the bearing stratum. The geotechnical engineer selects the design-bearing pressure and side shear based on soil and rock samples, tests, analysis, judgment, and experience, based on load character, tolerated settlements, and construc-tion methods. Material properties above and in the bearing stratum vicinity and disturbance from construction activity determine the feasibility of constructing a socket or bell without slurry. Soil permeability, groundwater conditions, and soil gradation are factors that determine whether casing, slurry, or dewatering is required. These parameters also dictate the concrete placement method, and may influence ground loss considerations. Shear strength and deformation, including shrink and swell characteristics of the subgrade material penetrated by the shaft, determine whether side resistance in these materials should be a design consider-ation. Side resistance could act to resist superstructure loads or a major applied downward drag in consolidating subgrade or uplift load in swelling subgrade on the shaft.

The location and characteristics of rock on the site can have a significant impact, particularly on construction cost and duration. The geotechnical engineer should define rock for each project. Sedimentary, metamorphic, and igneous rock have highly varying capacity, and high-capacity piers may require special investigations of the rock character, sometimes below each pier. The use of drilled piers bearing in limestone susceptible to solution activity requires caution due to the risk of voids or other discontinuities.

3.3.2 Site conditions—Available construction area, site access, headroom, and existing facilities requiring protec-tion against settlement, ground loss, noise, or contamination, influence the construction method and design. The choice of design and construction methods used for new piers may lead to subsidence, which can be caused, for example, by uncontrolled excavation or by removing fine-grained mate-rials from the surrounding soil through water flow from dewatering or consolidation. These effects on adjacent and new structures should be design considerations.

3.3.3 Construction geotechnical engineering and quality control—Perform geotechnical engineering to confirm the validity of simplifying design assumptions and to assess compatibility of construction means and methods with design requirements. Scope and method of observation, obtainable tolerances, and quality control influence the degree to which the design can reasonably be refined.

Drilled pier design and installation are multiphase tasks where proper quality control and assurance in construction are vital. Without them, the probability of a successful foun-dation is reduced. Even the highest quality structural element can have its capacity as a load-carrying member significantly reduced from improper installation details and relationship between the installed pier to the surrounding subgrade.

3.3.4 Constraints—Available construction expertise and equipment, available materials, and building code requirements, including construction limitations, influence construction and design.

3.3.5 Design considerations—The licensed design professional should compute vertical and lateral loads and moments imposed on the pier. Length and section properties of the pier, distribution of load on end bearing, lateral resis-tance, and side resistance are determined based on loads and subsurface conditions. The subgrade may be part or all of the driving force, particularly in situations involving slopes or retaining walls.

The pier stiffness EcI surrounding subgrade response and their interaction are important in the analyses of later-ally loaded piers. Subgrade response is the least predictable variable. Pier deflection is often the limiting factor in deter-mining acceptable performance in response to lateral loads and moments.

3.4—Pier typesPiers can be divided into types according to the manner in

which axial loads are transferred to the subgrade and the pier response to lateral load. The assignment of a given pier to a certain type can depend on the:





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a) Soil and rock qualities around the shaft and at the pier base;

b) Character of the contact surface between pier and subgrade;

c) Relative stiffness factor;d) Embedded length of pier.3.4.1 Axially loaded piers—With respect to axial load

capacity, there are three types of piers (3.4.1.1 through 3.4.1.3).

3.4.1.1 Bearing-type pier—A straight-sided or belled pier, drilled through weaker soils and terminating on subgrade of satisfactory bearing capacity, is considered a bearing-type pier if it is designed to achieve its capacity from that stratum.

The bearing area can be increased by a bell at the shaft bottom. However, the soils in which the bell is constructed should have sufficient cohesion to permit the excavation to stay open until the concrete is placed. In caving soils, bells are normally not used because the bell may require grouting or installation by the slurry displacement method, which involves increased risks for construction defects. Alterna-tively, the pier may be enlarged to eliminate the need for the bell, or extended into a material in which a bell is more easily excavated.

3.4.1.2 Combination bearing- and side resistance-type pier (Fig. 3.4.1.2)—A pier extended (socketed) into a bearing stratum in such a manner that part of the axial load is transferred to the sides of the shaft and the rest of the load is carried in end bearing.

3.4.1.3 Side resistance-type pier (Fig. 3.4.1.3)—A pier built into a bearing stratum in such a manner that the load is carried by side resistance.

3.4.2 Laterally loaded piers—Based on response to lateral load, there are two pier types (3.4.2.1 and 3.4.2.2).

3.4.2.1 Rigid pier (Fig. 3.4.2.1)—A pier so short and stiff in relation to the surrounding soil that lateral deflection is primarily due to the pier’s rotation about a point along its length, its horizontal translation, or both. A rigid pier’s rotational resistance is governed in part by the load defor-mation characteristics of the soil adjacent to and under the embedded portion of the pier, and by the restraint, if any, provided by the structure above. Piers shorter than approxi-mately 10 diameters behave as rigid piers.

3.4.2.2 Flexible pier (Fig. 3.4.2.2)—A pier of suffi-cient length and with flexural rigidity (EcI) relative to the surrounding soil such that lateral deflections are primarily due to flexure. Piers longer than 10 diameters typically behave as flexible piers. The depth below which the pier is considered fixed will vary depending on underground condi-tions, loading, and pier properties. Further discussion of flexible behavior is found in ACI 543R.

3.5—Geotechnical considerationsThe geotechnical engineer should determine the scope of

investigation needed for pier design, and should:a) Have adequate knowledge of the underground site

conditions, selecting a foundation system that resists imposed loads and is compatible to design, and is econom-ical and constructible;

b) Perform subsurface exploration that is thorough, and that has enough samples and data to establish that the soil, rock properties, or both are within the zones of interest;

c) During investigation, consider the influence of geologic features on foundation design and performance;

d) Consider and evaluate as needed: collapsing soils, random variation in fill character, shrinkage and swelling conditions, slope stability, soil liquefaction potential, rock cavities or other voids, potential rock collapse, progressive limestone solution, and weathering profiles.

The scope of the investigation should include the following (3.5.1 through 3.5.9).

3.5.1 Exploration—The geotechnical engineer should determine the type of exploration appropriate for the project needs including soil borings, depth and type of rock coring, cone penetrometer tests, dilatometer tests (DMTs), surface seismic profiling, and cross-hole seismic exploration. A sufficient number of explorations should be made to establish with reasonable certainty the subsurface

Fig. 3.4.1.2—Combination bearing-type and side resis-tance-type pier.





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stratification (profile) and water table location. Where the piers are to terminate in rock, exploration should include a bedrock surface profile and character. The structure type and

geologic and geomorphic conditions will govern the explo-ration program. Exploration of foundations for a structure that will rest on igneous rock that is known to be boulder- and joint-free will vary significantly from exploration of the same structure that will rest on limestone with severe cavi-

Fig. 3.4.1.3—Side resistance-type pier.

Fig. 3.4.2.1—Rigid-type pier (Davisson 1969) with notation altered.

Fig. 3.4.2.2—Flexible-type pier (Davisson 1969).





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ties and solution activity, even when the intact rock strength at both sites is similar.

3.5.2 Depth of explorations—Exploration depth should reach settlement of the bearing stratum below the pier. Where practical, at least one boring should extend into bedrock regardless of how far below it is to the anticipated bearing level of the piers.

3.5.3 Water table and dewatering—If water is encoun-tered within the zone of pier penetration, the site explora-tion should identify the water table elevation(s), potential for fluctuations, permeability, and gradient across the site.

3.5.4 Piers to bedrock level—If piers are to be founded on rock or socketed into bedrock, probes or cores should be extended into the bedrock to a depth of at least twice the diameter of the bearing area below the base level, but not less than 10 ft (3 m). This depth is necessary to determine rock strength and condition, and the potential for suspended boul-ders above bedrock. The minimum size cores preferred are NX size when pier capacity is high and the rock quality is crit-ical to establishing maximum pier capacity (ASTM D2113).

3.5.5 Boulders and obstructions—Geotechnical investiga-tions should pay particular attention to identifying potential obstructions and boulders because drilled piers are not often easily relocated and the presence of unexpected obstructions to drilling could increase the cost, time, or both to construct drilled piers. This is one of the more common causes of contractor claims.

Suggested subsurface investigation procedures include:a) If high standard penetration test (SPT) values are observed

and cobbles are suspected, use a larger sampling spoon;b) If boring refusal is encountered, do not terminate and

relocate the boring but, instead, core through the obstruction and determine its size and character;

c) Extend borings several feet (meters) below design penetration of the piers and do not terminate borings upon refusal at expected rock level. Instead, core into bedrock to determine if it is a disconnected boulder;

d) If obstructions are encountered at a site, add more borings at pier locations;

e) Shallow obstructions should be explored by test pits to uncover old foundations, abandoned and active utilities, and foundations of abutting buildings including left-in-place sheeting that may extend beyond their property line and obstruct the drilled piers.

Multiple test pier installations may be needed to finalize the foundation design. Specifications should be sufficiently flexible to allow design modifications during construction.

3.5.6 Subgrade strength—In cohesive soil, obtain the unit weight and strength parameters from sufficient samples taken at different depths to determine depth trends; indi-vidual samples may be erratic. In cohesionless soils, it is common practice to estimate the soil’s relative density to determine the allowable soil pressure based on SPT, cone penetration test (CPT), DMT, or pressure meter test (PMT). Where lateral load behavior of drilled piers is important, special attention should be given to the determination of subgrade properties within 10 pier diameters of the surface.

3.5.7 Axial load tests—For large projects or in cases of inadequate experience with subsurface conditions, load tests should be performed. Given the potential variability of capacity prediction methods, load tests remain the best method of optimizing pier design capacity.

Load test reactions are provided by belled or socketed piers designed for tension and positioned around the drilled pier being compression tested. The reaction piers transfer load through a reaction frame and jacks positioned above the test pier (ASTM D1143).

Heavily loaded drilled piers are often tested using other methods. One such method, developed by Osterberg (1989), consists of installing an embedded instrumented loading jack capable of applying a load in excess of the pier’s maximum design load within a full-scale test pier. Both end bearing and side resistance are tested. The maximum test load is defined by failure in either side resistance or end bearing. If maximum side resistance is exceeded, the test can be extended further conventionally with a surface reaction frame and dead weights or anchor piers to permit applying higher end-bearing load. Another common test method accelerates an independent reaction mass upward using pressure contained in a chamber. The resulting reac-tion is applied to the pier (ASTM D7383). Load tests may also be performed on small-diameter, instrumented, drilled piers; however, care should be exercised in extrapolating the results to larger-diameter piers.

Where water inflow is not a problem, it is also possible to drill a full-sized pier and perform a plate load test at the pier base level to establish bearing capacity. Although such testing is uncommon, it has the advantage of allowing visual observation of the subsurface conditions before testing.

3.5.8 Lateral response load tests—Lateral load testing has become common, particularly where earthquake or wind loading is important. Often performed by placing a jacking frame between two drilled piers and jacking them apart, it measures the relative and absolute movement of each pier. If drilled piers are to be installed from a level above a base-ment subgrade, it is often important to perform the lateral load test after excavating to subgrade. This becomes an issue when the subgrade is below the groundwater table and the lateral load test requires completion before production pier installation begins.

3.5.9 Design geotechnical report—The design geotech-nical report, written by the geotechnical engineer in nonman-datory language to guide foundation design and construction, may be included with the contract documents for information only. The design geotechnical report is distinguished from a geotechnical data report in that it provides design parameters and recommendations on construction methods. Wholesale inclusion of reports into specifications is not advisable. They should instead be listed as a reference document and used to prepare plans and specifications.

CHAPTER 4—DESIGN

4.1—LoadsPier design consists of two steps:





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(1) Determination of pier dimensions;(2) Design of the concrete pier element and any necessary

reinforcement.In Step (1), which involves interaction between the

subgrade and pier, all loads are traditionally service loads and all subgrade stresses are at allowable values. The applied service loads do not include load factors. If load and resis-tance factor design (LRFD) methods are used, factored loads are compared with soil resistances that have been reduced to account for their variability and uncertainty. Great care should be taken that soil resistances are calibrated. In either approach, it is critical that the stresses in the subgrade and the predicted deflections are compatible with the principles of soil mechanics.

In Step (2), the pier is structurally designed by the strength design method as adjusted in this report, although some prefer to use the alternate design method (4.6). The alternate design method employs the use of service loads in the struc-tural design. Pier-subgrade capacity compatibility is not part of the analysis. Normally, service loads are used to calculate the resulting moments, shears, and axial forces, which are multiplied by the appropriate load factors for the various cases of loading for structural design of the pier. If factored loads are applied to the pier, the soil pressures required to maintain equilibrium with these factored loads are used to obtain the moments, shears, and axial forces necessary for strength design of the concrete pier (4.6). Where moments or eccentric loading conditions are involved, the soil pressures required to resist factored loadings could have distributions different from those found for the service load conditions. The soil pressures used for evaluation of the geotechnical capacity should be compatible with the principles of soil mechanics.

4.1.1 Axial loads—Axial loads may consist of the axial components of D, Dg, LL, W, E, Sp1, Sn, Pq, Pup, and Pan.

4.1.2 Lateral loads and moments—Lateral loads may be caused by unbalanced subgrade pressures, thermal move-ment of the superstructure, wind, wave, ice, impact, or earthquake-generated forces. Moments may be caused by axial loads applied with eccentricity, by lateral loads, and may be induced by the superstructure through connections to the pier. Moments or lateral loads may also be applied by the subgrade directly or indirectly.

4.2—Loading conditionsPier forces are determined from whichever combination

of loading produces the greater value for the item under investigation.

4.2.1 Axial loads—Maximum and minimum loading conditions should be investigated for pertinent stages of construction and for the completed structure. Load factors should be incorporated in loading equations as indicated in the applicable code.

4.2.1.1 Maximum loading—Excess weight of the pier foundation over the weight of the excavated soil, negative side resistance or downdrag, loads applied due to swelling soils, and long-term redistribution effects on side resistance

should be considered. For example, an initial upward acting side resistance may lessen, disappear, or reverse with time to act as downdrag.

a) Dead load, live load, side resistance, and uplift, if consistently present:

When positive, upward acting side resistance is present:

(D + L + Pup) < (Pq/FS1 + Sp1/FS2) (4.2.1.1a)

If negative side resistance is present:

(D + L + Pup) < ((Pq + Sp1)/FS – Sn) (4.2.1.1b)

Equation (4.2.1.1a) or (4.2.1.1b), whichever applies to the condition investigated, should always be satisfied.

b) Dead load, live load, side resistance, uplift and wind or earthquake:

If positive side resistance is present:

(D + 0.75(L + (W or E) – Pup)) < (Pq/FS1 + Sp1/FS2) (4.2.1.1c)

If negative side resistance is present:

(0.6D + (L + (W or E) – Pup)) < ((Pq + Sp1)/FS – Sn) (4.2.1.1d)

Often, a factor of 1.33 may be applied to the resistance under short-term loading, but this factor may range from 1.0 to greater than 1.33, depending on the confidence in the resistance and the factor of safety adopted. Some codes may not permit factors greater than 1.0. In Eq. (4.2.1.1c) and (4.2.1.1d), W or E should be entered at its maximum down-ward acting value. Side friction resistance and end bearing develop at different displacements and are dependent on subgrade properties.

