Axial Load Testing for Pile Design

Ralph Rollins, performed geotechnical
investigations for over 5000 structures
I took Soil Mechanics class from my Father
Rachel Rollins is a Civil Engineering student
Rachel took Soil Mechanics class from her Father
Granddaughter, Ella, shows early
interest in soil behavior…
Axial Load Testing for Pile Design
Kyle Rollins
Brigham Young University
One good test is worth a
thousand expert opinions.
Werner Von Braun
Designer of Saturn V Moon Rocket
Space Shuttle Columbia Disaster
Analyses based on impact
of small ice particles imply
styrofoam impact won’t be
a problem.
Full-scale test shows a problem!
Healthy Skepticism for Tests
 A theory is something nobody believes,
except the person who proposed it.
 An experiment (test) is something
everybody believes, except the person who
performed it
--Albert Einstein
“The trouble with
quotes on the
internet is that it’s
difficult to discern
whether or not they
are genuine.”
--Abraham Lincoln
Pile Load Testing Desirable for:
 Projects with large number or piles, small
improvements yield large cost savings.
Pile Groups for Bridges on I-15
World’s Largest Solar Power Plant-Ivanpah, California
400 MegaWatt- 153,000 Support Piles for Solar Reflectors
Pile Load Testing Desirable for:
 Projects with large number or piles, small





improvements yield large cost savings.
Increased reliability for unfamiliar soils or new
pile types.
Critical structures with significant loss due to pile
failure.
Lower factor of safety (Higher Resistance Factor)
Research to improve design equations
Pile setup may be important
Pile Set-up Behavior on I-15
Pile Load Testing Desirable for:
 Projects with large number or piles, small





improvements yield large cost savings.
Increased reliability for unfamiliar soils or new
pile types.
Critical structures with significant loss due to pile
failure.
Lower factor of safety (Higher Resistance Factor)
Research to improve design equations
Pile setup may be important
Axial Static Load Tests
 Conventional Static Jacking Tests
 Statnamic Testing (Rapid Load Testing)
 Osterberg Cell Testing
Static Jacking
Tests
Courtesy Dan Brown
Conventional Static Tests
 Test Setup - Equipment and Measurements
 Instrumentation
 Loading & Interpretation
 Advantages & Limitations
Test Setup
 Reaction System
 Loading & Load
Measurement
 Reference System
& Displacement
Measurements
Static Vertical Load Test Setup
Stiffeners
Reaction beam Static Load Tests
Load cell
Hydraulic jack
LVDT
Stem reaction
plate
Mirror
Plate
Spherical bearing
Ram
Bourdon Gage
Dial Gage
Bracket attached
to pile
Wire
Scale
Grade
Test Pile
Kentledge (Dead Wt.) System
Reaction Pile System–Vancouver, BC
400 ton Frame for
Liquefaction Downdrag Test
Reaction Pile System
 Up to 600 tons pretty common
 Up to 1200 tons reasonable, 4000 tons possible.
 Care in alignment!
 Spacing (>6D ASTM, >3.5D Reese & O’Neill)
 Design by licensed civil engineer
BYU/UDOT Load Frame
 1200 Ton Capacity
 Used for I-15 project
to test to failure
AMEC Load Frame
 Mobile Arrangement
 1000 Ton Capacity
Caltrans’ $1 Million Load Frame
64 ft long, 9.3 ft high, 6 ft wide
 World’s largest reaction frame (4000 tons)
 Expected to save $6 to $10 million per year
on construction costs
Load Test on
Santa Clara
River Bridge
on I-5 in N.
Los Angeles
$14 million
cost saving
Loading
 Calibrated hydraulic jack
 Calibrated load cell w/ bearings
 Avoid misalignment!
Loading Procedures
 Quick load test method (most common in US)
 Load applied incrementally (10%) and held for constant
2.5 to 15 minute period.
 Slow Maintained test method
 Load applied in 25% increments to 200% of design
load
 Each load held 2 hrs. min. and until δ/t < 0.002 in/min
 Constant Rate of Penetration
 0.01 to 0.05 in/min for clay; 0.03 to 0.10 in/min for
sand
Reference System
 Reference Beam Stability &





