cfa and dd piles in earthquake country --...

P.O. Box 309 Jasper, TN 37347 (423)942-8681 www.danbrownandassociates.com

DANBROWN

ASSOCIATESAND

CFA and DD Piles in Earthquake Country D. Mike Holloway, Ph.D., P.E.

SEAOSC Seminar, Long Beach, CA March 10, 2012

Link to Presentation Handout: http://danbrownandassociates.com/wp-content/uploads/2012/03/CFA-and-DD-Piles-in-Earthquake-Country-DMHolloway_handout.pdf Link to FHWA GEC 8: Design and Construction of Continuous Flight Auger Piles: (includes sample performance specifications) http://danbrownandassociates.com/publications Other Presentations: http://danbrownandassociates.com/presentations

1

Cast-in-Place Piles in Earthquake Country

D Michael Holloway

Dan Brown and AssociatesMHolloway@DanBrownandAssociates.com

Outline

• Basic Concepts of CFA and DD Piles

• Factors Affecting Performance

• Applications:– Favorable Conditions

– Unfavorable Conditions

• Inspection Issues

• What about Seismic Loading?

• Summary & Conclusions

2

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

3

DrillingDrilling

Grouting/Concreting

4

Concreting/Grouting

Grout Concrete

Insert Rebar

5

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

6

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”!

7

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.

8

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

9

Photograph and figures courtesy of Pile Dynamics Inc

Automated Monitoring Systems

PIR System

10

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

11

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

12

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?

13

Auburn Test Program 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 drilled 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

14

Applications for CFA and DD Piles

• Foundations– Alternative to driven piles and bored piles

– Nearby vibration-sensitive structures

– Low headroom applications

– Sound walls

• Walls

• Pile supported embankments

15

Seismic Loading Issues

First St Viaduct

16

PCPS Concrete Pile

17

PCPS Concrete PileCasting Yard -- Bulkhead

18

PCPS Concrete Pile Stack

CFA PileConventional Reinforcement

19

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, et al.

DOWNLOAD LINKS

• Handouthttp://danbrownandassociates.com/wp-content/uploads/2012/03/CFA-and-DD-Piles-in-Earthquake-Country-DMHolloway_handout.pdf

• GEC 8 -- second publication on this page: http://danbrownandassociates.com/publications

20

Thank you for your attention!