In Eq. (4.2.1.1d), only the value of W that exceeds Sn should be used because at the point of bearing capacity failure, wind load and downward friction cannot be addi-tive. Side resistance is often developed at low displace-ments of 0.1 to 0.4 in. (3 to 10 mm) whereas tip resistance is developed at large displacements (2 to 5 percent of pier diameter in cohesive soils and elastic parts of the resis-tance in granular soils) (O’Neill and Reese 1999). Factors of safety should be applied separately to these resistances when considering relative displacement. The value of Sn in Eq. (4.2.1.1d) is sometimes reduced or reversed due to pile strain from applied temporary vertical loading (Terzaghi et al. 1996). A more complete discussion of negative side fric-tion is found in Davisson (1993).

In Eq. (4.2.1.1a) through (4.2.1.1d), uplift Pup should be entered at its lowest permanent value only.

4.2.1.2 Minimum loading—In Eq. (4.2.1.2a) through (4.2.1.2c), uplift Pup is entered at its maximum value. If

(Pup + (W or E)) < Dg (4.2.1.2a)





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no further investigation is needed. Otherwise, Eq. (4.2.1.2b) and (4.2.1.2c) should be satisfied.

(Pup – 0.9Dg) < Sn = (Pan/FS2) (4.2.1.2b)

Pup – 0.9Dg + 1.25W) < (Sn + Pan/FS2) (4.2.1.2c)

If sufficient side resistance is available, anchors to rock or soil (Pan) will not be necessary. In Eq. (4.2.1.2a) and (4.2.1.2c), W should be entered at its maximum upward acting value.

4.2.2 Combined loadings—The lateral loads and moments should be superimposed on simultaneously occurring axial loads in any of the combinations listed in 4.2.1.

4.3—Vertically loaded piers4.3.1 Capacity from subgrade—The total ultimate axial

capacity may be a combination of end bearing and side friction. The theoretical ultimate capacity is expressed in Eq. (4.3.1a)

Q = Sp1 + Pq (4.3.1a)

The geotechnical engineer should consider strain compat-ibility and deflection in determining the safety factor. Safety factors could vary from 1.5 to 5 for side friction or end bearing, depending on the subsurface conditions, structural loads, and degree of confidence in the subsurface param-eters. The side friction and end bearing may be described further by the following equations

Sp1 = fp · Ap and Pq = qb · Ab (4.3.1b)

The geotechnical engineer should provide values for fp and qb using the soil, rock, or both soil and rock properties and construction method. The values of fp and qb vary widely and are depth dependent. Determination of these values may require iterative estimates of the allowable capacity of the drilled pier foundation in collaboration with the structural engineer to satisfy both factor of safety and allowable settle-ment requirements. Due to strain compatibility requirements, the total ultimate capacity will be less than the maximum theo-retical determined from the individual end and side resistance components because peak side resistance typically develops much faster than maximum end-bearing resistance.

Load testing continues to be the most reliable method of developing or confirming design capacities for drilled piers because load-carrying capacity can be impacted by construc-tion means and methods, and no single accepted method of developing service load capacity exists.

4.3.2 Pier settlement—The subgrade compression proper-ties should be determined to permit estimates of total and differential settlement. In-place tests, such as cone penetrom-eter, DMT, PMT, or plate load at pier subgrade, full-scale load tests, and laboratory tests of undisturbed pier subgrade are commonly used. Total pier settlement is the sum of pier base movement plus elastic pier shortening considering side resistance. The geotechnical engineer’s settlement estimate

should consider the effects of adjacent foundations, which can influence the behavior of individual piers (Ulrich and Ehlers 1995; Ulrich 2007).

4.4—Laterally loaded piersAt present, the most complete method for evaluating

lateral response of piers is a beam on an elastic founda-tion mathematical model using a computer and a nonlinear soil response (Reese 1977, 1984; Reese and Wright 1977). Commercial lateral analysis software is available and widely used. The major variables are the subgrade response and pier stiffness (EcI). Subgrade reaction may be modeled as a linear spring or as an elastic-plastic material using p-y data (Reese 1977, 1984; Reese and O’Neill 1988; Reese and Wright 1977). Because no unique method of modeling the subgrade response is universally accepted, the geotechnical engineer should develop the subgrade response model based on the model for which the licensed design professional has rele-vant local experience.

4.4.1 Lateral loads and moments—Drilled piers will be subject to large lateral loads along the pier length in cases when piers are used as retaining walls; walls to arrest slope movement; anchors; and foundations for power poles, elevated tanks, and tall buildings. When the subgrade pres-sures on the basement walls are unequal or insufficient to resist the lateral loads within allowable deformation of the superstructure, the foundations should provide the necessary resistance. The piers will then be loaded with lateral forces at the top. Axial forces result from overturning and some-times moments at the top.

The allowable pier head deflection in each design case may be a few tenths of an inch or a few inches (mm) depending on the project requirements.

Piers that resist lateral load have been designed success-fully by approximate methods. The allowable lateral load on a vertical pier is obtained from a table of presumptive values found in some handbooks, building codes, or from simpli-fied solutions that assume a rigid pier and one soil type. These allowable loads, however, might not be appropriate compared with values computed by the method recom-mended in 4.4.4, and they provide no information on pier deflection. Simplified solutions could be misleading for many drilled pier foundations.

4.4.1.1 Inclined piers—To avoid analyzing a pier for lateral loads, some licensed design professionals assume, according to the approach used for driven piles, that the lateral loads are resisted by the lateral component of axial loads taken by piers installed on an incline. Most methods available for the analysis of a pier group that includes inclined piers are approximate; the movements of the pier head under load are not considered. Inclined piers in design should be used with caution because the contractor often cannot install the piers at the angle desired. Inclined piers provide a much stiffer response at small deflections, often attracting the entire lateral load and causing high localized shear in the pier cap and rotation of the cap. Designs should provide for these loads and deflections.





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4.4.1.2 Beam on elastic foundation—Theory and experi-ence have shown that a more rational and satisfactory solu-tion of the lateral loaded pier design is obtained by using the method of soil-structure interaction with the theory of a beam on an elastic foundation. Variable pier stiffness and multilayered soil systems are fundamental parame-ters addressed in the analysis using the beam on an elastic foundation theory. Because soil or subgrade response and considerations of construction means and methods are the most critical elements of analysis, the geotechnical engineer should develop the subgrade response model. Although the geotechnical engineer or structural engineer can design the pier, analysis and design by the geotechnical engineer is recommended to minimize possible miscommunication or misinterpretation of the subgrade response model.

4.4.2 Laterally loaded pier problem—A laterally loaded pier is a soil-structure interaction problem. The solution requires that numerical relationships between pier deflection and soil reaction be identified and considered in obtaining the deflected shape of the pier. Application of a lateral load to the head of a pier causes lateral deflection of the pier. Soil reactions satisfy the equations of static equilibrium and should be consistent with deflections. In addition, because no pier is completely rigid, the amount of pier bending should be consistent with the soil properties and pier stiffness.

Lateral forces on piers, which depend on the environment and function of the supported structure, can be produced by wind, waves, ships, ice action, subgrade pressures, seismic action, or mechanical causes.

The ability of piers to resist lateral loads depend on construction means and methods, material, and stiffness; subsoil conditions; embedment of pier, pier cap, and founda-tion wall in the soil; degree of fixity of pier to cap connec-tion; pier spacing; and the existence and magnitude of axial loads. Group-effect limitations are more severe for laterally loaded piers than for those with axial loads only (Davisson 1969; Reese and Van Impe 2001).

In evaluating the lateral capacity of vertical piers, the soil resistance against the pier, pier cap, and foundation walls should be considered. Soil resistance can contribute substan-tially to the lateral capacity of a pier group or pier founda-tion, provided that soil is present for the loading conditions under consideration. The presence of axial compressive loads can contribute to the pier’s lateral or bending capacity by reducing tension stresses caused by bending due to lateral loads. Design methods for lateral loading of concrete piers should consider axial loads, whether compression or tension, and lateral soil resistance. Cracked or uncracked stiffness properties of the concrete pier should be consid-ered (4.4.4.13 through 4.4.4.16). If lateral load capacity is critical, it should be investigated or verified by field tests under predicted in-service loading conditions, including the vertical dead load that can be considered permanent.

Research (O’Neill and Reese 1999; Reese 1984) has promoted modeling a concrete pile as a system of finite differences and the subgrade reaction as nonlinear responses called p-y data. The analysis and design by this approach allows the consideration of multiple strata and slope condi-

tions. The method originated in response to the need to design large-diameter offshore piles for combined axial and lateral loading, and was later applied to piers and concrete piles. The use of the finite difference method promoted by O’Neill and Reese (1999) is probably the most widely used approach for concrete piers and piles subjected to lateral loads. Confirming load test results are few rather than abundant.

Modeling the lateral subgrade reaction as an elastic system is reliable for small top deflections, but the use of the p-y approach may be more helpful for large deflections and layered profiles. Technological advances have allowed the mathematical problem to be solved with relative ease, but the subgrade response characteristics are still uncertain in part because of the limited database used to develop the p-y response models. Strain gauge measurements have made it possible the determine soil response during the testing of full-scale piers, and numerical solutions allow the deflected shape or lateral deflection of a pier to be computed rapidly and accurately even though the soil reaction against the pier is a nonlinear function of pier deflection and of depth below the ground surface.

Although several methods (GAI Consultants 1982; Borden and Gabr 1987; Poulos and Davis 1980) are available for the analysis of drilled piers, the method reported by Reese (1984) is shown in 4.4.3 through 4.4.6, along with approxi-mate methods that may be used for preliminary analysis. The approximate methods are more suitable in a single layer.

4.4.3 Pier-subgrade interaction—The soil-structure inter-action problem is illustrated by considering the behavior of a strip footing, as shown in Fig. 4.4.3a.

Fig. 4.4.3a—Soil structure interaction for a strip footing.





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It is usually assumed that the bearing stress is uniform across the base of the footing, as shown in Fig. 4.4.3a. However, under the stress distribution shown, the cantilever portion of the footing will deflect and the downward move-ment at b will be less than at a. The footing is probably stiff enough that the deflection of b, with respect to a, is small; however, the concept is established that the footing base does not remain planar. Therefore, the bearing stress across the footing base conceptually should not be uniform. Two examples of soil-structure interaction illustrate the same kind of problem to resolve for drilled piers (Fig. 4.4.3b and 4.4.3c).

Figure 4.4.3b is a model of an axially loaded pier with soil replaced by a set of mechanisms.

The mechanisms show that the loads transferred in side resistance and in end bearing are nonlinear functions of the downward movement of the pier. A nonlinear curve showing axial load versus pile head movement can easily be obtained (Reese 1984) if the mechanisms are described numerically (4.4.3b).

A model for a laterally loaded pier is shown in Fig. 4.4.3c.A pier is shown with lateral loading at its top (Fig. 4.4.3c).

Again, the soil has been replaced by a set of mechanisms that conceptually define subgrade-response curves. Such curves give the subgrade resistance per unit length along the pier p as a function of pier deflection y. The mechanisms vary with position along the pier, as p is a nonlinear function of both y and x. The p-y concept, though two-dimensional, is based

on the synthesis of full-scale pile and pier load tests and subgrade properties. Shear at the base of the pier is neglected because the pier is considered sufficiently long that lengths are assumed to extend below the theoretical depth of fixity. Determination of p-y curves by the geotechnical engineer and the selection of pier stiffness are the two most important considerations in the analysis of laterally loaded piers.

4.4.4 Five general methods of solution for an individual pier—Among the available methods, five are considered for the solution of a single pier under lateral loads:

(1) Computer with nonlinear soil response using a beam on an elastic foundation;

(2) Static;(3) Nondimensional curves;(4) Elastic;(5) Curves and charts.Methods (3), (4), and (5) are considered simplified.

Methods (4) and (5) will not be discussed, as the elastic method has a limited application and a large number of curves and charts would be required.

Using the beam on an elastic foundation method (1) is preferred in the design of piers under lateral load. Although the method is easy to employ with modern computers, the subgrade response characteristics remain complex. A unique method with consistent parameters is not available. Geotech-nical experience and judgment are the principal elements of analysis, preferably requiring analysis by the geotechnical engineer.

Fig. 4.4.3b—Model of a pier under axial load (Reese and O’Neill [1988] with notation altered).





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The static (2) and nondimensional (3) methods have a place in the design process, but they are primarily for small-diameter piles. They can be employed for preliminary design or as a check of the computer output in simple cases. Simplified methods are limited in that multilayered systems and complicated ground geometries cannot be considered. Frequently there is uncertainty regarding some of the param-eters that enter design computations—for example, in the strength and deformation characteristics of supporting soil. The computer method allows the geotechnical engineer to investigate the influence of these uncertainties, because the response of the pier to small variations in parameters is readily seen.

4.4.4.1 Preliminary design— For preliminary design, several methods of analysis are used to evaluate the capacity and deformation of laterally loaded piers (Broms 1965). Many licensed design professionals use a finite difference solution of the beam-column equation with the p-y method reported by Reese (1984) for both preliminary and final design. Any of the following methods described can be used by the licensed design professional for preliminary design. The preliminary design can be adequate for final design, depending on the complexity of the project, the licensed design professional’s local experience, and project details.

4.4.4.2 Subgrade reaction analysis—Displacement of laterally loaded piers is based on the beam-on-elastic subgrade theory using simplifying assumptions regarding

soil stress-strain behavior. The method of Singh et al. (1971) is used to compute the lateral capacity, displacement, and maximum moment of piers in cohesive and cohesionless soils as a function of pier dimensions, type of loading, and fixity of the head. The method is applicable, provided the ratio of pier length to the relative stiffness factor (T) is greater than 5.

4.4.4.3 Finite difference method with nonlinear soil response—Preliminary design of laterally loaded drilled piers may be based on the results of computer methods with nonlinear soil response as reported by Reese (1984). The nonlinear flexural rigidity of the pier may be incorporated into the analysis to consider the composite properties of the pier.

4.4.4.4 Nondimensional solutions—Preliminary design of laterally loaded drilled piers may be based on using nondi-mensional solutions reported by Reese (1984).

4.4.4.5 Final design—Although several methods are available (GAI Consultants 1982; Poulos and Davis 1980; Borden and Gabr 1987), the analysis and design method reported by Reese (1984) is presented in 4.4.4. Selection of the design analysis parameters should include consider-ation of the limitations of both the empirical database for the subgrade response and the construction methods that might be used. The project specifications should be prepared so that the permitted construction methods are consistent with design.