Supports
Redundancy!
LVDT or Linear Pots w/ data
acquisition system
Dial Gauges
Optical measurements (scale
& mirror w/ piano wire)
Surveying instruments
Rapid
Load Testing
Statnamic Testing
load
displacement
strain
acceleration
load
displacement
14
Load Test Type Definitions
 Static Test – Duration, T > 1000 L/c
 Dynamic Test – Duration, T < 10 L/c
 Rapid Test - 10 L/c < T < 1000 L/c
 Typical Axial Statnamic Duration 100 to 120
msec
Schematic of Vertical Statnamic Test
Exhaust
Vent/Silencer
Reaction
Weights
Load Piston
Combustion
Chamber
Load Cell and
Laser Window
5000 Ton Gravel Backfill Device
2000 Ton Hydraulic Catch Device
Statnamic Test Firings
30 MN (6750 kip) Device within
Gravel Filled Cylinder
Courtesy of Applied Foundation
Testing, Green Coral Spring, Florida
14 MN (3100 kip) Device with
Hydraulic Catch Mechanism
1000 Ton Statnamic in Utah
Instrumentation
RAPID LOADING APPARATUS
LOAD CELL
STATIONARY REFERENCE
DISPLACEMENT
TRANSDUCER
ACCELEROMETER
PILE FOUNDATION
SIGNAL CONDITIONING
STORAGE AND
DISPLAY
Load Cell
Calibrated
Full Scale
Laser
Displacement
Sensor
Laser Projector
Capacitive Accelerometers
Force & Settlement Time Histories
-0.40
800
-0.30
600
Displacement
-0.20
400
-0.10
200
0.00
0
0.10
-200
0.20
-400
0.30
0.0000
0.40
0.1000
0.2000
0.3000
0.4000
Time (sec)
0.5000
0.6000
-600
0.7000
-800
Force (kips)
Settlement (in)
Force
Unloading Point Method Model-Axial Loads
Stanamic Force, (Fstn)
Pile Mass, M
(Fa)
Sping, K
(Fu)
Dashpot, C
(Fv)
Fstn = Fu + Fv + Fa
or
Fu = Fstn - Fv - Fa
Fu = Fstn - Cv - Ma
Unloading Point Analysis Procedure
3000
1. Pick Fstn where v=0.
Maximum Fstn
2500
2. (Fu)max = Fstn - Ma
Force, kN
2000
1500
1000
Fstn @ v=0
(Fstn)max-Ma-(Fu)max
3. C =
v
500
4. Fu = Fstn - Cv - Ma
0
0
5
10
15
Deflection, mm
20
25
30
Statnamic Results
0
Load (lbs)
100,000 200,000 300,000
400,000
Deflection (in)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Statnamic
Interpreted Static
Segmental Unloading Point Method
 Method for long shafts, utilizing strain
gauge or embedded accelerometer data
F1meas.
Gage
Level 1
Cv skin.
m2
kx skin.
Gage
Level 2
F2meas.
FStatnamic
Nw = 5
Strain
Gage
Levels
Static versus…
Statnamic…
1900 Ton Statnamic Test of 54” Cylinder Pile
(self supported by pile 32 feet above water).
Courtesy Dan Brown
Load Rate Tests in Salt Lake City, Utah
SML
16 hrs.
QML
50 min.
2 min.
20 sec.
Statnamic
Effect of Loading Rate on Pile Capacity
Failure Load/(54 Minute Failure Load)
2.5
2
1.5
1
0.5
0
0.0001
0.0010
0.0100
0.1000
1.0000
Velocity, v (mm/sec)
10.0000
100.0000
Allowance for Rate of Loading Effects
Strain Rate Reduction factors for Axial Load
(Mullins et al, 2002)
 0.95 for Rock
 0.91 for Sand
 0.69 for Silt
 0.65 for Clay
Factor of 0.55 for SLC tests in Clay
Rate Effect Corrections in Clay
Weaver and Rollins, ASCE JGGE Oct. 2009
Avg. Rate Reduction Factor = 0.47
Advantages of Statnamic Testing
 Large load capacity, applied at top of pile
 Reaction system not needed
 Economies of scale for multiple tests
 Useful for proof testing on production piles
 Can mobilize large toe displacements
 Long wave length
Limitations
 Capacity high, but still limited
 Rapid loading method: rate effects are
significant in clay soils
 Mobilization costs for reaction weights
Fundex PLT Capability
 Mobile testing unit
capable of 6 to 10
compression tests/day
 Up to 800 kips load
capability
 Self-sufficient unit
Fundex PLT Operation
 25,000 kg weight dropped from progressively





