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

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

B A C K G R O U N B A C K G R O U N D 2 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

BACKGROUND 1 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. 3 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

O B J C T I V E O B J C T I V E S S 4 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

OBJECTIVES 2 Case study, Type B micropile (Han and Ye, 2006) Model calibration Uncertainties PLAXIS 2D in soil testing and numerical Quantifying modeling adhesion properties Micropile enlarged portion length Effect of micropile diameter 5 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

CASE STUDY (HAN and Ye, 2006) 3 Overview Micropile: Type B, Diameter = 0.15m, Length = 8.0m, Grouting pressure = 0.2 – 0.5MPa. Drill and Drill and Grout Grout under injected water injected water pressure 6 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

CASE STUDY (HAN and Ye, 2006) 4 Overview (0.0) Micropile Site soils: Vane shear C u (kPa) Top soil 10 20 30 40 (-1.0) Lean clay crust 3 γ = 19.1 kN/m sat (-2.6) Soft lean clay 3 γ = 17.6 kN/m sat PL = 10.8 – 13% I L > 1.0 (-10.6) Soft fat clay PL = 21.5% γ = 17.1 kN/m I L > 1.0 sat 7 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

NUMERICAL MODELING 5 Geometrical Modeling 10m Axisymmetrical model. 15-noded triangular element. Randolph and Wroth (1978): - Horizontal boundary placed at 2.5 L. 20m - Vertical boundary placed at r = 2.5L(1- ν ) Quick maintained load test. 8 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

NUMERICAL MODELING 6 Material Modeling Lean clay Soft lean Soft lean Soft fat ` E crust clay (1) clay (2) clay Micropile Mohr- Mohr- Mohr- Mohr- Linear- Model Coulomb Coulomb Coulomb Coulomb Elastic Behaviour Undrained Undrained Undrained Undrained Non-porous C ( kPa) 41.3-29 29 35 23.5 -  φ o 0 0 0 0 - E` (MPa) 13.30 13.30 27 9.40 31400 ψ o 0 0 0 0 - ν ` 0.35 0.35 0.35 0.35 0.15 K v (m/sec) 2.55x10 -9 1.85x10 -9 1.85x10 -9 1.85x10 -9 - K h (m/sec) 6.48x10 -9 2.93x10 -9 2.93x10 -9 2.93x10 -9 - R int Variable Variable variable variable - Lateral earth pressure, K o =(1 – sin φ `) OCR sin φ ` C a = α . C u C a = R int . C u 9 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

INTERFACE PROPERTIES 7 Soil Parameters First Second Methodology Methodology For three enlarged Use L en from first portion lengths, L en , methodology (0.25, 0.5, and 1.0m) Vary soil Vary soil Vary the α value parameters to parameters to increase its decrease its Get α and L en that stiffness stiffness gives the best match between the Get lower bound Get upper simulated and the of α bound of α field curves Practical range of α 10 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

INTERFACE PROPERTIES 8 First Methodology Load (kN) Load (kN) 0 50 100 150 200 0 50 100 150 200 0 0 5 5 Displacement (mm) Displacement (mm) 10 10 15 15 α=0.8 α=0.8 20 α=0.9 20 α=0.9 α=1.0 α=1.0 Field Field 25 25 a) Case of enlarged portion length = b) Case of enlarged portion length 0.25m = 0.5m 11 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

INTERFACE PROPERTIES 9 First Methodology Load (kN) 0 50 100 150 200 0 5 Displacement (mm) 10 15 α=0.8 20 α=0.9 α=1.0 Field 25 c) Case of enlarged portion length = 1.0m 12 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

INTERFACE PROPERTIES 10 First Methodology Load (KN) Load (KN) 0 50 100 150 0 50 100 150 0 0 1 3 1 3 Q Q Q Q Q Q 1 1 ult ult ult ult ult ult 2 4 2 4 2 2 3 3 Depth (mm) Depth (mm) 4 4 5 5 6 6 Field Field 7 7 FEM FEM 8 8 a) Case of enlarged portion length = b) Case of enlarged portion length = 0.25m 0.5m 13 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

INTERFACE PROPERTIES 11 First Methodology Type Ult. Toe Shaft % toe % Load (KN) Load resist resist. resist. shaft 0 50 100 150 0 (kN) . (kN) (kN) resist. 1 3 Q Q Q 1 Numerical 135 11 124 8.0 92 ult ult ult 2 4 2 Field 135 11.7 123.2 8.7 91.3 3 Depth (mm) 4 Enlarged portion length (Len)= 1.0m 5 Neg. skin friction Adhesion coefficient α = 0.9 6 Field 7 Failure of surrounding soil (see related FEM slides) 8 d) Case of enlarged portion length = 1.0m 14 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

INTERFACE PROPERTIES 12 Second Methodology Load (kN) Load (kN) 0 50 100 150 200 0 50 100 150 200 0 0 5 5 Displacement (mm) Displacement (mm) 10 10 15 15 α=0.7 α=0.9 20 α=0.8 20 α=1.0 α=0.9 Field Field 25 25 a) Lower bound ( α ) b) Upper 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 15 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

EFFECT OF SHAFT DIAMETER 13 Diameter Ult. Load Toe Shaft % Unit (m) (kN) resist. resist. increase shaft (kN) (kN) in ult. resist. load (kPa) 0.15 135 11 124 - 30.8 0.17 150 7.3 142.7 11 32 0.19 163 8.8 154.5 21 31.5 0.228 181 12.6 168.4 34 29.5 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 16 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

CONCLUSIONS 14 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. 17 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

REFERENCES 14 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. 18 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

THANK YOU.. Mohamed Elkasabgy 19 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

CASE STUDY (HAN and Ye, 2006) Overview 0.15m L = 8.0m Enlarged L en base 0.228m 20 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007

CASE STUDY (HAN and Ye, 2006) Overview Undrained shear strength C u (kPa) 0 10 20 30 40 50 0 Top soil Lean clay crust 2 4 Soft lean clay (1) Depth (m) 6 8 10 Soft lean clay (2) Soft fat clay Soft fat clay 12 Measured C u =11.w c -1.13 Idealized 14 Gao (1994) 21 Department of Civil & Environmental Engineering, University of Western Ontario, Canada, 2007