Post on 11-Oct-2020
Advanced Interpretation of Instrumented Micropile Load Tests
International Workshop on Micropiles, TorontoSeptember 28, 2007
Terence P. Holman, Ph.D., P.E.Senior Engineer-Geotec, MORETRENCH
Thomas J. Tuozzolo, P.E.Vice President, MORETRENCH
Introduction• Two case histories of strain gauge
instrumented micropile load tests– Case History No. 1 – 167 Johnson Street– Case History No. 2 – Dublin Road Pump
Station (DRPS)– All piles Type B pressure grouted with
typically developed pressures of 345 kPa• Highlight aspects of pile mechanics
– Degradation of secant pile modulus– Nonuniform load distribution– Generation of micropile tip resistance and
shaft resistance
Case History No. 1-167 Johnson St
Dense to v. dense m/cSAND (NYCBC 7-65)
δv
• 40+ story residential high rise on mixed mat/spread footing foundations
• Dense to v. dense sand and sand/gravel deposits
• Excessive δv beneath heavily loaded elevator core
• Minimize δv incorporatemicropiles to create “piled raft” effect– Allow high δ and low F.S.
• 2 strain gauge instrumented load tests– 14 gauges per pile
Ground Conditions and Pile Design
SW Strain Gauge
0 m
6.1 m
9.1 m
12.2 m
13.7 m
15.2 m
17.8 m
384 mm Isolation Casing
EB Strain Gauge
SR-2SR-1
SG-1
SG-2
SG-3
SG-4
SG-5
SG-1
SG-2
SG-3
SG-4
SG-5
273 mm Casing
DL=1112 kN, TL=2224 kN
0 25 50 75 100 125 150Uncorrected SPT N-value (blows/0.3 m)
24
21
18
15
12
9
6
3
0
Dep
th b
elow
Gro
und
Surf
ace
(m)
LB-1LB-2LB-3LB-4LB-5LB-6LB-7LB-8LB-9
UR
BA
N F
ILL
SAN
D, S
M T
O T
R G
RA
VE
L (S
P)
Instrumentation• Spot-weldable gauges on bar (10 ea.)
– Accuracy=15 µε, Sensitivity=0.4 µε• Embedment gauges in grout (4 ea.)
– Accuracy=15 µε, Resolution=1.0 µε• Grout strength and unconfined modulus testing
– E=13.5 to 14.5 GPa (Unconfined secant at ε=0.11%)– f’c=44.8 MPa (cylinders) to 62.1 MPa (cubes)
Test Pile Installation
Left - Installation of 273 mm test element (pile)Top – Installation of 194 mm diameter reaction anchor, 1334 kN capacity
Test Pile Construction
Left - Installation of 273 mm test element (pile)Top – Buried old foundation wall and obstructions
Load Testing Data
0 1000 2000 3000Applied Pile Top Load (kN)
50
40
30
20
10
0
Pile
But
t Set
tlem
ent (
mm
)
SR-1 (6.1 m Bond)SR-2 (9.1 m Bond)
Net PermanentSettlement=29 mm
Net PermanentSettlement=8 mm
Plunging Failureat 2224 kN
Max. TestLoad= 2669 kN
0 200 400 600 800 1000Measured Strain (µε)
18
16
14
12
10
8
6
4
2
0
Dep
th b
elow
Gro
und
Surf
ace
(m)
P=556 kNP=1112 kNP=1669 kNP=2224 kNPost Failure (1771 kN)Residual
Tip Isolation CasingGeometry Change
Tip Pile CasingGeometry Change
0 400 800 1200 1600 2000Measured Strain (µε)
18
16
14
12
10
8
6
4
2
0
Dep
th b
elow
Gro
und
Surf
ace
(m)
P=556 kNP=1112 kNP=1669 kNP=2224 kNP=2669 kN
Tip Isolation CasingGeometry Change
Tip Pile CasingGeometry Change
SR-1
SR-2
Case History No. 2-DRPS
• Ground loss, heave, and settlement around 3 pump station structures following excavation and pile driving
• Complex ground conditions– Excess head/high
groundwater levels– Marine glauconitic
silty fine sand deposits
PUMP CHAMBER
WET WELL INLET CHAMBER
SILT, SAND,AND CLAY
FILL
GLAUCONITIC F/M SAND
SHEET PILES
(BUILT) (NOT BUILT) (NOT BUILT)
Ground Conditions and Pile Design
0 10 20 30 40 50 60Uncorrected SPT N-value (blows/0.3 m)
24
21
18
15
12
9
6
3
0
Dep
th b
elow
Gro
und
Surf
ace
(m)
B-1 (pre-failure)B-2 (pre-failure)B-3B-4B-5B-6
SIL
TY
SAN
D(S
M)
GL
AU
CO
NIT
IC F
/M S
AN
DSA
ND
, SIL
T, A
ND
CL
AY
(SM
, ML
, CL
)Zone of GroundLoss/Disturbance SW Strain
Gauges
0 m
13.6 m
16.8 m
20.3 m
DL=534 kN, TL=1067 kN
Isolation Casing
194mm OD Casing
1.5 m
Load Testing Data
0 200 400 600 800 1000Applied Pile Top Load (kN)
50
40
30
20
10
0
Pile
But
t Set
tlem
ent (
mm
)
