Post on 28-Jul-2018
Drilled Shafts:
Who, When, What, Where
How and Why?
Presented by:
W. Tom Witherspoon
23 April 2010
Overview
• Who: Parties of Interest
• When: Project Sequence
• What: Drilled Shafts
• Where: Applicability
• How: Design/Installation
• Why: Cost, Time, Quality,
Claims, Risk
• ADSC-IAFD: What is it?
Who?
Parties with Interest:
• Owners / Developers
• Architects
• General Contractors
• Designers
• Geotechnical Engineers
• Structural Engineers
• Experienced and Inexperienced Designers
When? (sequence of non-D/B Projects)
Concept (Owner)
Design (Arch./Eng’r)
Management (Arch./C.M.)
Project Const. (General Cont.)
Foundations (Specialty Cont.)
Pre-Bid Involvement
Exc. & Fdns (Sub & Specialty Cont.)
Site Work and Prep (G.C. & Sub Cont.)
Substructure (G.C. & Sub Cont.)
Superstructure (G.C. & Sub Cont.)
Occupancy (Owner)
Post-Bid Involvement
Who by the Numbers
• Owners / Developers: 100%
• Design: 5-10%
• Geotechnical: 1-2%
• Construction: 85-90%
• Specialty Contractor (Foundations): 10%
• QA/QC & Inspection: <5%
• Claims: can be more than 100%
What?
• Drilled and concreted foundation systems
• Diameters: >12 inch, <12 ft
• More common range: 24 to 60 inch (verify)
• Can accommodate a variety of lengths (~100s ft)
• Capable of supporting large loads
• Axially (xx kip) and Laterally (xx kip)
• Installed in soils and rock
• Environmentally Friendly
• Less noise & vibration than pile driving
Over-
burden
Rock
What?
What? …Advantages
• Reliability – historically proven method
• Low noise and minimal vibrations
• Can visually inspect bearing stratum
• Can monitor vertical alignment
• Structural strength in bending
• For seismic and wind loading
• Eliminate need for pile cap
• One drilled shaft in lieu of multiple piles
Where?
• Variety of subsurface conditions:
• In sands, clays and rock
• Above or below GWT
• Deposits with cobbles
• Karst areas
• Collapsible soils
• Shrink/swell clays
• Compressible strata
• Soft clays, loose sands
• Liquefiable soils
Where?
• Variety of loading conditions:
• Axial loading
• Lateral loading
• Lateral with axial loading
• Uplift loading
• Seismic loading
• Impact loading
• Open areas and low headroom
• Difficult access areas
• Settlement-sensitive structures
Where?
Where? …Applicability
• Building foundations
• Single column per drilled shaft
• Transportation structures
• Bridge foundations
• Retaining walls
• Sound walls
• Underpinning
• Slope stabilization
• Seismic loading conditions
…Applicability
…Limitations
• Requires careful QC/QA
• Construction method is sensitive to
ground conditions
• Requires experienced, capable
subcontractor
• No direct field control of axial capacity
• Concerns for structural integrity, rebar
placement
How? …design elements
Side Resistance, RS
Axial Load in Compression, Qc
W
Base Resistance, RB
Axial Design Shown
• Axial Loads are
resisted by
• Side resistance, RS
• Base resistance, RB
• Force Equilibrium:
QC + W = RS + RB
• Lateral Loads are
resisted by
• Soil strength
• Bending Stiffness of
Shaft, EI
How? …design elements
• How are the soil strength and side/base
resistances determined??
• Assumptions about soil strength made based on
field and laboratory testing
• Field tests: SPT, CPT, PMT, DMT, etc.
• Lab tests: drained and undrained shear strength tests
• Ultimately, develop load-deformation relationships
• Strength, resistance and expected
behavior is dependent upon soil type
(cohesionless, cohesive or rock)
How? …design elements
• Soil Types: (general terms only)
• Cohesionless – sands, gravels, some silts
• Cohesive – clays, some silts
• Rock – unweathered and weathered rock
• Side and Base Resistance = f(soil)
• Displacements to mobilize resistance
• Side Resistance:
• Cohesive and Cohesionless Soils: 0.4 to 0.6 inch
• Base Resistance:
• Cohesive Soils: ±4% shaft diameter
• Cohesionless Soils: ± 10% shaft diameter
How? …installation
• General Construction Methods
• “Dry” Method
• Drilling, cage setting, concreting
• Installation performed as quickly as possible to minimize
the effects of stress relief in the soil or rock
• “Wet” Method
• Drilling, slurry head, cage setting, concreting
• Bond between concrete and rebar not reduced if cage is
placed in slurry before concreting
• Casing Method
• Many procedures can be followed
• Casing is removed during concreting
How? …Dry Method
• Used when boreholes will stand open
• Relatively strong cohesive soil or rock with few cohesionless soil seams
• Minor water infiltration can be tolerated as long as no sloughing is occurring
• Can visually inspect the borehole
• Less expensive and faster than wet / casing methods
Drill Borehole Set Cage
How? …Dry Method
Place Concrete Completed Shaft
How? …Dry Method
• Used if caving or water-
bearing soils are
present
• Direct, visual inspection
of borehole not possible
• Contractor skill and
experience are essential
• Careful control of the
chemical and physical
properties of the slurry
How? …Wet Method
Set Cage in SlurryExcavate Borehole
under Slurry
How? …Wet Method
Place Concrete with
Gravity Tremie / Pump
Completed Shaft
How? …Wet Method
• Used when near-surface
soils may cave / deep
soils or rock are stable
and dry
• Otherwise, full-depth
casing (or wet method)
is considered
How? …Casing Method
Possible Sequence
• Drill until caving soil is reached
• Introduce slurry (and maintain head)
• Drill through caving soil
• Insert and seal casing
• Continue drilling below casing, if needed
• Set cage on bottom of borehole
• Place fluid concrete well up into casing
• Pull casing slowly and steadily, continuing to add
fluid concrete until shaft is completed
How? …Casing Method
Why? …The reasons to be interested
• Cost
• Time
• Quality
• Claims
• Risk
ADSC-IAFD
• A trade association established in 1964
comprised of people working together to
advance all aspects of the industries it
serves.
• Comprised of
• Contractor Members: foundation drilling, anchored
earth retention and geo-support contractors
• Associate Members: manufacturers and suppliers
• Technical Affiliates: engineering and design
professionals
ADSC-IAFD
• Work of the ADSC includes
• Establishing standards & specifications for the
industries it serves
• Promoting ethical practice
• Conducting design, construction and inspection
seminars worldwide
• Developing technical materials
• Funding and conducting original research
• Providing a forum for the free flow of ideas
• Stimulating industry growth
• Interfacing with corresponding industries and
agencies (FHWA, OSHA, DOTs, etc.)
Information about Industry
• Get information from Industry Survey to
highlight the size of drilled shaft market,
the experience of members, safety,
training, the quantity performed, etc.
Available thru ADSC
• Experienced Member Contractors
• Training
• Regional seminars
• In-house seminars
• Video
• Design, Safety, Personnel
• Technical Library
• Technical Papers
• Design Manuals
• Foundation Drilling Magazine
• Specifications
Recap
• Who: All parties involved with the
project
• When: Get geo-contractor involved
as soon as possible during
planning/design
• What: Drilled Shafts applicable in
various loading, ground,
environmental conditions
• Where:
• How:
• Why:
Thank You for
Your Attention!!