The computer design procedure considers the soil-struc-ture interaction problem using relationships (p-y curves) to define the ground reaction (p) versus pier deflection (y) along the length of the pier. The use of p-y curves requires both static equilibrium and compatibility of reaction between the pier and ground, with pier deflections consistent with stiff-ness of the pier and ground. Drilled piers are classified as either long and flexible, or short and rigid, and as fixed against rotation or free to rotate at the ground surface. The extent of head fixity depends on relative stiffness between the pier(s) and cap if present.

Analysis is first performed using a stiffness based on the concrete modulus of elasticity and gross moment of inertia. The analysis is then refined using reduced pier stiffness values based on the composite section, loading conditions, and code requirements.

4.4.4.6 Response of pier and subgrade to lateral loads and moments—Friction along the bottom of the cap should be disregarded for design purposes because the slightest soil consolidation beneath the cap eliminates it unless special measures are taken to maintain continued lateral soil shear resistance. The passive pressure against the cap should also be disregarded wherever excavation for repair or altera-tion of underground installations will render it ineffective. Passive soil pressures mobilized against pier and pier cap are effective in resisting lateral loads, provided the displace-ments to mobilize them can be tolerated. In areas subject to freezing, the influence of frost heave and freezing-and-thawing cycles at the ground surface should be considered.

The solution to the theory of a beam on elastic foundation following the procedures in Reese (1984) for a pier under

Fig. 4.4.3c—Model of a pier under lateral loading showing concept of bilinear soil response curves (Reese and O’Neill [1988] with notation altered).





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lateral load should meet two general conditions: 1) The equations of equilibrium should be satisfied; and 2) deflec-tions and deformations should be consistent and compatible. Details related to the solutions of the laterally loaded pier using p-y subgrade response are given in Reese (1984).

4.4.4.7 Scour—For structures located in rivers, oceans, or bays, the potential for loss of lateral capacity due to scour should be a design consideration. Richardson et al. (1991) provides general guidelines and methods to estimate and design bridge structure foundations to resist scour.

4.4.4.8 Cyclic loading—The influence of cyclic loading on the load-deformation behavior of laterally loaded drilled piers should be considered in the design. The effects of cyclic lateral loading are pronounced for free-headed piers in stiff cohesive soils. Cyclic loading in loose granular soils also causes reduced resistance to lateral loading, but the effect is much less pronounced than in clays (Reese 1984). In general, cyclic loading progressively increases deflections of piers in clays due to strain softening. Cyclic loading tends to reduce the lateral subgrade modulus, which may be reduced to approximately 30 percent of the initial value. The combi-nation of group action and cyclic loading may reduce it to as little as 10 percent of the initial ks value (Davisson 1969). The stress level, expressed as a percentage of stress at maximum deflection, should be considered. Small stress levels could result in negligible reduction in subgrade modulus due to cyclic loading. The actual reduction should be determined by the geotechnical engineer using tests where feasible.

4.4.4.9 Group action—Drilled piers in a group are consid-ered to act individually when the center-to-center spacing perpendicular to the direction of the applied load is greater than three diameters and when the spacing parallel to the direction of the applied load is greater than or equal to eight diameters (Fig. 4.4.4.9). When the pier layout does not conform to these spacing requirements, pier interaction should be considered in the design. For the case of closely spaced drilled piers in a group, the interaction behavior is typically accounted for indirectly using empirical proce-dures proposed by Reese (1984) and Poulos and Davis (1980). Reese’s procedure assumes a reduction in the coef-ficient of lateral subgrade reaction for a pier in a group from that of a single pier using the ratios shown in Table 4.4.4.9.

In addition to row spacing, pier spacing perpendicular to the load as well as the depth and magnitude of pier deflection should also be considered when evaluating group effects

and interference. Several authors have indicated that the recommended ratios of lateral resistance to center-to-center spacing can be refined. Reese and Van Impe (2001) offer a review of methods for estimating effects of spacing on the lateral capacity of piers in a group.

4.4.4.10 Combined axial and lateral loading—The ground-line deflections and maximum moments of drilled piers subject to lateral loads increase with increasing axial load. The effects of combined axial and lateral loading are most pronounced for free head, short, small-diameter piers in loose or soft ground conditions. Combined loadings can be accounted for in computer analyses (Reese 1984).

4.4.4.11 Sloping ground—For drilled piers that extend through or below sloping ground, the potential for addi-tional lateral loading should be considered during design. Placement of drilled piers above or on slopes will reduce the lateral capacity and increase displacement compared with similarly sized and loaded piers constructed in level ground. Methods of analysis for piers in stable slopes are given by Borden and Gabr (1987) and Reese and Van Impe (2001). Additional consideration should be given for piers in slopes having low factors of safety such as marginally stable slopes or those showing ground creep, as well as for piers extending through fills overlying soft soils bearing into more compe-tent underlying subgrade formations. Lateral loading from slope movement can be large and fail piers in bending and shear. The geotechnical engineer should consider the slope condition and its effects on soil reaction and lateral load.

4.4.4.12 Allowable lateral displacements—Allowable lateral displacements for drilled piers should be developed by the structural engineer and be consistent with the func-tion and type of structure, anticipated service life, and conse-

Fig. 4.4.4.9—Plan view depiction of pier spacing for consideration of group effects (Baker and Gnaedinger 1960; Kiefer and Baker 1994).

Table 4.4.4.9—Group effect coefficientsCenter-to-center pier spacing, parallel to direction of applied

load, in pier diameters, dRatio of lateral resistance of pier

in group to single pier*

8d 1.00

6d 0.70

4d 0.40

3d 0.25*Although reduction does not apply to the lead pier, it does apply to all piers in its shadow. For other spacing, interpolation may be used (Davisson 1969; Prakash 1962; Reese and Van Impe 2001).





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quences of unacceptable displacements on the structure’s performance.

4.4.4.13 Pier stiffness—The flexural behavior of a drilled pier subjected to bending is dependent on its flexural stiff-ness EcI. The value of EcI is the product of the pier mate-rial modulus of elasticity and the moment of inertia of the cross section about the axis of bending. Although EcI is essentially constant for the level of loading to which a struc-tural steel member is subjected, but both Ec and I vary as the stress conditions change for a reinforced concrete member (Reese 1984). For concrete, the value of Ec varies because of nonlinearity in stress-strain relationships, and the value of I is reduced because the concrete in the tension zone becomes ineffective due to cracking. The tensile weakness of concrete and the ensuing cracking is the major factor contributing to the nonlinear behavior of reinforced concrete (Wang and Reese 1987).

The EcI of a reinforced concrete pier is often assumed constant for simplicity in the analysis, although this assump-tion is not rational. Analyses by Wang and Reese (1987) demonstrate that the magnitude of error associated with this assumption can be large. The effects of variable EcI and cracked shaft sections are evaluated using commercially available software, or as subsequently discussed in 4.4.4.14.

4.4.4.14 Crack mechanism—Cracks form when the flex-ural stress due to bending exceeds the tensile strength of concrete. Immediately after formation of the first crack, stresses near the cracking zone in the concrete are redis-tributed. As loading continues, additional cracks open up occasionally, but in general, the initial cracks penetrate more deeply with an increase in flexural stress.

Many variables influence the development and character-istics of cracks. The major ones—percentage of reinforce-ment, bond characteristics, and tensile strength of concrete where cracks occur at random; and the location and spacing of cracks—are subjected to considerable variation.

4.4.4.15 ACI 318 recommendations—The stiffening effect due to tension in the concrete should be considered in making a realistic prediction of short-term deflections of reinforced concrete beams and, likewise, laterally loaded piers. ACI 318 provides an approximate method of computing stiffness considering the effect of cracking.

The flexural stiffness computed directly from the trans-formed cracked section, in terms of the EcI at the instant of cracking, does not represent the stiffness of the entire pier. The flexural stiffness varies with load and deformation. With the 318 modification, flexural stiffness first shows a constant range of stiffness for an uncracked section. After the cracks are initiated by a higher moment, a smooth transition of EcI, in terms of progressive cracking, is represented. When a section is fully cracked due to tensile stresses, EcI reaches a minimum value that is calculated by the transformed cracked-section method.

4.4.4.16 Recommended stiffness—Pier flexural stiffness EcI should be calculated as an upper bound using the gross concrete section as a first approximation. If the applied moments and lateral loads are of sufficient magnitude to cause cracking when acting with concurrent axial loads, then

the flexural stiffness using the effective moment of inertia should be reduced as provided in ACI 318 or in Reese (1984) and O’Neill and Reese (1999).

Use of a flexural stiffness between the gross cross section and cracked section values, in accordance with analysis simplified by the computer program PMEIX (Reese 1984), should provide a legitimate pier analysis model and associ-ated deflection prediction when the reduced section proper-ties are considered in the analysis. For assessment with the interaction diagram of ultimate moment versus axial load, the compressive strain at failure is taken as 0.003. The rela-tionship of EcI and moment can be computed for different pier reinforced sections to define the relationship between bending moment and available uncracked section along with the consideration of axial load on the section as an interac-tion diagram.

4.4.5 Factor of safety—A simple numerical factor of safety is not appropriate in the design of a drilled pier because of complications the soil-structure interaction introduces to the design. Two principal types of failure associated with soil-structure interaction are subgrade and pier structural failure. Subgrade failure is characterized by excessive deflection of the pier under lateral load that is usually associated with short piers. The factor of safety against lateral load failure of a short pier can be increased significantly by increasing the pier penetration. A pier structural failure occurs when the bending moment becomes greater than the bending resis-tance of the pier.

The factor of safety should consider the loading types applied to the foundation, subgrade properties, and the struc-ture’s significance. Generally, four types of lateral loadings on piers are recognized:

(1) Short-term static;(2) Repeated;(3) Sustained;(4) Dynamic.The associated subgrade responses are different for each

loading type. In dealing with seismic loading, the subgrade should be tested to determine its susceptibility to collapse or liquefaction, the reduction of strength under rapid cyclic loading and, where possible, the p-y characteristics under rapid oscillation. These data may be used in the dynamic analysis of the structure as a whole to calculate the transverse loads on piers. The dynamic analysis of a pier-supported structure involves considerations beyond the scope of this document.

The effects of sustained loading should also be considered when using p-y curves for analysis. A universally accepted procedure for designing the effects of sustained loading is not available because of the many parameters involved, including the consolidation characteristics of a specific clay or silt. The available p-y criteria (O’Neill and Reese 1999) are formed based on load tests; the associated p-y criteria do not give a subgrade response related to long-term behavior. Published p-y data for long-term response condi-tions should be used with caution in the absence of local performance data. The selection of design to accommodate





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sustained loading should involve considerable judgment by the geotechnical engineer.

Design of a laterally loaded pier is principally influenced by the confidence of the geotechnical engineer in the values of subgrade properties selected for design. The geotechnical engineer should formulate judgments on subgrade proper-ties based on interpolations, extrapolations, and geological continuity. If the design load is derived from the soil as in a landslide or earth pressure (O’Neill and Reese 1999; Ulrich and Ehlers 2000), then the geotechnical engineer should formulate judgments related to the design load as well.

Uncertainties often arise in design because of the limita-tions in establishing the geometry related to the load appli-cation and subgrade resistance along with the limitations of exploration and laboratory testing.

4.4.6 Design organization—The geotechnical engineer may provide subgrade-response curves to the structural engi-neer who would compute bending moment and deflection. A preferred arrangement is to allow the geotechnical engineer to perform the lateral load analysis and the structural engi-neer to furnish the applied loads. The geotechnical engineer can furnish the reinforcing design for the structural engineer to check and adjust. The geotechnical engineer may vary the pier dimensions in a parametric study of the subgrade response. In this latter case, the structural engineer should review the lateral load analysis to verify that the pier design and estimated response are compatible with the superstruc-ture design.

4.5—Piers socketed in rockThis pier type is socketed into rock to a depth of one to

six times the diameter of the pier, or in some cases deeper, for the purpose of developing high capacity. A column cap is designed to transfer loads from the superstructure to one or more rock socketed piers. The pier is designed to resist all vertical loads and transfer those loads to the rock.

4.5.1 Load transfer to rock—Test loading on high-quality rock has demonstrated that when the depth of socket exceeds twice the diameter, service loads are usually fully resisted on the sides of the rock socket (Horvath and Kenney 1979; Koutsoftas 1981) before mobilizing the available end-bearing capacity. One of the following design assumptions is typically used:

a) The capacity is estimated based on end bearing only, including the effects of embedment within the rock;

b) Capacity is derived solely from side resistance, particu-larly if it is difficult to clean the bottom of the drilled pier;

c) The drilled pier capacity is estimated using both end bearing and side resistance.

The allowable end bearing is estimated by careful exami-nation of site-specific rock cores, compression tests on rock cores, and using local code limits or local practice. Pres-sure meter tests (PMTs) and compressive strength tests on rock cores, adjusted to account for discontinuities in the rock mass, are also used to estimate end-bearing capacity (Canadian Geotechnical Society 1985). Rock coring or rock drilling below the socket in each drilled pier should be performed where rock quality is variable.

The allowable unit shear within the rock socket is deter-mined using experience and judgment in examining rock cores and by relating rock quality and type to local code limits. The compressive strength of the rock and concrete has been used to estimate unit socket shear (Canadian Geotech-nical Society 1985). The unit socket shear developed is also dependent on the roughness of the socket sidewalls. Grooves are sometimes made in sidewalls to enhance shear. Correla-tions between a roughness factor and the ratio of unit socket shear to compressive strength of the weaker of the concrete or rock are available (Canadian Geotechnical Society 1985). Shear stresses, however, cannot exceed the allowable bond stress between the concrete and socket sides. The allowable bond stress may be given by local code or determined by load test (City of New York 2008).

Observing the rock socket is desirable to determine rock quality and sidewall roughness. Observation methods include diver surveys entering a dewatered hole, surface examination with a drop light, probing in shallow holes, and television survey.

4.5.2 Settlement of piers—Settlement of piers founded on rock generally does not significantly exceed the elastic compression shortening of the structural member. Settle-ment is the result of open joints or seams of compressible material in the rock. Settlement can also result from inad-equate removal of soft or weak material from the rock socket bottom. Pressure meter or plate load tests may be used as an aid in estimating drilled pier settlement in soft rock or hard soil; a jack may be used in harder rock (Goodman et al. 1968). Elastic solutions for estimating caisson settlement are included in the Canadian Foundation Engineering Manual (Canadian Geotechnical Society 1985).

4.5.3 Structural design of rock socketed piers—The pier is often designed as a composite column in accordance with the requirements of ACI 318. The steel core can be an H-section or reinforcing steel cage representing as much as 5 to 30 percent of the total pier area. Depending on local code requirements, casing is sometimes used as a structural member in soils that are noncorrosive. Concrete typically has a compressive strength of 3000 to 8000 psi (20 to 55 MPa) at 28 days. Higher strengths may be considered where the required concrete quality is readily available. Where the rock socket is below the groundwater table and the hole cannot be sufficiently dewatered, current practice is to place the concrete using tremie methods for the full height of the pier.

