Advanced Interpretation of Instrumented Micropile Load Tests - - PowerPoint PPT Presentation

▶
advanced interpretation of instrumented micropile load
SMART_READER_LITE
LIVE PREVIEW

Advanced Interpretation of Instrumented Micropile Load Tests - - PowerPoint PPT Presentation

Feb 26, 2024 238 likes •440 views

Advanced Interpretation of Instrumented Micropile Load Tests International Workshop on Micropiles, Toronto September 28, 2007 Terence P. Holman, Ph.D., P.E. Senior Engineer-Geotec, MORETRENCH Thomas J. Tuozzolo, P.E. Vice President,

slide-1
SLIDE 1

Advanced Interpretation of Instrumented Micropile Load Tests

International Workshop on Micropiles, Toronto September 28, 2007 Terence P. Holman, Ph.D., P.E. Senior Engineer-Geotec, MORETRENCH Thomas J. Tuozzolo, P.E. Vice President, MORETRENCH

slide-2
SLIDE 2

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

slide-3
SLIDE 3

Case History No. 1-167 Johnson St

Dense to v. dense m/c SAND (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 δvincorporate

micropiles to create “piled raft” effect

– Allow high δ and low F.S.

  • 2 strain gauge

instrumented load tests

– 14 gauges per pile

slide-4
SLIDE 4

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-2 SR-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

25 50 75 100 125 150 Uncorrected SPT N-value (blows/0.3 m) 24 21 18 15 12 9 6 3 Depth below Ground Surface (m)

LB-1 LB-2 LB-3 LB-4 LB-5 LB-6 LB-7 LB-8 LB-9

URBAN FILL SAND, SM TO TR GRAVEL (SP)

slide-5
SLIDE 5

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)

slide-6
SLIDE 6

Test Pile Installation

Left - Installation of 273 mm test element (pile) Top – Installation of 194 mm diameter reaction anchor, 1334 kN capacity

slide-7
SLIDE 7

Test Pile Construction

Left - Installation of 273 mm test element (pile) Top – Buried old foundation wall and

  • bstructions
slide-8
SLIDE 8

Load Testing Data

1000 2000 3000 Applied Pile Top Load (kN) 50 40 30 20 10 Pile Butt Settlement (mm)

SR-1 (6.1 m Bond) SR-2 (9.1 m Bond) Net Permanent Settlement=29 mm Net Permanent Settlement=8 mm Plunging Failure at 2224 kN

  • Max. Test

Load= 2669 kN

200 400 600 800 1000 Measured Strain (µε) 18 16 14 12 10 8 6 4 2 Depth below Ground Surface (m)

P=556 kN P=1112 kN P=1669 kN P=2224 kN Post Failure (1771 kN) Residual Tip Isolation Casing Geometry Change Tip Pile Casing Geometry Change

400 800 1200 1600 2000 Measured Strain (µε) 18 16 14 12 10 8 6 4 2 Depth below Ground Surface (m)

P=556 kN P=1112 kN P=1669 kN P=2224 kN P=2669 kN Tip Isolation Casing Geometry Change Tip Pile Casing Geometry Change

SR-1 SR-2

slide-9
SLIDE 9

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)

slide-10
SLIDE 10

Ground Conditions and Pile Design

10 20 30 40 50 60 Uncorrected SPT N-value (blows/0.3 m) 24 21 18 15 12 9 6 3 Depth below Ground Surface (m)

B-1 (pre-failure) B-2 (pre-failure) B-3 B-4 B-5 B-6

SILTY SAND (SM)

GLAUCONITIC F/M SAND

SAND, SILT, AND CLAY (SM, ML, CL)

Zone of Ground Loss/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

slide-11
SLIDE 11

Load Testing Data

200 400 600 800 1000 Applied Pile Top Load (kN) 50 40 30 20 10 Pile Butt Settlement (mm)

Net Permanent Settlement=26 mm Plunging Failure at 933 kN

200 400 600 800 1000 Measured Strain (µε) 21 20 19 18 17 16 15 14 13 Depth below Ground Surface (m)

P=267 kN P=534 kN P=801 kN P=934 kN Post Failure Full Unload

SG-3 Level SG-2 Level SG-1 Level

slide-12
SLIDE 12

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

slide-13
SLIDE 13

Composite Micropile Behavior

200 400 600 800 1000 Measured Strain (µε) 20 22 24 26 28 Estimated Secant Modulus (GPa)

Esec (MPa)= -0.0063ε + 27.46

200 400 600 800 1000 1200 Measured Strain (µε) 10 20 30 40 50 60 70 Estimated Secant Modulus (GPa)

Cased Zone Secant Modulus

  • Approx. 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)
slide-14
SLIDE 14

Load Distribution

200 400 600 800 1000 Interpreted Load (kN) 21 20 19 18 17 16 15 14 13 Depth below Ground Surface (m)

P=267 kN P=534 kN P=801 kN P=934 kN Post Failure Full Unload

DRPS

400 800 1200 1600 2000 2400 Interpreted Load (kN) 18 16 14 12 10 8 6 4 2 Depth below Ground Surface (m)

P=556 kN P=1112 kN P=1669 kN P=2224 kN Post Failure (1771 kN) Residual

SR-1

500 1000 1500 2000 2500 3000 Interpreted Load (kN) 18 16 14 12 10 8 6 4 2 Depth below Ground Surface (m)

P=556 kN P=1112 kN P=1669 kN P=2224 kN P=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)

slide-15
SLIDE 15

Generation of Tip Resistance

  • Total pile deformation
  • Tip resistance mobilizes

nonlinearly for piles with short bond length

– Initial yield at settlement ratio

  • f 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

10 20 30 40

  • Calc. Pile Tip Settlement (mm)

200 400 600 800 Mobilized Tip Load (kN)

SR-1 SR-2 DRPS

0.02 0.04 0.06 0.08 0.1 0.12 Normalized Tip Settlement δt/Db 0.2 0.4 0.6 0.8 1 1.2 Normalized Tip Load Qt/Qtmax

SR-1 SR-2 DRPS

t b c

δ δ δ δ + + =

∫

= +

L b c

εdz ) δ (δ

slide-16
SLIDE 16

Generation of Bond Resistance

10 20 30 40

  • Calc. Shaft Compression

and Tip Settlement (mm) 100 200 300 Average Bond Shear Stress (kPa)

SR-1 SR-2 DRPS

  • 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
slide-17
SLIDE 17

Summary and Conclusions

  • Strain gauges can point out changes in

pile geometry

  • Composite, nonlinear nature of micropiles

complicates stress-strain response

  • Resistance distribution is nonuniform

along bond length

  • Significant micropile tip resistance may be

mobilized for shorter bond length piles

  • Instrument for better understanding!!
slide-18
SLIDE 18

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