THE AXIAL CAPACITY OF MICROPILES By Mohamed Elkasabgy Prof. M. H. - - PowerPoint PPT Presentation

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THE AXIAL CAPACITY OF MICROPILES By Mohamed Elkasabgy Prof. M. H. - - PowerPoint PPT Presentation

Oct 15, 2022 205 likes •474 views

FINITE ELEMENT ANALYSIS OF THE AXIAL CAPACITY OF MICROPILES By Mohamed Elkasabgy Prof. M. H. El Naggar Associate Dean Research Assistant Faculty of Engineering Geotechnical Research Centre University of Western Ontario University of

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FINITE ELEMENT ANALYSIS OF

THE AXIAL CAPACITY OF MICROPILES

By

Mohamed Elkasabgy

  • Prof. M. H. El Naggar

Associate Dean Research Assistant Faculty of Engineering Geotechnical Research Centre University of Western Ontario University of Western Ontario INTERNATIONAL WORKSHOP ON MICROPILES

Toronto, Canada, 2007 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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B A C K G R O U N B A C K G R O U N D

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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BACKGROUND

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Micropiles: small-diameter (typically less than 30 cm), drilled and grouted replacement piles that are typically reinforced . Types of Micropiles (FHWA Classification): a) Philosophy of behaviour Case 1: micropiles ar directly loaded. Case 2: Support and stabilization by interlocking. b) Method of grouting Type A: grouting under gravity head. Type B: grouting pressure between 0.3 and 1.0 MPa. Type C: grouting pressure 1.0 MPa. Type D: grouting pressure between 2.0 and 8.0 MPa.

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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O B J C T I V E O B J C T I V E S S

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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OBJECTIVES

Case study, Type B micropile (Han and Ye, 2006) Model calibration PLAXIS 2D Quantifying adhesion properties Uncertainties in soil testing and numerical modeling Effect of micropile diameter Micropile enlarged portion length

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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CASE STUDY (HAN and Ye, 2006)

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Overview Micropile: Type B, Diameter = 0.15m, Length = 8.0m, Grouting pressure = 0.2 – 0.5MPa.

Drill and injected water Drill and injected water Grout Grout under pressure

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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CASE STUDY (HAN and Ye, 2006) Overview Site soils:

sat

(0.0) (-1.0) (-2.6) (-10.6) Top soil Lean clay crust γ = 19.1 kN/m

sat

3 Soft lean clay γ = 17.6 kN/m

sat

3 Soft fat clay γ = 17.1 kN/m Vane shear Cu (kPa) 10 20 30 40 PL = 10.8 – 13% IL > 1.0 PL = 21.5% IL > 1.0 Micropile Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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NUMERICAL MODELING

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Geometrical Modeling

20m 10m

Axisymmetrical model. 15-noded triangular element. Randolph and Wroth (1978):

  • Horizontal boundary placed

at 2.5 L.

  • Vertical boundary placed at

r = 2.5L(1-ν) Quick maintained load test.

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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NUMERICAL MODELING Material Modeling

`

E

  • variable

variable Variable Variable Rint

  • 2.93x10-9

2.93x10-9 2.93x10-9 6.48x10-9 Kh (m/sec)

  • 1.85x10-9

1.85x10-9 1.85x10-9 2.55x10-9 Kv (m/sec) 0.15 0.35 0.35 0.35 0.35 ν`

  • ψ o

31400 9.40 27 13.30 13.30 E` (MPa)

  • φ o
  • 23.5

35 29 41.3-29 C (kPa) Non-porous Undrained Undrained Undrained Undrained Behaviour Linear- Elastic Mohr- Coulomb Mohr- Coulomb Mohr- Coulomb Mohr- Coulomb Model Micropile Soft fat clay Soft lean clay (2) Soft lean clay (1) Lean clay crust

Lateral earth pressure, Ko=(1 – sinφ`) OCRsinφ` Ca = Rint . Cu Ca = α . Cu

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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INTERFACE PROPERTIES

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Soil Parameters For three enlarged portion lengths, Len, (0.25, 0.5, and 1.0m) Vary soil parameters to increase its stiffness Vary soil parameters to decrease its stiffness Get α and Len that gives the best match between the simulated and the field curves Get lower bound

  • f α

Practical range of α Vary the α value First Methodology Use Len from first methodology Second Methodology Get upper bound of α

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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INTERFACE PROPERTIES

First Methodology

5 10 15 20 25 50 100 150 200 Load (kN) Displacement (mm) α=0.8 α=0.9 α=1.0 Field 5 10 15 20 25 50 100 150 200 Load (kN) Displacement (mm) α=0.8 α=0.9 α=1.0 Field

a) Case of enlarged portion length = 0.25m b) Case of enlarged portion length = 0.5m