4.5.4 Tension capacity of rock socketed piers—Structures that impose tensile loads on piers include tall structures subjected to high wind or earthquake load, tanks positioned well below the groundwater table, or flood level emptied for maintenance, and structures subject to high eccentric loading or applied moments. Pier tension is commonly resisted only by rock socket shear. Due to inclined fractures and joints in the rock, the licensed design professional should consider an alternative mode of failure by pulling a conical plug of rock formed along these planes of weakness. Resistance consists of the weight of the rock plug, the soil above the plug, the weight of the pier, and any friction that develops from roughness along the failure surface of the rock plug.





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Tension capacity is often confirmed through load test to twice the design capacity neglecting the temporary resis-tance of nonbearing soils. Tensile capacity is transferred to the structure using reinforcing steel or wide flange sections embedded in the pier concrete extending from the pier base to above pier head where it is connected directly to the struc-ture above. Tension loads are sometimes resisted by sockets into the soil bearing strata in the same manner as discussed previously.

4.6—Strength design of piersDrilled piers embedded in soil of sufficient strength to

provide lateral support (4.4.3) may be constructed of plain or reinforced concrete. Design of plain concrete piers should be guided by the provisions in ACI 318. Piers that cannot be designed using plain concrete with practical or desirable dimensions may be designed using reinforced concrete in accordance with the provisions in ACI 318 and applicable local building codes. If permitted by local code, piers may be designed using the alternate design method from Appendix A, ACI 318-99.

All loadings on the left side of the equations in 4.2, whether axial, transverse, or moment, are multiplied by the appropriate load factors given as follows. All reactions on the right side of the equations in 4.2 are evaluated from them. These reactions have no relationship to ultimate soil values and are only intended to balance the factored load-ings (4.1). The pier should also satisfy the compatibility requirements of subgrade reaction with upper estimates of working load. The strength design method should be used for analysis regarding load capacity. As for settlement and lateral motions, no load factors should be incorporated and only service loads used.

4.6.1 Load factors for strength design—Load factors are given in ACI 318.

4.6.2 Strength reduction factors—Strength reduction factors are given in ACI 318.

4.6.3 Pier reinforcement—Pier reinforcement may be required to resist tension caused by uplift forces, to resist flexure caused by lateral loads or moments applied at the top of the pier, or to transfer load from structure to pier. Rein-forcement used for tensile forces may consist of a single bar, or a series of bars centered and installed in the full depth of the pier. Reinforcement used for flexural forces typically consists of vertical bars installed in the perimeter of the pier enclosed with horizontal ties. Flexural reinforcement need not extend the full depth of the pier if the pier is consid-ered flexible and the flexural stresses are dissipated as the lateral load is transmitted into the soil adjacent to the pier. The applicable building code may define minimum rein-forcement requirements other than that needed to transfer the load. Codes may also define details regarding connection to the pier or cap, for piers supporting structures designed to accommodate construction tolerances. Pier reinforcement may also be required for higher seismic design categories.

4.7—Pier configuration4.7.1 Bells—The bell sides should slope at an angle of not

less than 45 degrees with the horizontal for piers installed in the dry. Table 4.7.1 gives limitations for minimum bell slope angle as a function of end bearing.

Thickness at the bell’s edge should be at least 6 in. (150 mm). The diameter of the bell should not exceed three times the diameter of the shaft. The contractor should be required to provide a positive means of demonstrating that the belled section has not caved in and is acceptably clean of loose materials.

4.7.2 Caps—Where used, the depth of the cap should be sufficient to accommodate development of the vertical rein-forcement from the shaft and the dowels or anchor bolts for the column. Caps are often formed with removable steel or disposable cardboard forms.

4.7.3 Permanent steel casing—Permanent steel casing, if used for confinement or as a structural member, should be designed for loading conditions on the casing including placement and should have a minimum thickness of 0.0075 of the diameter of the pier shaft in inches (mm). This thick-ness is for casings placed and grouted in the rock and neither driven nor screwed into the rock.

The critical buckling stress varies inversely with the cube of the radius (Broms 1964). Poor welds and damage from placement or handling also reduce the ability of the casing to withstand external pressure. The casing should not be included in calculations of area or moment of inertia of the pier section, except for full length, continuous, noncorru-gated casing.

4.7.4 Joints—Construction joints in the shaft should be avoided. The remedy for an unavoidable cold joint depends on the relationship of the as-designed pier to the applied loads and the vertical position of the joint. When a construc-tion joint becomes unavoidable, the surface of the concrete should be cleaned and roughened and, in unreinforced piers, vertical dowels of sufficient length to develop the bars above and below the joint should be set in the fresh concrete. The cross-sectional area of the dowels should be not less than 0.01 times the gross area of the section in the shaft (1 percent). Where vertical reinforcing steel is already present, the dowels need only bring the percentage of steel up to 1 percent. Where vertical reinforcement equals or exceeds 1 percent, no dowels are required.

Where a structural discontinuity becomes unavoid-able during concrete placement, it may be more desirable to remove the concrete before it reaches initial set than to install replacement piers. Adequate cleaning of the pier and

Table 4.7.1—Minimum bell angle (O’Neill and Reese 1999)

Maximum allowable bearing pressure

Minimum bell angle from horizontal

6 ton/ft2 (600 kPa) 45 degrees

6 to 10 ton/ft2 (600 to 1000 kPa) 50 degrees

10 to 15 ton/ft2 (1000 to 1500 kPa) 55 degrees

Greater than 15 ton/ft2 (1500 kPa) 60 degrees





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concrete soil interface may not be a legitimate option where slurry is used and the joint is located at significant depth (Ulrich and Ehlers 1995; Ulrich 2007).

In high seismic risk areas, the cap and upper part of the pier may be subjected to moments of magnitude matching the ultimate moment capacity of the supported column and high shear loads. This combination may constitute a severe splitting condition, requiring careful attention to the arrangement of splices in vertical reinforcing steel at the two joints (pier to cap and cap to column) and to the provision of adequate spiral or hoop reinforcement in the cap and the effected part of the pier.

4.7.5 Unbraced piers—Soil having a standard penetration value N > 1 or undrained shear strength greater than 100 lb/ft2 (5 kPa) may be considered to provide lateral support and effectively prevent buckling of piers at service loads. Piers extending above the soil surface or penetrating caverns or large voids and standing in air, water, or material not capable of providing lateral support, should be designed as columns. Effective column length should be estimated based on end fixity. Assume the pier is pin-connected at a point two diam-eters into the competent subgrade material.

4.7.6 Interconnecting ties—Design for earthquake and other lateral loading may require individual piers or pier caps be interconnected by ties in accordance with the applicable building code. Ties may also be required to resist bending moments. The bending moments will redistribute horizontal loading on the piers, resulting in a need for additional anal-ysis. Ties are also used to develop passive pressure in the soil under wind loads and to reduce differential settlements.

4.7.7 Concrete protection for reinforcement, steel column stubs or cores—Where permanent casing is used, the protec-tive concrete thickness should be no less than 3 in. (75 mm), or five times the maximum size of the coarse aggregate, whichever is larger. Larger clearance between reinforcement and the inside edge of casing should be considered where the casing is to be removed, particularly when the reinforcing cage and casing are long. This will reduce the chance that during removal the casing will bind against the reinforcing cage.

CHAPTER 5—CONSTRUCTION MEANS AND METHODS

Aside from familiarity with subsurface conditions, construction means and methods are the most important vari-able in design because their effects can ruin an apparently successful design. Because the design geotechnical report only guides the design and construction of foundations and is not a contract document, careful preparation of contract documents is vital to project success. Relevant project docu-ments for construction should be prepared by the geotech-nical engineer and reviewed by the structural engineer.

5.1—Excavation and casingExcavation methods used should produce a pier hole

that is properly located and plumb, will not disturb the soil adjacent to the hole beyond design expectations, and will produce a clean hole of specified dimensions for its entire

length. Each pier should be founded into or on the desig-nated bearing stratum.

5.1.1 Excavation equipment—Excavation can be performed using any equipment that will obtain the specified results including by hand, auger drill, bucket drill, clamshell, or any combination thereof. Overdrilling should be avoided. Where a good bearing stratum of limited thickness overlies poorer material or a water-bearing stratum under signifi-cant head, drilling to excessive depths may require addi-tional time and equipment, or result in reduced pier-bearing capacity. Definitions of refusal, obstruction, and rock should be included in the contract documents based on input from the geotechnical engineer.

During excavation of the shaft, the contractor should make frequent checks that the shaft is plumb by placing a carpen-ter’s level flush to the Kelly bar. Some causes of excessive deviations from plumb are:

a) Failure to initially position and then hold the drill rig and auger on the design center of the shaft. Occasionally rotate the drill rig or move from the hole during excavation. Reposition and plumb the auger before resuming the drilling;

b) When the auger encounters obstructions such as boul-ders, old foundations, or rubble fill, the auger may veer off and slant the hole;

c) If the drill rig is situated on soft ground, uneven settle-ments could cause the Kelly bar to veer out of alignment;

d) The additional force or torque applied when drilling in very dense soils may change the Kelly bar alignment.

5.1.2 Casing—In firm soils with little or no groundwater seepage, casing may not be required except for personnel safety. Loose casings, used solely for the purpose of protecting personnel, should be at least 0.25 in. (6 mm) thick steel, and are normally removed from the hole when observa-tion is completed. Under other circumstances, depending on the method of installation, the ground and water conditions, and the surrounding facilities, a tight casing, slurry, or other means may be required to retain the earth. Where subsid-ence is to be prevented or kept to a minimum, retaining of the earth should be required in the contract documents. Side friction may be affected by the use of casing.

5.1.3 Belling—Belling may be performed by machine or hand. The soil formation, groundwater conditions, available equipment, and construction experience govern the belling method and potential design influence. Special belling procedures may be required to accommodate soil conditions or if groundwater is present. Machine-belled piers through driven casings with slurry have been used successfully offshore (Ehlers and Bowles 1973). Other successful proce-dures are described in Baker (1986).

5.1.4 Spacing—A drilled pier should not be excavated too close to another drilled pier in which freshly placed concrete has had insufficient time to set. The minimum distance to prevent a blow-in or collapse of fluid concrete from one hole to another should be determined by the geotechnical engineer. Considerations should include soil properties, pier geometry, concrete setting time, and the installation method used to drill the pier hole.





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5.1.5 Special procedures for rock installation—The following methods (5.1.5.1 through 5.1.5.3) require careful evaluation and proper planning for installation of drilled piers in rock.

5.1.5.1 Excavation technique—When specified, penetra-tion into rock should be obtained by approved methods such as drilling, coring, chipping, and chopping. Blasting should not be performed in confined areas where it may cause damage to casing or the surrounding subgrade or property.

5.1.5.2 Bearing area—When piers are to be founded on rock, the shaft should be cut into the rock and the bottom stepped or leveled so that the bearing is obtained on surfaces sloping not more than 10 degrees.

5.1.5.3 Side resistance—When straight shaft piers are designed for bearing in rock or other hard formations, all or a portion of the bearing load may be transferred to the forma-tion by side resistance developed between the formation and lower part of the pier. In some localities it is common practice to cut a series of groove or key rings into the sides of the pier hole near the bottom (Fig. 3.4.1.3). Under other circumstances, shear rings are not required to develop side resistance. Grooving is accomplished by cutters attached to the boring equipment.

5.2—Placing reinforcement5.2.1 Assembly—Steel reinforcement, steel stubs, or core

sections should be accurately placed and supported in the correct locations. Should the method of pier construction employed require removal of casing, prevent exposure of the reinforcement or other embedded metal to surrounding soil during the removal process. Spacers, capable of sliding on the casing, should be securely attached to the reinforcement.

5.2.2 Spacing—The clear spacing between vertical rein-forcing bars should be at least four times the size of the maximum coarse aggregate or 4 in. (100 mm), whichever is larger. The casing to reinforcing cage clearance can impact the concrete placement and should be considered in the design. Reducing the cage diameter may be necessary to allow the same minimum clearance as the reinforcing bars.

5.2.3 Splices—Vertical splices of reinforcing bars should conform to ACI 318. Generally, no more than 50 percent of the bars should be lap spliced at one location.

5.3—Dewatering, concreting, and removal of casing

Casing should be used to create a seal and cut off water infiltration to the pier when the casing can be installed into an impervious stratum. Where such a seal against ground-water is not possible, an accepted slurry displacement method should be used, or a dewatering system should be installed that will permit proper excavation, observation, and concreting of the pier. Should the dewatering system employed contemplate pumping inside the pier, the unbal-anced water head created should not cause a blow, where there is a bottom heave or quick condition that disturbs the proposed bearing stratum or surrounding facilities. In cohe-sionless soils, such as silts and fine sands, where the hydro-static head is higher than the excavation level created by pier

drilling, it may be necessary to install a filtered dewatering system to lower the water level without pumping soil fines from the formation.

5.3.1 Groundwater infiltration—Infiltration from a source at or near bottom, at a rate of less than 0.25 in. (6 mm) rise per minute at the bottom of the pier, should be considered a dry pier. Concrete can be placed by buckets, chuting, tremie pipe, or elephant trunks in a manner that minimizes aggre-gate segregation; however, the total height of water in the bottom of the pier should not exceed 2 in. (50 mm) at the time that sufficient concrete has been placed to balance the water head (Baker and Gnaedinger 1960; Baker and Keifer 1994; Brown et al. 2010; Suprenant 2001). It is also permis-sible to allow free fall of concrete if it is directed vertically on the centerline of the shaft, avoiding the sides of the shaft or the reinforcement cage. If a leak occurs some distance above the bottom, an appreciable amount of water could enter the pier in the time needed to fill the pier up to the point of leak. Casing through the permeable stratum is recom-mended in this situation. Every reasonable effort should be made to obtain a dry hole. If these efforts fail, or if economy dictates, it may be necessary to tremie concrete the pier.

If infiltration of groundwater exceeds a rise of 0.25 in. (6 mm) per minute, the pier should be considered a wet pier, and concrete should be placed by an approved slurry displacement method in accordance with this report, the requirements of ACI 336.1, and project documents.

5.3.2 Compaction of concrete—The free unobstructed fall of concrete with a minimum slump of 4 in. (100 mm) produces adequate compaction up to the top 5 ft (1.5 m) of depth. Vibration of the concrete is required only for this upper depth. Segregated concrete and laitance should be removed before proceeding with construction of the cap. Discharge of concrete through a hopper with a short vertical pipe care-fully centered on the pier shaft is recommended for free, unobstructed fall (Litke 1992; Suprenant 2001). The pres-ence of a reinforcing cage in a pier of small diameter may require dropping the concrete through a long vertical pipe. This could then require a greater depth of top vibration, or proportioning of the concrete mixture for a higher slump and smaller maximum aggregate size similar to tremie concrete. Concrete compaction is not always needed for piers installed by the slurry displacement method because consistency of the concrete reaching the design elevation is fluid.

