CFA and DD Piles in Earthquake Country - DM Holloway - SEAOSC Seminar - Long Beach, CA - March10,...
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Transcript of CFA and DD Piles in Earthquake Country - DM Holloway - SEAOSC Seminar - Long Beach, CA - March10,...
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Cast-in-Place Piles in Earthquake Country
D Michael Holloway
Dan Brown and [email protected]
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
4
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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17
18
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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!