increasing heights
Heavy coiled springs diffuse impact and spread
energy over a 200 ms period
Weight is hydraulically caught on upward stroke to
allow only a single blow
Deflection is recorded by optical receiver from
LED transmitter fastened to test pile.
Load measured by load cell above pile.
Family of individual drops is compiled to create
classical Load Deflection Curve.
Hydraulic Clamp
25,000 kg Weight
Damping Springs
Receiver
Load Cell
LEDs
Test Pile
DATA AQUISISION SYSTEM
Displacement Measurement
System
LED Transmitter
Load Cell
Fundex Pile Load Tester
Courtesy of American Pile
Driving, Inc. Antioch, California
Fisherman’s Wharf, San Francisco
 22-Inch Diameter
Tubex Pile
 45 Feet Deep
 End-Bearing Into
Dense Sand
FILL
Dense SM
BAY MUD
CH
V. Dense
SM
Static vs. PLT, Fisherman’s Wharf
Bay Street, Emeryville
16”
13
3
Pre-stressed piles
PLT’s in 2 shifts
static tension tests
Estimated
saving to project: $500,000
Osterberg Cell Testing
 Calibrated, embedded,
sacrificial jack within
the test pile
Qs
 Concept: Load base of
the pile against the side
shear, & eliminate
reaction system
O-cell
Qp
Osterberg Load Testing
Typical O-CellLoad-Movement Curves
4
Extrapolated
Curve
Movement (inches)
3
Measured Side
Shear Curve
2
Maximum
Load from
O-Cell Test
1
0
Pile
wt.
-1
Measured EndBearing Curve
-2
-3
-4
0
100
200
300
400
Load (kips)
500
600
700
800
MRT-NEL C701, Singapore
Courtesy Jack Hayes
Multi-cell assembly -- attaching O-cells to bottom plate
COMPARISON TEST, MRT-NEL C701, Singapore, March 1998.
Comparison test Curves
Kentledge Test versus O-cell equivalent top load-settlement curve
0
-10
-20
Settlement (mm)
-30
O-cellTest
Test
O-cell
Kentledge
Test
Kentledge Test
-40
-50
-60
-70
-80
-90
-100
-110
-120
0
2
4
6
8
10
12
14
16
18
Top Load (MN)
20
22
24
26
28
30
O-Cell Test Advantages
 Elimination of Load Frame
 Separation of End-Bearing and Side Shear
 High Load Capacity
(World Record 36,000 Tons! – St. Louis)
 Improved Safety
O-Cell Disadvantages
 Must arrange for simultaneous end-bearing
and side shear failure or extrapolate.
 Upward shear may be somewhat lower.
 Expensive for lower capacity piles.
Interpretation of Load-Settlement
 Variation in Capacity Interpretations
 Davisson Failure Criteria Most Common
Load, Q
D/120 + 0.15 in
Settlement
QL/AE
L/AE
Interpretation of Load-Settlement
 Double Tangent Failure Criteria
Settlement
Qu
Load, Q
Interpretation of Load-Settlement
 Problems with Double Tangent
Load, Q
Settlement
Qu B Qu A
B
A
Pile Instrumentation
Strain Gauges
Instrumentation
 Sister-bar (embedment) or weldable strain gauges
 Resistance type gauges
 Vibrating wire type gauges
 Tell-tale rods
 Extensometers
Interpretation of Instrumentation
 Load = ε(AE)
 What is area?
 What is concrete modulus?
 What is precision of strain measurement?
 Residual Stresses?
Load vs Depth
Depth
Load, Q
L
Q3 Q2
Load Transfer (t-z Curves)
 Load Transfer, levels 2-3, = Q2 – Q3
 Unit Load Transfer, t = (Q2 – Q3)/(πDL)
 Displacement, z = (Displ at top) – Σ(ε*ΔL)
t
z
Residual Stress in Driven Piles
Q
Residual
Load
Q
Calculated
From Instruments
Q
True
Distribution
Load vs Depth
Depth
Load, Q
Q4a
Q4b
Load Transfer (q-z Curves)
 End Bearing Pressure,
q = Q4 /(πr2 )
 Toe Displacement,
z = (Disp. at top) – Σ(ε*ΔL)
q
z
3.5
t-z Curves
21 ft
Unit Side Friction (ksf)
3
46 ft
61 ft
2.5
2
1.5
1
0.5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
M id-Point Displacement (in)
End-Bearing Pressure, Q (ksf)
350
q-z Curves
Pile 5
300
Pile 3
250
200
150
100
50
0
0
0.05
0.1
0.15
0.2
0.25
Pile Tip Displacement, Z (in)
0.3
0.35
0.4
Load-Settlement from q-z & t-z curves
Layer 2
t2
q
Qp = Apq
Depth
t1
End Bearing, q
Layer 1
Side shear, t
Side shear, t
z1=
Load, Q, in Pile
Layer
1
z2+(Qp+Q2)L1
Q1=
2 AE
ΔQ s1= As1t1
t1
Q2+ ΔQs1
zmid-layer=
z2+(Qp+Q1)L1
Mid-layer disp., z
Q2=
z2= 4 AE
Iterate for compatible load-displacement
zp+(Qp+Q2)L2
Qp+ ΔQs2
Layer 2
L2
2 AE
ΔQ s2= As2t2
t2
zmid-layer=
Qp
zp+(Qp+Q2)L2
Mid-layer disp., z
4 AE
Iterate for compatible load-displacement
Base
q
Toe Disp., z
Assume initial toe displacement
Load-Settlement Curve
Pile Head Load, Q1
That process gives one point
on the curve
Pile Head Settlement, z1
Assume larger tip
displacement and repeat
process for additional points
Additional Uses for q-z and t-z curves
 Downdrag conditions
where the soil around
the pile may move
down relative to the
pile
 Liquefaction
 Fill settlement
 Lateral pile groups to
account for rocking
Liquefaction Induced Downdrag