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