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ENCE 4610ENCE 4610Foundation Foundation

Analysis and Analysis and DesignDesign

Interpretation of Boring Logs and Field Data

AASHTO LRFD Method and Factors

Boring Logs and Their Interpretation

Overview of Boring Logs and Their Use

● Boring Logs are at the center of geotechnical foundation design

● Without an adequate testing program, we are forced to guess at the type and configuration of foundations, which leads to additional expense and risk

● Although this section centers on boring logs, there are other information sources of importance:– Laboratory tests– Geophysical methods– Site plans for boring and

testing– Data from tests such as

CPT logs

Sample Boring Log

Sample Boring Log

Sample Boring Log Key

Sample Boring Logs

Sample Boring Log

Information Found on Boring Logs● Information Found on

Boring Logs– Date and time of

testing– Location of soil surface– Location of elevation

datum– Layering of Soils and

Rocks– RQD Data

● Information Found on Boring Logs– Classification and

Characterization of Soils

– SPT and CPT Data● CPT can have their

own profile

– Location of Water Table

Other Types of Data in Site Exploration

● Boring logs are not the only types of exploration that can be used in site investigation; we also have, for example, test pits/hand auger results and CPT Tests

Site Plans● Boring logs and other

investigations are not prepared or used in isolation; they are a part of the site testing program and plan

● Their location and relationship to each other is crucial in understanding the nature of a site, and the variations in stratigraphy that are found there

Before Boring: Sources of Site Data

Field Reconnaissance

Subsurface Exploration Program

Guidelines for Minimum Number of Exploration Points and Depth of Exploration

Subsurface Profile

Subsurface Profile

From Boring Logs to Idealized Soil Profile

● Once the boring logs are used to outline the stratigraphy of a site, the next step is to develop an idealized profile for design purposes

● In most cases, the desired result is a profile with the “Mohr-Coulomb Triple” (φ, c, γ) for each layer– Boring logs seldom give this

information directly, especially the unit weight

– Most data from boring logs comes from disturbed samples, thus empirical correlations are common

● SPT data is still the most common type of data to appear on boring logs beyond the few feet/meters below the surface– We discussed the limitations of

SPT data in Soil Mechanics

● CPT data offer some additional opportunities– Soils can be classified with CPT

data– Methods for deep foundation

capacity using CPT data directly have been developed

Correlations for Cohesionless Soils

Which one you use—and there are others—depends on the reliability of the correlation. Table 8-1 is probably the least reliable and best for preliminary work. The others are better but must be used with care as with anything.

Correlations for Cohesive Soils● Correlations such as Table 4-2

are not very reliable and should only be used for preliminary work

● In some cases boring logs will include field vane shear test results, which are much more useful

● Unconfined compression strength tests are best for unremoulded purely cohesive soils but require an undisturbed sample

Example of Boring Log Interpretation● Given

– Apple Freeway Project (SFH Appendix A)

– Boring UDH BAF-4

● Find– Idealized soil profile

– po diagram

– Preconsolidation Pressures and consolidation properties

● Solution– The example presents

many of the results as coming from “laboratory tests”

– Although this is common (except for unit weight,) we will attempt to correlate these results by other means

Boring UDH BAF-4

Water Table

Vane Shear Results

Undisturbed Samples for Laboratory Tests RQD Results

SPT Results

Idealized Soil Profile● Layer 1

– Corresponds with “Black Organic Silt” layer– Assume unsaturated because it is above

the water table– Assuming SPT values are N60 corrected

(not always the case,) N60 = 1+1 = 2

– This is a very soft soil; previous correlations show around 100 pcf, since it is organic 90 pcf is reasonable

● Layer 2– Because of the water table, layer has to be

split in idealized profile

– N60 = 8 + 9 = 17

– Soil is probably medium, thus unit weight 110-130 pcf unit weight value given at lower range

● Layer 3– No SPT data; however, we have vane shear

data, which indicates (see below) that cohesion is 1050-1250 psf (triaxial tests in same range)

– Cohesion range indicates a stiff soil, thus unit weight around 120-140 pcf, 125 pcf is reasonable

● Submerged Unit Weight– This profile computes the submerged unit

weight by an intuitive process– This is a reasonable approximation but a better

way is to subtract the unit weight of water from the total unit weight of saturated soil