5.3.3 Casing removal—If ground conditions are such that the casing may be removed during the concreting of piers, the equipment and procedures used should not disturb, pull apart, or pinch off by subgrade movement to concrete. It is of extreme importance to establish that this can be accom-plished before the removal of the casing and to check concrete level during removal. The concrete level should always be maintained a minimum of 5 ft (1.5 m) above the bottom of the casing during concrete placement, but some-times a much higher head is required, depending upon the fluid head outside the casing. Proper consideration should be given to mixture proportioning, including cement factor, slump, and admixtures, in the placing of concrete and the pulling of casings. A trial batch should be made for concrete





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mixtures used with casing removal to confirm the mixture properties will be as designed.

Casings should be maintained in good shape and free from old concrete on the inside surfaces where it would tend to make removal difficult. When ground movements are antici-pated, the contractor should make frequent measurements of the shaft diameter in at least two perpendicular horizontal directions and at reasonably spaced elevations to assure that the specified minimum shaft diameter is maintained and that warping does not indicate imminent collapse. Casing should be of sufficient length to extend past caving soil. The casing diameter should closely match the diameter of the hole when removal of casing is intended.

5.3.4 Unstable strata—When piers penetrate unstable strata such as peat, soft clay, or loose sand and silt, and the casing is required to maintain the shape of pier through these layers, then the casing should not be removed.

5.3.5 Theoretical volume—The theoretical volume of concrete required to fill the pier should be computed. If the actual volume installed—estimated by delivery tickets, for example—is appreciably less than the theoretical volume, the pier may have experienced pinching, collapse of side-walls, or contamination of tremie concrete. If a pier defect is suspected, options include immediate reinstallation before the concrete sets or investigation of the as-installed pier in accordance with 6.6 of this report. Rejection of an unaccept-able pier may require installation of one or more replace-ment piers at locations that will facilitate load transfer from the structure above.

Monitoring pier concrete placed by collecting and analyzing truck delivery tickets has limitations unless provi-sions are made to accurately measure the volume of concrete delivered for each placement. Accordingly, the size of a defect can be large enough to reduce pier capacity but small enough to go undetected by the theoretical volume calcu-lation; the theoretical volume calculation without precise knowledge of the emplaced volume is very limited.

Concrete in a pier should be placed in one continuous operation. If a construction joint is unavoidable, it should be treated in accordance with 4.7.4. If the casing is to be removed and the joint was unintended, it may be necessary to cut the casing at the joint and leave the portion below the joint in place permanently, or immediately drill out the pier and start again.

5.4—Slurry displacement methodThe slurry displacement method is typically used to

construct piers in soil below the groundwater table. Appro-priate construction means and methods along with diligent construction engineering and observation are vital during construction of piers installed by slurry displacement method.

5.4.1 Planning—Where drilled piers are to be installed below groundwater level, in caving or sloughing soils, or in sand, a casing or slurry should be used to stabilize the exca-vation. Slurry level in the excavation should be maintained at a minimum of 10 ft (3 m) above the static groundwater level and above any unstable zones at a vertical distance that prevents caving or sloughing of those zones into the excava-

tion. In some circumstances, it may be necessary to raise the drill platform to accommodate the minimum head require-ments and begin the drill process.

Where the purpose of the slurry is only to maintain an open hole until a casing is placed, such as the wet hole method, the initial drilling fluid may be water unless experience has shown that a slurry is required.

If no casing is planned, then control of the slurry is much more critical. In some soil profiles, it may be possible to use only water. Increasing the slurry viscosity by mixing various mineral solids or chemical polymers with the water will be required if water alone does not stabilize the pier excavation.

5.4.2 Excavation—Methods and equipment used for pier excavation should leave the sides and bottom of the hole free of loose material that would prevent intimate contact of the concrete with firm, undisturbed subgrade. Slurry should be introduced into the pier shaft before the water table is reached or unstable soils are encountered. Although a minimum head of slurry should be 10 ft (3 m) above the groundwater level, maintaining the slurry level at top of pier hole is considered good practice. The auger should be raised and lowered at a slow enough rate that the slurry does not swirl or cause suction on the sidewalls as the tool is withdrawn. Swirling of the slurry can cause scouring. Suction can cause cave-ins.

All spoil and excavated materials should be kept away from each open pier excavation to avoid contamination of the excavation after final cleanout.

5.4.3 Installation method—The slurry level in the excava-tion should be maintained at a minimum of 10 ft (3 m) above the static groundwater level and above any unstable zones to prevent caving or sloughing of those zones into the excava-tion. The contractor should be required to demonstrate that stable conditions are being maintained. This can be accom-plished by multiple depth measurements and sounding the bottom when no drilling is occurring.

Where the purpose of the slurry is only to maintain an open hole until a casing is placed (wet method), the initial drilling fluid may be water unless experience has shown that slurry is required. If no casing is planned, then slurry control is much more critical. Although in some soil profiles it may be possible to use only water, this is uncommon. Increasing slurry viscosity by mixing various mineral solids or chem-ical polymers with water will be required if water does not stabilize the pier excavation and lift drill cuttings.

Slurry should consist of water or a stable colloidal suspen-sion of various pulverized solids or polymers thoroughly mixed with water so that appropriate properties are main-tained. Attapulgite and bentonite should meet the American Petroleum Institute (API) Specification 13A. The type of various solids used will depend on the subsurface conditions and characteristics of the mixing water. A test report from the supplier giving the physical and chemical properties of the additive should be supplied to the construction geotech-nical engineer at the start of the work.

The slurry should be mixed, stored, and transported using equipment designed for the purpose. Slurry made with mineral solids for use in the slurry displacement method should be mixed in tanks on-site or arrive at the site





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premixed. Mixing in the shaft is permissible for the appli-cation of liquid polymer slurry. Water used to mix slurry should be clean, fresh water of a quality approved by the construction geotechnical engineer.

Slurry should meet the criteria in Table 5.4.3.The reference values in the table apply to bentonite or

attapulgite slurries. While polymer slurries are widely available, there are no standard test methods available that apply to these products. Some guidance may be sought from published DOT specifications, but caution should be exer-cised, as polymer slurries exhibit a wide range of proper-ties when measured using the methods listed in Table 5.4.3. Test installations should be made to confirm acceptable use in environments where adequate experience does not exist, as polymer slurries are not suitable for all formations and drilled pier excavations. Different polymer slurries may react differently to the minerals present in the subgrade, and they may have a significant effect on the concrete-to-subgrade interface and on the concrete itself. As-built concrete prop-erties can be different from expectations under some condi-tions. In some cases, these reactions have prevented the tremied concrete from adequately displacing the slurry. The polymer reaction that prevents displacement can be inconsistent across a site; consequently, test installations may not be helpful. Water may be used in place of mixed slurry, but the pier excavation should maintain stability and water use should be subject to approval of the construction geotechnical engineer. Slurry testing should be performed and recorded for quality control and quality assurance. The frequency of slurry testing from down-hole samples should be a minimum of two sets of tests per work shift, the first test being done at the beginning of the shift. Field condi-tions and requirements of the drilled pier contractor and construction geotechnical engineer may make more frequent testing necessary. One example is multiple test require-

ments per pier to assure acceptable slurry. The drilled pier contractor should have an on-site slurry sampler capable of obtaining slurry samples at any depth within the excavation if requested by the construction geotechnical engineer.

The contractor should use drilling tools and excavation procedures to prevent excessive negative fluid pressure during excavation. At the completion of excavation, the bottom should be cleaned with an air-lift system or a clea-nout bucket equipped with a one-way flap gate that prevents spoils in the bucket from reentering the shaft, or other suit-able tools. The type of cleanout equipment will depend on the formations and pier design. Slurry should meet specifica-tions before concreting. If cleaning, recirculating, desanding, or replacing the slurry is necessary, the contractor should be prepared to do so.

For piers with end bearing, the slurry sand content should be limited to 4 percent; otherwise, the sand content should be limited to 20 percent. The limiting of sand content or mud weight will depend on the pier design and end-bearing component. Sand content values less than 4 percent are probably unreliable and likely require replacing the slurry before concreting begins. The slurry density should not exceed 75 lb/ft3 (1200 kg/m3) for end-bearing piers, and it may be necessary to replace the slurry after drilling the pier, but before concreting. The 75 lb/ft3 (1200 kg/m3) value is a threshold and may vary with the pier design. Closer restric-tions in the case of end-bearing piers are necessary to facili-tate sounding the bottom with a rod or weighted tape.

Slurry should be sampled and tested from the mud tank and from within 12 in. (300 mm) of the bottom of the drilled pier.

The bottom of the excavation should be checked to confirm that drill cuttings and hole sides are not falling to the bottom. Sounding a tremie pipe or air-lift pipe, or feeling with a weighted line are acceptable methods for checking bottom buildup of material, but the method of sounding should be defined in the contract documents by the geotech-nical engineer. The permissible amount of drill cuttings at the bottom of the pier should be less than 6 in. (150 mm) in depth for piers designed without end bearing. The bottom cleanliness will depend on the ratio of end bearing to side friction and is the judgment of the construction geotechnical engineer based on the project needs and limitations of pier construction. If the required bottom cleanliness conditions are not met, the excavation bottom should be cleaned again and the slurry modified.

Concreting the drilled piers should be completed the day excavation is completed; if this is not possible, the excava-tion should be redrilled, cleaned, and the slurry tested before concreting. For piers with neat line volumes larger than one truckload, the theoretical concrete volume should be computed for each pier size and compared with the actual volume of concrete placed after each truckload.

5.4.4 Reinforcing steel—Reinforcing steel should be placed in a shaft excavation after the construction geotech-nical engineer has completed his observations and approved placement.

Table 5.4.3—Recommended slurry properties

Item to measureRange of results at

68°F (20°C) Test methodsDensity before concreting

(Mud balance)API RP 13B-1 Section 4

or ASTM D4380

a. Piers with design end bearing

75 lb/ft3 (1360 kg/m3) maximum

b. Piers without design end bearing

85 lb/ft3 (1200 kg/m3) maximum

Marsh funnel viscosity before concreting

26 to 45 seconds*(Marsh funnel and cup)

API 13B-1 Section 6.2 or ASTM D6910

Sand content by volume before concreting (Sand-screen set)

API 13B-1 Section 9 or ASTM D4381

a. Piers with design end bearing 4 percent maximum

b. Piers without design end bearing 20 percent maximum†

pH, during excavation 8 to 12

(Glass-electrode pH meter)API 13B-1 Section 11 or

ASTM D4972*May be increased to 60 for polymer slurries.†Higher sand contents have been successfully used in some locations.





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Following approval of the excavation, reinforcing steel should be centered in the hole at the correct position and elevation using spacers as necessary.

Reinforcing steel should not touch the sidewall of the exca-vation and should be completely encased in concrete using appropriate spacers as necessary. Minimum concrete cover of 3 in. (75 mm) should be maintained and the minimum clear spacing between reinforcing steel should be 4 in. (100 mm) between horizontal reinforcement and four times the size of the maximum coarse aggregate between vertical rein-forcement (a minimum of 4 in. [100 mm]).

5.4.5 Concrete—All concrete should satisfy the following requirements:

a) Concrete used in the slurry displacement method should have a slump of 7 to 9 in. (175 to 230 mm).

b) Maximum aggregate size should be 0.75 in. (19 mm) and not more than one-quarter the reinforcement clear spacing.

c) Maintain a 6 in. (150 mm) or greater slump after 4 hours.d) The required parameters of the concrete mixture should

be confirmed by trial batch if the supplier cannot demon-strate adequate experience with the concrete mixture design submitted.

A trial batch of the drilled pier concrete is recommended to confirm the fluidity properties are consistent with the design. Slump tests should be taken hourly during the trial batch if the supplier cannot demonstrate adequate previous experience at achieving the recommended properties. Slump tests should be taken from every truck in the field.

Longer fluidity durations may be needed for large, deep drilled piers; in this case, laboratory slump-loss tests are recommended. Slump tests should be taken hourly, both during the trial batch and in the field, if the supplier cannot demonstrate adequate previous experience at achieving the recommended properties. The length of time needed to place concrete in the pier, with or without temporary casing, may also govern the retarding characteristics of the concrete mixture. Special mixture proportions, including the use of admixtures and careful control of cementitious material content, may be required.

The specification submittal requirements should include the construction geotechnical engineer as a reviewer of the concrete mixture proportions along with appropriate support information relating to all of the needed concrete properties and results of past trial batches with the mixture. Equipment and procedures should be part of the specification submit-tals given to the construction geotechnical engineer for prior approval.

5.4.6 Concreting methods—Concrete placement should be made following acceptance of the excavation by the construction geotechnical engineer and placement of the reinforcing steel.

If the design includes end bearing, the bottom should be sounded again with a measuring tape with a weight attached to the end after the steel placement and just before concrete placement. All concrete should be placed while the excava-tion remains clean and stable and the concrete remains fluid.

Placement should not begin until adequate concrete supply is assured.

Concrete may be placed by tremie methods or by pumping. In either case, a plugged, capped, or pig-plugged tremie should be inserted and seated in the excavation. The tremie should extend to the bottom of the shaft before the commencement of concrete placement, and care should be taken to expel all slurry suspension from the pipe during the initial charging process if the pig plug approach is used. The tremie pipe should be embedded in fresh concrete a minimum of 10 ft (3 m) and maintained at that depth throughout concreting to prevent entry of slurry into the pipe. Pump tubes should be embedded at least 15 ft (4.5 m) below the top of concrete.

Minimum tremie diameter should be 10 in. (250 mm). Pump tube minimum diameter should be 5 in. (125 mm), although 4 in. (100 mm) diameter pump tubes have been successfully used with concrete mixture proportions designed with appropriate aggregate sizes.

All tremies should be maintained clean and smooth on the interior. The exterior should be as smooth as practical and free from materials that might contaminate the slurry. The latter is accomplished by rinsing the tremie, or by storing the tremie in a hole filled with slurry.

The concrete should be placed while keeping the tremie tip embedded in the concrete. The upper portion of concrete flow should be wasted by overflow at the top of the pier, as it is often contaminated with mud. Rapid raising or lowering of the tremie should not be allowed.

In the capped tremie pipe approach, the tremie pipe or pump line should have a seal, consisting of a bottom plate or approved equal, that seals the bottom of the pipe until the pipe reaches the hole bottom and enough concrete has been placed to seal off water flow into the tremie pipe. The use of a pig inserted in the pipe to separate the concrete from the slurry is acceptable.