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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INTERFACE PROPERTIES

First Methodology

5 10 15 20 25 50 100 150 200 Load (kN) Displacement (mm) α=0.8 α=0.9 α=1.0 Field

c) Case of enlarged portion length = 1.0m

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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INTERFACE PROPERTIES

First Methodology

1 2 3 4 5 6 7 8 50 100 150

Load (KN) Depth (mm)

Field FEM

ult

Q 4 3

ult

Q 2 1

ult

Q

1 2 3 4 5 6 7 8 50 100 150

Load (KN) Depth (mm)

Field FEM

ult

Q 4 3

ult

Q 2 1

ult

Q

a) Case of enlarged portion length = 0.25m b) Case of enlarged portion length = 0.5m

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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INTERFACE PROPERTIES

First Methodology

91.3 8.7 123.2 11.7 135 Field 92 8.0 124 11 135 Numerical % shaft resist. % toe resist. Shaft resist. (kN) Toe resist . (kN) Ult. Load (kN) Type

1 2 3 4 5 6 7 8 50 100 150

Load (KN) Depth (mm)

Field FEM

ult

Q 4 3

ult

Q 2 1

ult

Q

  • Neg. skin friction

Enlarged portion length (Len)= 1.0m Adhesion coefficient α = 0.9 Failure of surrounding soil (see related slides) d) Case of enlarged portion length = 1.0m

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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INTERFACE PROPERTIES

Second Methodology

5 10 15 20 25 50 100 150 200 Load (kN) Displacement (mm) α=0.7 α=0.8 α=0.9 Field 5 10 15 20 25 50 100 150 200 Load (kN) Displacement (mm) α=0.9 α=1.0 Field

b) Upper bound (α) a) Lower bound (α) α varies between 0.8 and 1.0 with best estimate of 0.9 Bruce (1994): a varies between 0.6 and 0.8

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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EFFECT OF SHAFT DIAMETER 29.5 34 168.4 12.6 181 0.228 31.5 21 154.5 8.8 163 0.19 32 11 142.7 7.3 150 0.17 30.8

  • 124

11 135 0.15 Unit shaft resist. (kPa) % increase in ult. load Shaft resist. (kN) Toe resist. (kN)

  • Ult. Load

(kN) Diameter (m) Ultimate load increases by a factor of 2 Unit shaft resistance is approximately constant (Frassetto, 2004) Abrupt increase in axial load in pile near toe diminishes as shaft diameter approaches enlarged portion diameter

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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CONCLUSIONS Adhesion factor α ranges between 0.8 and 1.0, with the best estimate of 0.9. Estimated α values are highly dependent on factors such as site soils, method of construction, etc. The enlarged base can mobilize some negative skin friction. The failure of surrounding soft clay initiated at the toe and expanded upward and laterally along the shaft. Ultimate capacity increased approximately linearly with the increase of shaft diameter. Unit shaft resistance remained approximately the same with the increase of shaft diameter.

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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REFERENCES Bruce, D.A., (1994). Small-diameter cast-in-place elements for load bearing and in situ earth reinforcement, Chapter 6 in Ground Control and Improvement by P.P. Xanthakos, L.W. Abramson, and D.A. Bruce, John Wiley and Sons. Frassetto, J.C., (2004). Performance of micropiles, M.Sc. Thesis, Concordia University, Canada. Gao, D.Z., (1994). Ultimate bearing capacity of soft soil, Proceedings of the 7th Chinese Soil Mechanics and Foundation Engineering Conference: 300-304 (in Chinese). Han, J., and Ye, S., (2006). A field study on the behavior of micropiles in clay under compression or tension, Canadian Geotechnical Journal, 43(1): 19-29. Randolph, M.F., and Wroth, C.P., (1978). Analysis of deformation of vertically loaded piles, Journal of Geotechnical Engineering, ASCE, 114(12): 1465-1488. Russo, G., (2004). Full-scale tests on instrumented micropiles, Geotechnical Engineering, ICE, 157(GE3): 127-135.

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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THANK YOU..

Mohamed Elkasabgy

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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CASE STUDY (HAN and Ye, 2006) Overview 0.15m Enlarged base 0.228m L = 8.0m Len

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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CASE STUDY (HAN and Ye, 2006) Overview

2 4 6 8 10 12 14 10 20 30 40 50 Undrained shear strength Cu (kPa) Depth (m) Measured Idealized

Top soil Soft lean clay (2) Soft fat clay Soft lean clay (1) Lean clay crust

Cu=11.wc-1.13 Soft fat clay Gao (1994)

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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IENTERFACE PROPERTIES Micropile Failure

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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IENTERFACE PROPERTIES Failure

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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IENTERFACE PROPERTIES Failure

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

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IENTERFACE PROPERTIES Failure

Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007