Effective Stress Diagram

Consolidation Data● Consolidation data is

presented– It is based on lab testing,

which is the ideal way of doing this

– Most soil borings will include Atterberg limits, which will allow estimates of consolidation coefficients in the absence of laboratory tests

Application of Fellenius’ Method for Driven Piles

● Given– Soil profile just derived– 12” square concrete

pile, 4’ perimeter, 1 sq.ft. Toe area, 45’ long, groundline

● Find– Ultimate Static

Capacity of Piles using Fellenius’ Method

Layer Area, ft2 Effective Stress,

psi

Beta or N

t

Qs or Q

t

for Layer, kips

1 (4)(3) = 12 270/2 = 135 0.0 0

2 (4)(3) = 12 (600+270)/2 = 435

0.45 2.35

3 (4)(4) = 16 (600+800)/2 = 700

0.45 5.04

4 (4)(35) = 140

(3075+800)/2 = 1938

0.32 86.8

Toe 1 3075 16 49.2

Total 143.4

CPT Methods for Deep Foundations Static Capacity Analysis

● Although it doesn’t need to be pushed too hard, the analogy between a driven pile going into the ground and a CPT probe doing the same has led to direct application of CPT data to estimate static pile capacity

● These methods are becoming more popular as CPT becomes more widely utilized

● We will use the method of Eslami and Fellenius (1999) as it accounts for pore water pressures and simple to implement

Eslami and Fellenius Method (from Fellenius (2015))

Eslami and Fellenius Method

Example of Eslami and Fellenius Method

● Given: Profile shown below, 12” concrete pile, 45’ long, groundline● Find: Ultimate static capacity using E&F method

Example of E&F Method● Pile configuration

– Perimeter = 4’– Toe area = 1 sq.ft.

– Ct = 1/1 = 1

● Layering– Phreatic surface at approx. 6’– Layer 1 (0-3’) soft soils, neglect– Layer 2 (3’-6’) Sands

● qt = 160 tsf● U2 = 0 (above water table)● qE = 160 – 0 = 160 tsf● Cs = 0.010 (silty sand)● rs = (0.010)(160) = 1.6 tsf = 3.2 ksf

● Layering– Layer 3 (10’-20’)

● qt = 20 tsf● U2 = (13’)(62.4) = 0.811 ksf

● qE = (20)(2) – 0.811 = 29.2 ksf● Cs = 0.015 (silt)● rs = (0.015)(29.2) = 0.44 ksf

– Layer 4 (20’-28’)● qt = 25 tsf● U2 = (20’)(62.4) = 1.248 ksf

● qE = (25)(2) – 1.248 = 48.75 ksf● Cs = 0.025 (silty clay)

● rs = (0.025)(48.75) = 1.22 ksf

– Layer 5 (28’-45’)● qt = 20 tsf● U2 = (125’)(62.4) = 7.8 ksf (distance directly from CPT

data)● qE = (20)(2) – 7.8 = 32.2 ksf● Cs = 0.05 (clay)● rs = (0.05)(32.2) = 1.61 ksf

Example of E&F Method● Pile Toe

– The soil at the toe is relatively strong; thus we use the data 8’ above the toe (we don’t have data below the pile toe

– We will use the same data as the shaft

– Equation for rt calls for using qEg; unfortunately, we don’t have effective stress data, so we’ll use qE = 32.2 ksf

– rt = (1)(32.2) = 32.2 ksf

● Applying unit resistance to compute ultimate pile static capacity– Qs = (4’)((3.2)(3) +

(0.44)(10) + (1.22)(8) + (17)(1.61)) = 204.52 kips

– Qt = (1)(32.2) = 32.2 kips

– Qu = 204.52+32.2 = 236.7 kips

LRFD Methods

Solving the “Factor of Ignorance” via Probability

• LRFD is an attempt to define that “factor of ignorance” based on probabilistic considerations

• Principal Source: Design and Construction of Driven Pile Foundations, based on AASHTO LRFD Bridge Design Specifications, 7th Edition, 2014, with 2015 Interim.