In the pig plug approach, the open tremie pipe should be set on the bottom, the pig inserted at the top, and then concrete placed, pushing the pig ahead to separate the concrete from the slurry. When the pipe is fully charged, the pipe should be lifted off the bottom only enough to start the concrete flowing.

During tremie placement, the bottom of the tremie pipe should not be lifted above the concrete level. If the seal is lost, the pipe should be withdrawn, the seal replaced, and the tremie operation restarted using the capped tremie approach.

Aluminum pipe or equipment should not be used for placing concrete.

Exposed concrete should be protected against damage and should be cured and protected to prevent moisture loss and temperature extremes in accordance with ACI 301.

5.5—SafetyThe following safety provisions should be regarded as a

minimum. Government regulations such as the Occupational Safety and Health Administration (OSHA) may impose stricter measures.





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5.5.1 Gas in hole—All personnel involved in the exca-vation of a pier hole should be alert to toxic and explosive gases that may be released into the hole. Gas masks, gas detectors, adequate first aid equipment, and blowers to force fresh air to the bottom of the hole should be readily avail-able on the jobsite to assist personnel in emergencies. If gas is encountered or anticipated, absolutely no personnel entry should be permitted until the shaft has been properly vented and tested.

5.5.2 Fall protection—The use of full-body safety harness and safety rope should be worn by any person entering a pier hole. The top of any pier hole should be covered when excavation work is discontinued or finished and the hole left open for any reason. The cover should be strong enough to prevent a person from falling into the hole.

5.5.3 Collapse protection—Protective steel casing to retain soil in the shaft walls should be employed when any person is in the shaft or working at the bottom. A protective cage or diving bell may sometimes be used instead of full-length casing.

CHAPTER 6—CONSTRUCTION ENGINEERING AND TESTING

6.1—ScopeThe purpose of construction engineering and testing

is to assess the suitability of the in-place formations and construction means and methods for conformance with the pier design, assess the need for design modifications, guide the design modifications, and confirm the pier is constructed in accordance with the design and specifications. Given the difficulties associated with assessing post-construction suit-ability of a drilled pier, construction geotechnical engineering is an essential part of design (Ulrich and Ehlers 1995; Ulrich 2007). Construction geotechnical engineering involves, but is not limited to, the evaluation of the construction means and methods proposed by the contractor if different from those specified; assessment of the concrete mixture propor-tion; checking the location, excavation, plumbness of the shaft, shaft and bell dimensions if applicable; determining the proper depth of excavation and the strength of bearing stratum; assessment of underground conditions; guiding the pier design modifications in the event that conditions are not as designed; and observing that proper materials and concrete placement procedures have been used. It is expected that the construction engineer will furnish an opinion that each as-installed pier will perform as designed.

6.1.1 Common causes for faulty piers—Although there are multiple conditions that lead to faulty piers, the condi-tions are often detectable during construction. Some condi-tions that can lead to defects are:

a) Development of voids in a concrete shaft due to improper pulling of casing and the use of concrete having too little slump;

b) Concrete placed in seepage water in the pier;c) Side cave-in of soil, resulting in contaminated concrete;d) Improper location or plumbness of pier, or improper

reinforcement;

e) Surface cave-in of soil resulting in contaminated concrete or soil-filled voids;

f) Improperly placed tremie concrete resulting in segre-gation, discontinuity, or intermixing concrete with slurry or water;

g) Squeeze of shaft or bell excavation before concreting or during concreting;

h) Casing collapse;i) Excess water or contaminated concrete at cold joints

resulting in a reduction in concrete quality;j) Migration of water, resulting in dilution of the concrete

mixture and a reduction in concrete quality;k) Poor concrete delivered to site;l) Inadequate bell sizes;m) Inadequate bearing material;n) Incomplete slurry displacement during concrete tremie

placement;o) Sand settling out of slurry during concrete placement,

resulting in stiffening of the concrete (slump loss) and folding over of concrete trapping the sand sediments;

p) Reaction of polymer slurry with some formations;q) Air entrapment in piers installed by the slurry displace-

ment method.

6.2—Geotechnical field representativeThe construction geotechnical engineer should be respon-

sible for observation of drilled pier installation and asso-ciated construction engineering. The geotechnical field representative should be qualified in pier construction work and have sufficient technical educational background to interpret observations correctly and to communicate what is observed. The construction geotechnical engineer may delegate some or all construction field services to a geotech-nical field representative working under his/her supervi-sion. Continuous full-time observation of each pier instal-lation is recommended and associated office engineering is needed to respond to the project submittals associated with construction. Construction engineering and testing should be performed under the direct supervision of the construc-tion geotechnical engineer.

6.3—Preconstruction activitiesBefore design, a geotechnical investigation should be

made under the direction of a licensed professional engi-neer experienced and qualified in geotechnical engineering using experienced personnel to perform a field investigation and laboratory testing. The field investigation may include borings.

In addition to determining the location, strength, and compressibility of bearing materials at boring locations, it is desirable to anticipate problems that may be encountered during construction. Performing explorations at each pier location is sometimes necessary—for example, when piers pass through fills containing concrete and other obstructions, or where bearing depths or bearing materials are variable. In some cases, large exploratory auger holes or test piers are required to assist in the evaluation of actual conditions to be encountered.





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Rational project documents prepared jointly by the structural engineer and geotechnical engineer are essential for a successful project. Because construction means and methods have a dominant role in design and the construction geotechnical engineer provides the construction engineering services, the geotechnical engineer should have a major role in developing the documents and preparing the project specifications.

The project document package includes all design geotech-nical reports. The potential for obstructions and unforeseen conditions should be addressed in the project documents. Negotiated contracts with qualified foundation contractors are recommended.

The contractor should be required to submit a plan for foundation construction work for review by the geotech-nical engineer and licensed design professional, and the plan modified to address reviewer comments.

A preconstruction meeting should be held before contractor mobilization to review the construction procedure. Meeting minutes should be kept to record issues not addressed in the project documents. Adjustments or modifications to the construction documents should be documented and issued following the meeting.

6.4—Construction geotechnical engineering procedures

Adequate construction engineering may require down-hole observations and tests on each end-bearing pier excava-tion (The International Association of Foundation Drilling (ADSC) and the Deep Foundations Institute (DFI) 2004). Observation from the ground surface may be acceptable because of experience or geologic continuity, or may be required due to unsafe conditions or slurry placement.

The following procedures should be observed.6.4.1 Location—Horizontal deviation of the actual pier

center at the top from the design center should be measured and recorded. If a protective casing is used, the top should be checked with a plumb bob from staked reference lines, not referenced to the surface casing.

6.4.2 Plumb deviation—Plumb deviation refers to hori-zontal deviation from the vertical of the pier, also known as bottom plumbness. Horizontal deviation of the lower end of the shaft or bell should be referenced to the vertical plumb line of the pier design center at the top.

If the pier cannot be entered, the plumb deviation is esti-mated by moving the plumb line carefully from the design center at the top to the edge of the shaft at the bottom in all four directions and measuring distance from center. A plumb bob suspended from a long line will tend to move exces-sively, or may adhere to the pier walls at the bottom.

6.4.3 Removal of obstructions—The construction geotechnical engineer should participate in identifying and defining an obstruction, and should log all observed obsta-cles removed by the contractor.

6.4.4 Casing—Where casing thickness, length, diameter, or other properties have been specified, the geotechnical representative should verify these requirements are satisfied.

6.4.5 Loss of ground—Ground subsidence can occur from surface caving, squeezing clays into the pier excava-tion, flow of saturated silts and sands into the hole, and from removal of soil during pumping operations.

The contractor and the geotechnical field representative should check water discharge from the pumps at agreed intervals to estimate the amount of fines carried in the water. Sediment tanks with overflow weirs are useful, and frequently necessary, for this purpose. Soil movements that could cause significant loss of ground should immediately be brought to the contractor and the licensed design profes-sional’s attention by the geotechnical field representative. Difficulties in inserting and removing the drilling tools in the shaft excavation may indicate squeezing soils. Volume comparison of theoretical versus actual concrete quantities can also be helpful.

6.4.6 Control of groundwater—Water in the pier excava-tion can occur from loss of seal around the temporary casing or surface inflow if no top casing is used, seepage from a granular saturated layer in the shaft or bell, or inflow from the bottom of the excavation. The construction geotechnical engineer should observe the water control methods and report any inadequacies. For proper observation of bottom clean-up, testing of bearing material and accurate placement of concrete, the water level in the bottom of the hole should not exceed a depth of 2 in. (50 mm). Where the water flow cannot be controlled, special procedures will be required (5.3.1). The special procedures adopted should be approved by the licensed design professional and the construction geotechnical engineer before implementation.

A representative coring of concrete placed by special techniques in or through water or slurry may be required to check the soundness and continuity of the concrete. Sonic and gamma logging techniques used in preplaced access tubes have also proven useful for this purpose.

6.4.7 Depth of pier—The construction geotechnical engi-neer should decide when the bearing material has been reached and pier depth is adequate. This decision is made by observation of excavated materials, tests on samples of the bearing material, interpretation of the design geotechnical report, or all of these.

6.4.8 Belled piers—Bell bottoms should be reasonably flat. Down-hole observation is advisable to check the bell shape, dimensions, and concentricity, and verify that the bell roof and shaft walls are stable. The construction geotech-nical engineer should make observations or measurements to confirm the bell meets specification requirements. When shaft and bell entry is considered unsafe, special remote or indirect measuring procedures are recommended as described in 6.4.11.

6.4.9 Cleanout—Good cleanout of soft, loose, disturbed soil in the bottom of an end bearing straight shaft or belled pier should be obtained. Generally loose, disturbed soil should cover not more than 10 percent of bearing area to a maximum depth of 2 in. (50 mm). Down-hole observation is recom-mended to see that the specified limitations are being met.

A final observation should be made just before the start of concrete placement because bell roof instability can





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occur without notice and may be undetectable from the drill surface. Any additional disturbed material should be removed at this time to the same quality of cleanup as that recommended.

If the contractor considers conditions unsafe to send workers into the hole because of water seepage and running silt or an unstable bell roof condition, special measures are required. Potential solutions depend on foundation design, site-specific conditions, and available equipment. One potential solution is casing off the problem area and belling below, if soil conditions permit. Another method, if the foundation equipment capabilities are suitable and the reinforcement is confined to the shaft, is enlarging the bell under slurry, tremie placing a cement grout to fill the bell, allowing the grout to set up overnight, and then redrilling the bell partly in the grout and partly below so that the remaining grout collar provides support for the unsuitable soil. The construction geotechnical engineer should observe these special procedures to see that the design foundation criteria are accomplished.

6.4.10 Bearing material—Close observation and testing of the bearing material is important, requiring experienced judgment. Upon reaching the bearing stratum and before belling and cleaning, a strength test or a penetrometer test should be made on the bottom material. One or more probe holes may be drilled below the design level where water or gas problems would not result and useful informa-tion is expected. Probe hole use should be defined in the project documents and the information gleamed from them discussed at the preconstruction meeting.

6.4.10.1 Clay, hardpan, or soft shale—Tests should be made for shear strength to check that the required bearing capacity is met. Sampling and testing should be performed on undisturbed material representative of the materials over the bearing area. Make several tests over large bottom areas to establish the degree of uniformity of material and the validity of the original test samples.

When piers are placed on or in formations of expansive clay or shale, the loads on the bearing material should be sufficient to resist uplift as determined by appropriate tests, or the pier bearing level should be situated at sufficient depth to preclude variations in moisture content at that level after construction. Surface sources of water should be controlled through proper drainage systems to prevent free water from percolating along the perimeter of the pier. Otherwise, the pier should be designed to resist uplift. The magnitude of uplift forces, as well as the ability of the pier shaft, bell, or both shaft and bell to resist the uplift, should be furnished by the geotechnical engineer.

6.4.10.2 Cohesionless subgrade—Where the bearing material is granular and contains less clay, in-place pres-sure meter tests (PMTs), split-barrel penetration tests, and cone penetration tests (CPTs) may be performed as part of the original exploration program to confirm strength proper-ties and to avoid field delays. In addition, triaxial shear tests or other specified suitable tests could provide meaningful data. Where the properties and the range of variability of a bearing stratum have been adequately determined during the

geotechnical exploration, visual corroboration of the bearing stratum may be sufficient. This is often more cost effective than time-consuming laboratory or in-place tests.

In some areas where cavernous rock conditions are present, test holes, borings, or multiple probe holes at each drilled pier location may be needed. The need for such addi-tional exploration and assessment should be weighed care-fully with the variability of site-specific conditions.

Drilling of small-diameter probe holes into the bearing strata to provide information on the adequacy or unifor-mity of the bearing stratum with depth should be done with caution in clay or hardpan when water-bearing sand and silt layers or lenses are suspected within or below the bearing stratum. If such layers are penetrated, serious construc-tion problems may result from infiltration and the washing of silt or sand into the pier. Probe holes are recommended only where soft layers are suspected underneath the bearing layer or when it is felt necessary to supplement original boring investigations. When probe holes indicate soft mate-rial below the bearing stratum, the pier should be extended deeper or the undisturbed samples of the soft soils secured, appropriate laboratory tests made, and the results analyzed by the construction geotechnical engineer to evaluate the allowable bearing capacity of the pier. The bell may need to be enlarged, or additional piers may need to be required.

6.4.10.3 Bearing material under slurry—If construction of the pier is performed under slurry so that physical bottom observation is impractical, the bottom can be checked by various methods. The method of assessing the bearing mate-rial for drilled piers installed by slurry methods will depend on the project, foundation design and end-bearing compo-nent of the design, geologic formations encountered, and the experience of the construction geotechnical engineer. On some projects, assessing the drill cuttings at the surface and sounding the bottom with a weighted tape is the approach used (Ulrich and Ehlers 1995; Ulrich 2007). The Phillips North Sea Platforms supported by belled footings (Ehlers and Bowles 1973) relied on other procedures and drilling equipment sounding. There are also various advanced tech-niques available, including down-hole camera with a sedi-ment thickness gauge (Drilled Shaft Inspector’s Manual 2004). A short length of steel rod attached to a wire has been used successfully. Alternately, a weighted tape can be used (Frizzi et al. 2004). If desired, samples of the bottom can be obtained by attaching special split-barrel sampling tubes either to the drill rig, Kelly bar, or the airlift pipe and drop-ping them on the bottom with sufficient force to penetrate the bearing material.

6.4.10.4 Bearing material hard rock formations—Because of the inherent variability of rock formations on site, it is often difficult to determine in advance the necessary depth of excavation into the rock. The construction geotechnical engineer should observe the pier excavation to confirm the pier is founded in the design stratum. Soft, weathered, and broken rock should be removed to hard solid formations unless the design is based on bearing directly on the weath-ered rock. In some cases, weathered rock may extend tens of feet (meters) deep. Where piers are socketed into the rock





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and part of the load is carried by side resistance (4.5), the rock properties assumed in the design should be known by the construction geotechnical engineer so that the adequacy of the rock can be assessed in the field. Rock consistent with design recommendations is required for the entire depth of the socket as well as the bottom-bearing material, except where engineering analysis indicates this is unnecessary.

