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!!