Net Permanent Settlement=26 mm
Plunging Failureat 933 kN
0 200 400 600 800 1000Measured Strain (µε)
21
20
19
18
17
16
15
14
13
Dep
th b
elow
Gro
und
Surf
ace
(m)
P=267 kNP=534 kNP=801 kNP=934 kNPost FailureFull UnloadSG-3 Level
SG-2 Level
SG-1 Level
Analysis and Interpretation
• Nonlinear σ−ε behavior of composite pile section
• Calculated load distribution along bond length
• Deformation-based generation of micropile tip resistance and bond resistance
Composite Micropile Behavior
0 200 400 600 800 1000Measured Strain (µε)
20
22
24
26
28
Est
imat
ed S
ecan
t Mod
ulus
(GPa
)
Esec (MPa)= -0.0063ε + 27.46
0 200 400 600 800 1000 1200Measured Strain (µε)
10
20
30
40
50
60
70
Est
imat
ed S
ecan
t Mod
ulus
(GPa
)
Cased Zone Secant ModulusApprox. Secant Modulus (Field Data)Bond Zone Secant Modulus
Esec (GPa)= -0.0081ε + 42.89 (Cased)
Esec (GPa)= -0.0104ε + 26.32 (Bond)
SR-1
DRPS-Bond Zone• Interpretation of load distribution– P=εApEp
• Composite pile has complex σ−ε behavior
• Secant modulus of composite pile degrades with increasing strain– Linear degradation
model invoked• Calculate Esec as f(ε)
Load Distribution
0 200 400 600 800 1000Interpreted Load (kN)
21
20
19
18
17
16
15
14
13
Dep
th b
elow
Gro
und
Surf
ace
(m)
P=267 kNP=534 kNP=801 kNP=934 kNPost FailureFull Unload
DRPS
0 400 800 1200 1600 2000 2400Interpreted Load (kN)
18
16
14
12
10
8
6
4
2
0
Dep
th b
elow
Gro
und
Surf
ace
(m)
P=556 kNP=1112 kNP=1669 kNP=2224 kNPost Failure (1771 kN)Residual
SR-1
0 500 1000 1500 2000 2500 3000Interpreted Load (kN)
18
16
14
12
10
8
6
4
2
0
Dep
th b
elow
Gro
und
Surf
ace
(m)
P=556 kNP=1112 kNP=1669 kNP=2224 kNP=2669 kN
SR-2
• Non-constant mobilized bond stress for piles with short bond length (SR-1 and DRPS)– Approaches constant value near failure– 16-23 kN/m for SR-1, 25-28.5 kN/m for SR-2, 8.5-12.1 kN/m for
DRPS• Significant ultimate tip resistance for SR-1 and DRPS
– 19-25% of total ultimate capacity (300-700 kN)
Generation of Tip Resistance• Total pile deformation
• Tip resistance mobilizes nonlinearly for piles with short bond length– Initial yield at settlement ratio
of 0.01 to 0.02– Limiting values at settlement
ratio of 0.08 to 0.10• Small tip resistance
developed for SR-2– No failure condition– Denser soils at pile tip
• Trends similar to larger deep foundations
0 10 20 30 40Calc. Pile Tip Settlement (mm)
0
200
400
600
800
Mob
ilize
d T
ip L
oad
(kN
)
SR-1SR-2DRPS
0 0.02 0.04 0.06 0.08 0.1 0.12Normalized Tip Settlement δt/Db
0
0.2
0.4
0.6
0.8
1
1.2
Nor
mal
ized
Tip
Loa
d Q
t/Qtm
ax
SR-1SR-2DRPS
tbc δδδδ ++=
∫=+L
bc εdz)δ(δ
Generation of Bond Resistance
0 10 20 30 40Calc. Shaft Compression and Tip Settlement (mm)
0
100
200
300
Ave
rage
Bon
d Sh
ear
Stre
ss (k
Pa)
SR-1SR-2DRPS
• Develops with compression of bond zone and tip displacement (δb+δt)
• 6 to 8 mm of deformation required to initiate failure for short bond length piles (≈0.1% Lb)
• Ultimate τ reached between 10 and 20 mm (≈ 0.2% Lb)• No failure for SR-2 with long bond length
Summary and Conclusions
• Strain gauges can point out changes in pile geometry
• Composite, nonlinear nature of micropiles complicates stress-strain response
• Resistance distribution is nonuniformalong bond length
• Significant micropile tip resistance may be mobilized for shorter bond length piles
• Instrument for better understanding!!
Summary and Conclusions
• Implications for analysis and design – Structural assessment of micropile response
should account for real σ−ε behavior (i.e. nonlinear material behavior)
– For controllable design scenarios micropile tip resistance could be considered
– Short bond lengths for micropiles should be used cautiously due to the relatively small bond movement/compression required to reach ultimate capacity