Inspection holes should be used to determine if seams of soft material exist in the rock close enough to the bearing level to influence the pier design. From one to three probe holes are recommended, depending on pier size, design-bearing pressure, and anticipated rock quality. Additional probe holes may help determine the extent and thickness of seams prior to making a decision to remove the rock down through the unacceptable layers. The probe holes are drilled with an air drill and should extend into the rock a minimum depth equal to the shaft diameter. The speed of air drilling should be noted by the construction geotechnical engineer and the drill hole sides checked with a feeler rod, borescope, or TV camera to determine the location and size of cracks and clay seams. Tolerable settlement usually governs the permissible size and location of cracks and clay seams.

6.4.11 Pier inspections—The contractor should check each pier hole before entering for methane and carbon dioxide gas. If gas exceeds acceptable limits, it should be purged before any personnel enter, or other provisions should be made for safe entry. No smoking or welding should be permitted until the absence of gas is determined. If direct observation of the bottom is not possible, making purging impractical, determine bell size by observing the position of previously calibrated markings on the Kelly or bell bucket when the bell bucket is fully extended during belling (ADSC and the DFI 2004; Federal Highway Administration (FHWA) 2002; Hertlein and Baker 1996). If there is any question about the adequacy of the mechanical clean-up at the bottom, the bell can be oversized and any remaining spoil back-bladed to the oversize area at the bell’s edge.

6.4.12 Safety precautions—All Occupational Safety and Health Administration (OSHA) safety regulations should be followed for personnel entering a shaft for observation, cleaning, or any other purpose.

6.5—Concrete placementA drilled pier is not complete until the concrete has

been properly placed. Concrete placement operations are as important to successful completion of the pier as to its drilling. Concrete materials and placement methods are often dictated by field conditions and should be selected to prevent the development of voids, segregation of the coarse aggre-gates during concrete placement, and to achieve compat-ibility with design. Concrete should be placed so there is uniform quality for the full design cross section throughout the pier length. As placed, the concrete should develop the required strength.

6.5.1 Construction engineering before concreting—Eval-uation of the pier should begin with the excavation process and extend through concreting. After a pier is excavated, it should be inspected, including the bell where applicable, to

verify that it has not been closed in or partially filled by soil movements or pressure. This also reveals the presence of any foreign material or excessive amounts of water, as well as detrimental damage to any casing used. Inspections may include visual observations with a mirror or high-intensity light; quantitative verification of inside length and diameter; and the depth of any water, soil, debris, or other obstructions to concrete placement that are present. Leaky, damaged, or otherwise obstructed piers that cannot be dewatered and cleaned to permit proper concrete placement should be iden-tified so that replacement piers can be installed while the drilling rig is still nearby, if necessary.

If there is a delay before the pier is concreted, the pier should be covered for protection from inflow of surface water, soil, spoil, and other debris until the concreting takes place. The pier should then be re-inspected immediately before concrete placement. When concrete placement is occurring simultaneously with pier installation, it is diffi-cult for a single inspector to observe both operations prop-erly. Be sure in these circumstances that the inspection and construction crews for both operations are properly staffed with qualified personnel.

6.5.2 Factors affecting placement—The placement of concrete is affected by several factors, including:

6.5.2.1 Subgrade and pier-installation conditions—Pier spacing, installation sequence, excavation methods, and subgrade conditions can affect the concrete placement techniques, as these items influence the potential soil pres-sures, leading to casing collapse and soil intrusion that affect placement.

6.5.2.2 Pier configuration—The potential for concrete segregation, arching, damage, and groundwater inflow are affected by the geometrical properties of the casing: diam-eter; wall thickness; pier shape (straight-sided, belled); interior roughness. Therefore, these geometrical properties influence the selection of the placement procedures and materials.

6.5.2.3 Reinforcement—The presence of reinforcing steel influences the placement techniques because the length, location, clearance, and spacing of longitudinal steel, lateral spiral or ties, and spacers holding the reinforcement in its design location can constrict flow and contribute to segrega-tion and arching during concrete placement. The bar spacing, casing to bar clearance, and bar-to-wall clearance should be considered in determining the maximum aggregate size and the vibration or rodding requirements to facilitate concrete flow through and around the reinforcement. The bar clear-ance of four times the size of the maximum coarse aggre-gate is a minimum and a spacing of five times the maximum coarse aggregate size may be needed, although in some cases, special mixture proportions and the use of admix-tures have allowed successful pier installations with bar spacing smaller than 4 in. (100 mm). While it is preferred that the cage be placed prior to concrete placement, piers have been successfully constructed by inserting reinforcing steel through plastic concrete; however, special detailing, mixture proportioning, and careful control of construction are recommended.





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6.5.2.4 Condition of pier—Conditions of the pier, including presence of water, soil, or other debris, ruptures, or leaks, affect the techniques required to clean the excava-tion in preparation for concreting. If the inflow of ground-water into the excavation cannot be controlled, it may dictate the use of special underwater placement techniques, such as tremie or pump placement.

6.5.2.5 Concrete mixture proportioning—Design mixture properties such as slump, ratio of coarse-to-fine aggregate, maximum coarse aggregate size, water-cementitious mate-rials ratio (w/cm), cement factor, and admixtures, influ-ence the workability and cohesiveness of the mixture and the quality of the placed material. When selecting or estab-lishing the design mixture, placement techniques and desir-able mixture properties combat the obstacles listed in the preceding four items should be considered.

If the concrete is not properly placed, pier defects can develop that could cause the proposed structure to settle excessively. Concrete defects that can develop include:

a) Voids resulting from entrapped water, water migration, or incomplete concreting caused by arching, blockages, or shell collapse;

b) Weak zones resulting from soil inclusions, foreign object inclusions, or a reduced pile cross section;

c) Aggregate pockets resulting from coarse aggregate segregation during placement or erosion of cement paste and fines by water migration;

d) Weak concrete zones resulting from bleeding mixtures, excessive water present during concrete placement, and segregation;

e) Separations, breaks, or displacements caused by surrounding construction activities, such as pile heave or lateral displacement caused by adjacent driving, lateral pres-sures, and displacements from adjacent construction traffic, and lateral pressures and displacements related to adjacent excavations or fills.

Sometimes, the potential presence of defects is indicated during construction by:

a) A drop in the concrete level at the pier head after concrete placement;

b) Water seepage to the pier head from somewhere below;c) Excessive accumulations of laitance at the pier head;d) Excessive variation between the theoretical placement

volumes and delivered concrete volumes;e) Load test failures or excessive settlement.The prevention of concrete defects and identification

of conditions conducive to their development depend on proper construction engineering before concrete placement, proper concrete materials and placement procedures, and experienced pier-concreting personnel. Close coordination between pile construction engineering and concrete place-ment personnel is required.

6.5.3 Concrete monitoring—Perform the following moni-toring during concrete placement:

6.5.3.1 Observe if the reinforcing steel is clean and properly placed, and whether bar sizes and lengths are as designed.

6.5.3.2 Observe the condition of the pier bottom just before concrete placement. If sediment has accumulated on the bottom, reclean and recheck before concreting.

6.5.3.3 Using safety precautions, observe the condition of the shaft walls or steel casing that will be in contact with fresh concrete, and note the water position level behind the casing. Concrete should be placed immediately after these observations.

6.5.3.4 Using safety precautions, observe the method of placing concrete in the pier to see that there is no segregation or contamination of the materials when such methods as free fall through a hopper, tremie pipe, or back chute are used.

6.5.3.5 Perform tests on fresh concrete such as slump, air content, and wet unit weight when required, and cast test specimens. In addition to a set of cylinders for each specific batch of concrete yardage placed, which is typically 50 to 100 yd3 [38 to 76 m3] (or a day’s placement if less), one test cylinder should be taken from every truckload and tested at 7 days to check for any indications of potential problems.

6.5.3.6 Observe that concrete is placed continuously without interruption or long delays, and that a sufficient head of concrete is maintained inside the casing to balance the head of water outside the casing. Continuous placement will prevent inflow of water and soil into the fresh concrete as the casing is withdrawn. Compare the theoretical to actual volume of concrete placed (5.3.5).

6.5.3.7 Observe the level of concrete during the initial casing pull. If the concrete is observed to rise, there is a potential for discontinuity in the concrete.

6.5.3.8 Stay constantly alert for evidence that soil is included in the concrete by any mechanism, including inad-vertent movement of soil into the top of the open hole.

6.5.4 Cave-ins—When placing concrete in uncased exca-vations of questionable stability, close observation of the hole during concrete placement is necessary to detect a cave-in. During winter months when there is an excess of vapor from the hot concrete, detection is more difficult. When a cave-in takes place, the pier should be cleaned out, cased, and the concrete placed again. It is always possible for a soil cave-in to go undetected.

Surface soil cave-in over the top of the casing can occur only when the casing does not protrude sufficiently above ground surface or when there is a spoil pile too close to the pier location.

Side soil cave-in as casing is pulled can result either from inadequate head of concrete in the casing at the time of pulling to balance the forces tending to cause soil cave-in, or it can occur if the concrete becomes hung up in the casing either by arching due to low slump or a fast set. The suction created as the casing and concrete are pulled tends to cause an influx of soil and water beneath the casing. These condi-tions should not occur. In the event that they do, they should be corrected. This could require redrilling the hole using proper procedures.

6.5.5 Concreting through slurry—When concrete is placed under slurry, either by tremie procedures or by pumping, carefully observe that:

a) The slurry level in the pier is static before concreting;





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b) There is adequate initial separation of concrete and slurry. Check the pig dimensions for a close fit, and if a plate is attached to the bottom of the pipe, check the release mechanism;

c) The placement of tremie or pump pipe is at the bottom of the pier;

d) The concrete head in the tremie is adequate to allow lifting of the tremie tip and that tremie tip embedment in concrete is at least 10 ft (3 m);

e) The level of concrete in the pier is monitored with a weighted tape, and the position of the tremie or pump pipe is watched. Continue monitoring the relative position of concrete and pipe to see that the pipe is always in concrete;

f) The calculated theoretical concrete volumes are compared with actual placed volumes after placement for piers whose theoretical volumes exceed one truckload.

If concrete is placed under slurry without casing, and specially designed slurry has been specified for a side-resis-tance pier design, measure viscosity, density, pH, and sand content of slurry to see that it meets specifications as well as follows the procedures outlined in 5.4.6.

6.6—Post-construction assessmentCritical soil or groundwater conditions, questionable

construction methods, or subsequent nearby construction activities may raise doubt about the integrity of certain piers on a project. Construction means and methods do not always produce flawless piers. The presence of flaws in piers can impact both the structural and geotechnical ability of the pier to support load. The most frequently used method for deter-mining the soundness of a pier is core drilling with associ-ated probing on the sides of the core hole, TV inspection, bore scope inspection, and sonic testing between core holes. Sonic devices and techniques without core holes, such as surface reflection techniques, parallel seismic techniques, or large strain dynamic capacity testing, have also been used (Davis and Hertlein 2006). Although sonic testing from the surface is unlikely to detect unsatisfactory bottom condi-tions, particularly if the depth-to-diameter ratio exceeds 30, large strain dynamic capacity testing can effectively test the bottom if sufficient energy is used. None of the nondestruc-tive testing methods are sufficient as the sole criteria for acceptance or rejection of a pier.

For concrete placement underwater or with slurry using appropriate tremie or concrete pumping procedures, a satisfactory and relatively convenient method for checking concrete integrity and quality after placement is to use either sonic testing or gamma logging techniques in preplaced access tubes extended to the base of the pier (Baker 1986; Baker et al. 1992).

The Deep Foundation Institute (DFI) has published a manual on assessment of drilled piers that provides a good summary of the available techniques and their limitations (Manual for Non-Destructive Testing and Evaluation of Drilled Shafts 2005). Additional guidance and practical experiences are provided by Hertlein (1997) and Davis and Hertlein (2006).

6.7—ReportsConstruction reports, which may be issued daily or weekly

by the construction geotechnical engineer and drilled pier contractor, should be given to the licensed design profes-sional, structural engineer, and contractor. These reports should cover the following items when applicable:

a) Location and dimensions of pier holes drilled;b) Top and bottom elevations (the degree of accuracy

necessary should have been previously specified by the structural or the geotechnical engineer);

c) Type of shaft-excavating methods used;d) Description of materials encountered during excavation;e) Description of groundwater conditions encountered;f) Description, location, and dimensions of obstructions

encountered and if removal was attained;g) Description of temporary or permanent casing placed,

including purpose, length, wall thickness, and anchorage or seal obtained, if any;

h) Description of any soil and water movement, bell and wall stability, loss of ground, methods for control, and pumping requirements;

i) Data secured for all shaft, bell, and shear ring measurements;

j) Description of cleanout methods and adequacy of initial cleanout;

k) Elevation at which bearing material was encountered;l) Description of bearing material, probe holes made,

method of probing, rate of drilling in rock, samples taken, tests made, and conclusions reached with regard to adequacy of bearing material;

m) Description of adequacy of cleanout just before concrete placement;

n) Record of depth of water in hole and rate of water infil-tration before concrete placement;

o) Record of reinforcing steel inspection for position and adequacy;

p) Method of concrete placement and removal of casing, if any; record head of concrete during removal of casing and elevation of concrete when vibration began;

q) Record of any difficulties encountered, including possible soil inclusion, voids, shaft constriction, or casing collapse;

r) Condition of concrete delivered to site, including record of slump, unit weight, air content, and other tests and cylin-ders made for compression testing;

s) Record of any deviations from the specifications and decisions required.

6.8—Criteria for acceptanceThe following minimum criteria for acceptance of

completed piers should be included in the specifications. The licensed design professional or geotechnical engineer may specify additional or more restrictive requirements in the contract documents. Any variations should be brought to the attention of the licensed design professional and geotech-nical engineer before acceptance.

6.8.1 Location and plumbness—Unless compensated for in the structural design, permissible construction tolerances





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for plumbness should be in accordance with ACI 336.1. The top of the pier should not deviate from the planned location more than 4 percent of the pier diameter or 3 in. (75 mm) in any direction, whichever is less. Tolerances smaller than 2 in. (50 mm) are difficult to obtain.

6.8.2 Shafts, bells, and shear rings—Measurements of shafts, bells, and shear rings should be made by the geotech-nical field representative. The area of the shaft and bell should not be less than 100 percent of that specified. For machine-made bells, the bell roof slope should be a straight line downward and not be concave upward. The bell design edge should be at least 6 in. (150 mm) thick.

6.8.3 Cleanout—All loose material and spoil should be cleaned out of shafts and bells before placing concrete. In the case of end-bearing piers, the volume of loose material and spoil should not exceed an amount that would cover 10 percent of the area to a depth of 2 in. (50 mm).

