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Cast-in-Place Piles in Earthquake Country

D Michael Holloway

Dan Brown and AssociatesMHolloway@DanBrownandAssociates.com

DANBROWNASSOCIATESAND

Outline

• Basic Concepts of CFA and DD Piles

• Factors Affecting Performance

• Applications:– Favorable Conditions

– Unfavorable Conditions

• Inspection Issues

• What about Seismic Loading?

• Summary & Conclusions

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Basic Concepts• Drill using continuous flight

auger, most often 14” to 24” and <75’ deep, can go to 36” with big equipment and depths to 100’

• Soil fills auger flights and maintains hole stability

• Pump grout/concrete through hollow center, while pulling augers

• Place rebar cage into fluid grout/concrete

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DrillingDrilling

Grouting/Concreting

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Concreting/Grouting

Grout Concrete

Insert Rebar

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Drilled Displacement Piles

Omega

DeWaal

Fundex

Berkel

Berkel IDD

Factors Affecting CFA Pile Performance

• Sidewall roughness

• Effect of installation on in-situ stresses

• Mobilization of tip resistance

• Construction control

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Factors Affecting CFA Pile Performance

• Sidewall roughness– Bonding of grout/concrete can be terrific in

cemented material, e.g. limerock (Miami), cemented sands

– Probably less remolding in the near field in cohesive materials compared to driven piles or driven casing

– Probably less contamination from bentonitedrilling fluid compared to drilled shafts

Factors Affecting CFA Pile Performance

• Effect of installation on in-situ stresses– Lateral stress is typically reduced by

installation

– Rate of auger penetration is critical in uncemented materials; this needs to be controlled and rig must have adequate torque capacity to penetrate without “flighting”!

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Effect of Over Excavation in

Cohesionless Soila) auger w/ rapid penetration

b) auger w/ slow penetration, insufficient base feed to keep flights full, auger feeds from side

Fleming, W.G.K. 1995. “The understanding of continuous flight auger piling, its monitoring and control” Proc. Instn Civ. EngrsGeotech. Engng, 113, July,157-165

CPT from nearby CFA pile in sandy soil

Van Weel, A.F. 1988. “Cast-in-situ piles – Installation methods, soil disturbance and resulting pile behaviour” Proc., 1st Int’l GeotechSeminar on Deep Foundations on Bored and Auger Piles, (Ghent, Belgium) Van Impe, ed, Balkema, Rotterdam, 219-226.

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Suggested Guidelines for Penetration Rates

• Clay: 2 to 3 Rev’s per auger pitch

• Sand: 1.5 to 2 Rev’s per auger pitch

Construction Control

• Monitoring and control is essential during installation & withdrawal of augers:

– Rate of penetration & torque

– Location of auger tip

– Grouting pressures

– Grouting volume

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Photograph and figures courtesy of Pile Dynamics Inc

Automated Monitoring Systems

PIR System

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Mobilization of Tip Resistance

• Tip resistance may require significant displacement to mobilize; very dependent upon construction technique

• Concern about loosening at base for cohesionless materials

CFA PilesAdvantages• Good side shear bond

in cemented materials• Low cost• Low noise/vibration• Easy construction in

cohesive soils• Fast• Possible low headroom

work

Limitations/Problems• Often inadequate QC/QA• Potential for lost

ground/subsidence in cohesionless soils

• No direct field control of axial capacity

• Concerns for structural integrity, rebar placement

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Favorable Conditions

• Lightly cemented soils

• Cohesive soils

• Deeper groundwater

• Large numbers of piles supporting axial loads

Unfavorable Conditions• Very loose granular soils, esp. with:

– Shallow groundwater

– Interbedded clay strata

– Underlying hard stratum

• Very soft organic soils

• Large lateral load requirements

a) sand overlying hard clay b) sand bearing stratum underlying stiff clay

loose sand

sand hard clay

stiff clay

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Drilled Displacement Piles

Base

Drilled Displacement Piles

• CFA Pile• Displaces soil rather

than extracts soil• Possible to add

material (sand or stone)

• Incr lateral stress?• Densification?

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Unit Side Shear

0

20

40

60

80

100

120

Control + Sand +Stone Group Group +Sand

Un

it S

ide

Sh

ear,

kP

a

Auburn Test Program Conclusions

• Group effects can be substantial and beneficial with displacement CFA piles

• Amelioration beneficial, but cost-effectiveness may be questionable

• Study limited to Piedmont silty soils, but similar effects would be expected in granular soils

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Applications for CFA and DD Piles

• Foundations– Alternative to driven piles

– Nearby existing structures

– Low headroom applications

– Sound walls

• Walls

• Pile supported embankments

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Seismic Loading Issues

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Summary & Conclusions• CFA and DD Piles can be a very efficient

and cost effective foundation alternative if conditions are favorable

• Performance is very sensitive to construction techniques

• Need to properly utilize instrumentation and monitoring for control of drilling, concrete/grout placement

• Seismic loading needs special attention to reinforcement details, etc.

Thank you for your attention!

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Potential Applications- Foundations

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Building Foundations & Pile Rafts

Primary Piles (drilled first)

Secondary Piles

Potential Applications – WallsSecant or Tangent Pile Walls

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Potential Applications:Pile Supported Embankments

Potential Applications:Pile Supported Embankments

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Tests on CFA Piles in Clay (BRE)

• T33: 120mm/rev advance (tight)

• 6 min. to install

• Ground moved 0 to 5 mm away from pile

• Measured capacity 700 kN

• T34: 50mm/rev advance (loose)

• 9 min. to install

• Ground moved 0 to 5 mm toward pile

• Measured capacity 625 kN

Skinner et al, “Results from a piling trial on bored, CFA, and rotary displacement piles in stiff clay” BGA 2004

Comparative Tests, Texas• Stiff Clay overlying sand

= 0.87, ACIP Pile; = 0.53, Prestressed Concrete

Driven Pile;

• End bearing:qb = 40 ksf, Prestressed Concrete

Driven Pile (terminated in stiff clay)qb = 0, ACIP Pile (terminated in

very dense sand)

Ref: Kim, O’Neill, Roberts, Yin (2003) Comparative behavior of ACIP and driven piles in Gulf coastal clays, Deep Foundations on Bored and Augered Piles IV

Stiff Clay

Dense Sand

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Auburn NGES

0

2

4

6

8

10

12

14

16

180 10 20

Dep

th (

met

ers)

N-value

B-1B-3B-4

0

2

4

6

8

10

12

14

160 2500 5000

Dep

th (

m)

qc (kPa)

Avg. Tip Resistance

0

2

4

6

8

10

12

14

160 2 4 6 8 10

Dep

th (

m)

Rf (%)

Avg. Friction Ratio

Test ConditionsTable 1. Pile Configurations

ID D L Description of Installation

(m) (m)

TP1 0.46 8.2 Group @ 3D / Ameliorated w/Sand

TP2 0.46 11.6 Group @ 3D

TP3 0.46 11.0 Isolated

TP4 0.46 7.6 Isolated / Ameliorated with #89Crushed Stone

TP5 0.46 8.2 Isolated / Ameliorated with Sand

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Load Test Results

-70

-60

-50

-40

-30

-20

-10

00 500 1000 1500 2000 2500

Def

lect

ion

(m

m)

Load (kN)

TP2TP1

TP3

TP4

TP5

Control

Group +Sand Group

+ Stone

+ Sand

Thanks for Listening!