• “It should be remembered, however, that these are not true factors of safety, but include a "factor of ignorance." The author suggests that when the ultimate resistance of any pile has been determined, in fixing the factor of safety...the most unfavorable conditions possible in the supporting strata should be judged (the range of conditions possible being narrowed with better knowledge of the subsurface conditions and of the possibility of disturbance from extraneous sources) and a proportion of the factor of safety -- a "factor of ignorance" -- then allowed in respect to these possible conditions, the manner of determining the ultimate load, and the type of loading to be borne. The remaining proportion of the factor of safety -- or true margin of safety -- should be approximately constant for all classes of loading and foundation conditions involving the same value of loss in case of failure; and the overall factor of safety...will then be equal to the product of the true factor of safety with the "factor of ignorance." (David Victor Isaacs, 1931)

From ASD to LRFDFrom ASD to LRFD• Limitations of ASD

– Does not adequately account for variability of loads and resistances. The FS is applied only to resistance. Loads are considered to be without variation (i.e., deterministic).

– Does not embody a reasonable measure of strength, which is a more fundamental measure of resistance than is allowable stress.

– Selection of a FS is subjective, and does not provide a measure of reliability in terms of probability of failure.

• History– Until early 1970’s all civil

engineering design was done using ASD

– Transition for superstructures was complete by mid-1990’s

– Transition to LRFD for substructures began around this time and has continued to the present

Advantages of Challenges of LRFDAdvantages of Challenges of LRFD

• Advantages– Accounts separately for

variability in load and resistance prediction

– Achieves more consistent levels of safety in structure and substructure design

– Does not require knowledge of probability or reliability theory

• Challenges– Implementation requires a

change for engineers accustomed to ASD

– Resistance factors vary with design methods and are not constant

– Rigorous calibration of load and resistance factors to meet individual situations requires availability of statistical data and probabilistic design algorithms

LRFD Design ApproachLRFD Design Approach

(usually) 1 ,11

i

n

m

iii RQ

Distribution of Load and Distribution of Load and ResistanceResistance

Empirical Rates of Failure for Civil Empirical Rates of Failure for Civil Works FacilitiesWorks Facilities

Loading for Loading for Substructure DesignSubstructure Design

AASHTO Load Designations

LRFD Equation as Used in AASHTO SpecificationsLRFD Equation as Used in AASHTO Specifications

Limit States and LoadsLimit States and Loads• Strength Limit States

o Involve the total or partial collapse of the structure

o Include bearing capacity failure, sliding and overall instability

• Service Limit Stateso Affect the function of the

structure under regular service conditions

o Include excessive settlement, excessive lateral deflections, and structural deterioration of the foundation or excessive vibration

• For a structure to be sound, Resistance > Effect of the Loads

• Definition of Limit Stateo A condition beyond which

a structural component, such as a foundation or other bridge component, ceases to fulfill the function for which it was designed

Limit Limit States, States,

AASHTO AASHTO SpecificatioSpecificatio

nn

AASHTO Strength and Service Load FactorsAASHTO Strength and Service Load Factors

Combination of Loads

Example of Load

Factoring and

Combination

Determination of Resistance FactorsDetermination of Resistance Factors

• Calibration of Resistance Factors

– Engineering judgment– Fitting to ASD– Reliability theory– A combination of

approaches

• Selection of Resistance Factors

– Variability of the soil and rock properties

– Reliability of the equations used for predicting resistance

– Quality of the construction workmanship

– Extent of soil exploration– Consequence(s) of a

failure

Resistance Factors

Resistance Factors

LRFD Driven Pile Example• Given

– Loose SM sand, water table 60’ from surface

– 14” square concrete pile– Dead Axial Load DD = 150 kips– Live Axial Load LL = 100 kips

• Find– Pile length that will develop

sufficiently large resistance load to meet LRFD requirements

– Settlement at Service I load– Consider Strength I-V cases and

Service I case

LRFD Driven Pile Example• Resistance Factors

– φ= 0.35 (beta method)

– Factored Resistance = 362.5/0.35 = 1036 kips

• By trial and error, using TAMWAVE the pile length comes to 130’

LRFD Driven Pile Example• Service I Load = 250

kips

• Required Resistance = 250/0.35 = 714 kips

• Based on load test, pile head deflection is approximately 1.1” (can be sharpened with linear interpolation)

QuestionsQuestions