6.8.4 Reinforcement—Refer to ACI 117.6.8.5 Pier acceptability for concrete placement—A pier

should not be considered acceptable for concrete place-ment until the inspection procedures described in Section 6 have been accomplished as applicable and accepted by the construction geotechnical engineer.

6.8.6 As-built acceptance—The licensed design profes-sional or geotechnical engineer may choose to require load testing or nondestructive testing before accepting the as-built pier. Load testing methods are described in 3.5.7 and 3.5.8. Post-construction assessment techniques are described in 6.6.

6.9—Corrective measuresIf the pier does not meet specification requirements,

corrective measures are needed (Baker and Khan 1971). There is no simple, economical method to determine conclu-sively if an as-installed pier is defective, although several methods are available to augment construction observations and coring (Davis 1991). If necessary, suspect piers can be load tested by either conventional static means or by large strain dynamic testing (Baker et al. 1992). In the case of a shaft being off location or out of plumb, if the out-of -toler-ance deviation is discovered before concreting, it may be possible to correct most economically by increasing struc-tural reinforcement of the pier, adding structural lateral restraint, or by performing both corrections. This approach requires analysis by the structural engineer. If the excavation is severely out of tolerance, this approach may not be prac-tical. Instead, it might be necessary to backfill the hole with a lean mixture proportion grout and carefully redrill. Periodi-cally check and make corrections for plumb tolerance during the redrill. In this case, the grout should be designed to be reasonably comparable to the strength and hardness of the ground being drilled. Alternatively, it may be possible that the shaft can be redrilled sufficiently oversize that a design size shaft of the correct size and plumbness falls within the overall dimensions of the new oversize excavation.

CHAPTER 7—REFERENCESACI Committee documents and documents published by

other organizations are listed first by document number, full

title, and year of publication followed by authored docu-ments listed alphabetically.

American Concrete Institute117-10—Specification for Tolerances for Concrete

Construction and Materials and Commentary301-10—Specifications for Structural Concrete318-99—Building Code Requirements for Structural

Concrete and Commentary318-11—Building Code Requirements for Structural

Concrete and Commentary336.1-01—Specification for the Construction of Drilled

Piers543R-12—Guide to Design, Manufacture, and Installa-

tion of Concrete Piles

American Petroleum Institute13A—Specification for Drilling Fluid Materials13B-1—Recommended Practice for Field Testing Water-

Based Drilling FluidsRP 13B-1—Field Testing for Water-Based Drilling Fluids

ASTM InternationalD1143/D1143M-07(2013)—Standard Test Methods for

Deep Foundations Under Static Axial Compressive LoadD2113-08—Standard Practice for Rock Core Drilling and

Sampling of Rock for Site InvestigationD4380-12—Standard Test Method for Density of Benton-

itic SlurriesD4381/D4381M-12—Standard Test Method for Sand

Content by Volume of Bentonitic SlurriesD4972-13—Standard Test Method for pH of SoilsD6910/D6910M-09—Standard Test Method for Marsh

Funnel Viscosity of Clay Construction SlurriesD7383-10—Standard Test Methods for Axial Compres-

sive Force Pulse (Rapid) Testing of Deep Foundations

Deep Foundations InstituteManual for Non-Destructive Testing and Evaluation of

Drilled Shafts, 2005

International Association of Foundation Drilling (ADSC) and the Deep Foundations Institute (DFI)

Drilled Shaft Inspector’s Manual, second edition, 2004

Authored documentsBaker, C. N., 1986, “Pier Design and Construction in

Water Bearing Soils,” Proceedings of the International Conference on Deep Foundations, Deep Foundations Insti-tute (DFI), DFI-CIGIS Symposium, Hawthorne, NJ, Sept.

Baker, C. N.; Drumright, E. E.; Briaud, J. L.; Mensah-Dwumah, F.; and Parikh, G., 1992, “Drilled Shafts for Bridge Foundations,” Report No. FHWA-RD-95-172, U.S. Department of Transportation, McLean, VA.

Baker Jr., C. N., and Gnaedinger, J. P., 1960, “Investi-gation of the Concrete Free Fall Method of Placing High Strength Concrete in Deep Caisson Foundation, Soil Testing Services, Inc.,” Chicago, 11 pp.





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Baker, C. N., and Keifer, T. A., 1994, “Effects of Free Fall of Concrete in Drilled Shafts,” ADSC Report No. TL112, Jan.

Baker, C. N., and Khan, F. R., 1971, “Caisson Construc-tion Problems and Correction in Chicago,” Proceedings, ASCE, V. 97, No. 2, Feb., pp. 417-440.

Borden, R. H., and Gabr, M. A, 1987, “Analysis of Compact Pole-Type Footings, LT Base: Computer Program for Laterally Loaded Pier Analysis Including Base and Slope Effects,” Department of Civil Engineering, North Carolina State University at Raleigh, North Carolina Department of Transportation and Federal Highway Administration, Raleigh, NC, 215 pp.

Broms, B. B., 1964, “Lateral Resistance of Piles in Cohe-sionless Soils,” Proceedings, ASCE, V. 90, No. SM3, May, pp. 123-156.

Broms, B. B., 1965, “Design of Laterally Loaded Piles,” Proceedings, ASCE, V. 91, No. SM3, pp. 79-99.

Brown, D. A.; Turner, J. P.; and Castelli, R. J., 2010, “Drilled Shafts: Construction Procedures and LRFD Design Methods,” Geotechnical Engineering Circular 10, FHWA NHI-10-016, NHI Course No. 132014, May.

Canadian Geotechnical Society, 1985, “Deep Founda-tions on Rock,” Canadian Foundation Engineering Manual, Richmond, BC, Canada, Second ed, Part 3, Section 20.1.

City of New York, 2008, Section 1808.2.8.4, “Allowable Frictional Resistance,” Building Code of the City of New York, Jan. 1 to Dec. 31.

Davis, A. G., 1991, “The Development of Small Strain Integrity Testing at Drilled Shafts: A Review,” Transporta-tion Research Board, Washington, DC, Jan.

Davis, A. G., and Hertlein, B., 2006, Nondestructive Testing of Deep Foundations, John Wiley and Sons, Ltd., West Sussex, England, Aug. 18, 292 pp.

Davisson, M. T., 1969, “Design of Deep Foundations for Tall Buildings Under Lateral Load,” University of Illinois Bulletin, V. 66, No. 78, Feb. 14, pp. 157-174.

Davisson, M. T., 1993, “Negative Skin Friction in Piles and Design Decisions” Proceedings, Third International Conference on Case Histories in Geotechnical Engineering, St. Louis, MO, June 1, pp. 1793-1801.

Ehlers, C. J., and Bowles, W. R., 1973, “Underreamed Footings Support Offshore Platforms in the North Sea,” Paper No.1895-MS, Proceedings of the Ninth Annual Offshore Technology Conference, Houston, TX.

Federal Highway Administration (FHWA), 2002, “Drilled Shaft Foundation Inspection,” National Highway Institute (NHI) Course No. 132070, Federal Highway Administration Publication No. FHWA NHI-03-018, Dec.

Frizzi, R. P.; Meyer, M. E.; and Zhou, L., 2004, “Full-Scale Field Performance of Drilled Shafts Constructed using Bentonite and Polymer Slurries,” Proceedings of GeoSup-port 2004, The Geo-Institute, ASCE, Reston, VA.

GAI Consultants, Inc., 1982, “Laterally Loaded Drilled Pier Research, Volume 1: Design Methodology,” Research Report EPRI EL-2197, Electric Power Research Institute, Palo Alto, CA, V. 1, Jan.

Goodman, R. E.; Van, T. K.; and Heuze, F. E., 1968, “The Measurement of Rock Deformability in Bore Holes,” 10th Symposium of Rock Mechanics, University of Texas, Austin, TX, May, pp. 523-545.

Hertlein, B. H., 1997, “Practical Experience with Non-Destructive Testing of Augered CIP Piles,” Proceedings – DFI Specialty Seminar, Augered Cast-In-Place Piles Seminar, Orlando, FL.

Hertlein, B. H., and Baker Jr., C. N., 1996, “Practical Experience with Non-Destructive Testing of Deep Founda-tions: A Drilled Shaft Inspector’s Guide to Detecting Anom-alies and Assessing the Significance of ‘Defects,’” Founda-tion Drilling, ADSC, Dallas, TX, Mar-Apr., pp. 19-26.

Horvath, R. G., and Kenney, T. C., 1979, “Shaft Resistance of Rock-Socketed Drilled Piers,” ASCE Annual Convention, Atlanta, GA, Oct., pp. 182-214.

Kiefer, T. A., and Baker Jr., C. N., 1994, “The Effects of Free Fall Concrete in Drilled Shafts,” ADSC International Association of Foundation Drilling, Dallas, TX, June/July, pp. 16-17.

Koutsoftas, D. C., 1981, “Caissons Socketed in Sound Mica Schist,” Journal of the Geotechnical Engineering Division, ASCE, V. 107, pp. 743-757.

Litke, S., 1992, “Concrete Free Fall Tested in Alabama Highway Department Project,” Foundation Drilling, ADSC: The International Association of Foundation Drilling, June-July, pp. 14-16.

Occupational Safety and Health Standards, Construc-tion, (29 CAR 1926), U.S. Department of Labor, Occupa-tional Safety and Health Administration, U.S. Government Printing Office, Washington, DC.

O’Neill, M. W., and Reese, L. C., 1999, “Drilled Shafts: Construction Procedures and Design Methods,” Publication No. FHWA-IF-99-025, U.S. Department of Transportation, McLean, VA.

Osterberg, J. O., 1989, “New Device for Load Testing Driven Piles and Drilled Shafts Separates Friction and End Bearing,” Proceedings: International Conference on Piling and Deep Foundations, London, A. A. Balkema Publishers, Rotterdam, Netherlands, May, p. 421.

Poulos, H. G., and Davis, E. H., 1980, Pile Foundation Analysis and Design, John Wiley & Sons, New York, 397 pp.

Prakash, S., 1962, “Behavior of Pile Groups Subjected to Lateral Load,” PhD thesis, Department of Civil Engineering, University of Illinois, Urbana, IL.

Reese, L. C., 1977, “Laterally Loaded Piers: Program Documentation,” Journal of the Geotechnical Engineering Division, V. 103, ASCE, pp. 287-305.

Reese, L. C., 1984, Handbook on Design of Piles and Drilled Shafts Under Lateral Load, Federal Highway Administration, Office of Implementation, U.S. Department of Transportation, McLean, VA.

Reese, L. C., and O’Neill, M. W., 1988, “Drilled Shafts: Construction Procedures and Design Methods,” Publication No. FHWA-HI-88-042, U.S. Department of Transportation Federal Highway Administration, McLean, VA.





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Reese, L. C., and Van Impe, W. F., 2001, Single Piles and Pile Groups Under Lateral Loading, A. A. Balkema Publishers, Rotterdam, The Netherlands, 463 pp.

Reese, L. C., and Wright, S. J., 1977, Drilled Shaft Design and Construction Guidelines Manual, V. I and II, U.S. Department of Transportation, McLean, VA.

Richardson, E. V.; Harrison, L. J.; and David, S. R., 1991, “Evaluating Scour at Bridges,” U.S. FHWA, Office of Research and Development, Hydraulic Engineering Circular No. 18, Publication No. FHWA-IP-90-017, U.S. Department of Transportation, McLean, VA.

Singh, A.; Hu, R. E.; and Cousineau, R. D., 1971, “Lateral Load Capacity of Piles in Sand and Normally Consolidated Clay,” Civil Engineering Magazine, ASCE, V. 41, No. 8, Aug., pp. 52-54.

Suprenant, B., 2001, “Free Fall of Concrete,” Concrete International, V. 23, No. 6, June, pp. 44-45.

Terzaghi, K.; Peck, R. B.; and Mesri, G., 1996, Soil Mechanics in Engineering Practice, second edition, John Wiley and Sons, Inc., New York, 549 pp.

Turner, C. D., 1970, “Unconfined Free-Fall of Concrete,” ACI Journal Proceedings, V. 67, No. 12, Dec., pp. 975-976.

Ulrich, E. J., 2007, “The Tallest Building in the Texas Medical Center: Memorial Hermann Medical Plaza.” Proceedings, GeoDenver, Jan.

Ulrich Jr., E. J., and Ehlers, C. J., 1995, “Tallest Building in the Texas Medical Center: St. Luke’s Medical Tower,” Design and Performance of Mat Foundations—State-of-the-Art Review, SP-152, E. J. Ulrich, ed., American Concrete Institute, Farmington Hills, MI, pp. 219-244.

Ulrich, E. J., and Ehlers, C. J., 2000, “Arrest of a Texas Landslide,” Concrete International, V. 22, No. 1, Jan., pp. 57-64.

Wang, S. T., and Reese, L. C., 1987, “The Effect of Non-Linear Flexural Rigidity on the Behavior of Concrete Piles Under Lateral Loading,” Texas Section, ASCE, Houston, TX.





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As ACI begins its second century of advancing concrete knowledge, its original chartered purpose remains “to provide a comradeship in finding the best ways to do concrete work of all kinds and in spreading knowledge.” In keeping with this purpose, ACI supports the following activities:

· Technical committees that produce consensus reports, guides, specifications, and codes.

· Spring and fall conventions to facilitate the work of its committees.

· Educational seminars that disseminate reliable information on concrete.

· Certification programs for personnel employed within the concrete industry.

· Student programs such as scholarships, internships, and competitions.

· Sponsoring and co-sponsoring international conferences and symposia.

· Formal coordination with several international concrete related societies.

· Periodicals: the ACI Structural Journal, Materials Journal, and Concrete International.

Benefits of membership include a subscription to Concrete International and to an ACI Journal. ACI members receive discounts of up to 40% on all ACI products and services, including documents, seminars and convention registration fees.

As a member of ACI, you join thousands of practitioners and professionals worldwide who share a commitment to maintain the highest industry standards for concrete technology, construction, and practices. In addition, ACI chapters provide opportunities for interaction of professionals and practitioners at a local level.

American Concrete Institute38800 Country Club DriveFarmington Hills, MI 48331Phone: +1.248.848.3700Fax: +1.248.848.3701

www.concrete.org



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38800 Country Club Drive

Farmington Hills, MI 48331 USA

+1.248.848.3700

www.concrete.org

The American Concrete Institute (ACI) is a leading authority and resource

worldwide for the development and distribution of consensus-based

standards and technical resources, educational programs, and certifications

for individuals and organizations involved in concrete design, construction,

and materials, who share a commitment to pursuing the best use of concrete.

Individuals interested in the activities of ACI are encouraged to explore the

ACI website for membership opportunities, committee activities, and a wide

variety of concrete resources. As a volunteer member-driven organization,

ACI invites partnerships and welcomes all concrete professionals who wish to

be part of a respected, connected, social group that provides an opportunity

for professional growth, networking and enjoyment.



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