SEISMIC PERFORMANCE OF A SINGLE MICRO- PILE IN LAYERED SOIL Alper - - PowerPoint PPT Presentation

SEISMIC PERFORMANCE OF A SINGLE MICRO- PILE IN LAYERED SOIL

Alper Turan,

• M. Hesham El Naggar, Ph.D. P. Eng., Sean Hinchberger, Ph.D. P. Eng.

The University of Western Ontario

ISM Workshop, Toronto, September 27, 2007

1

INTRODUCTION Finite element analyses of seismic response of a single micro-pile performed considering soil nonlinearity. Factors Considered:

• soil plasticity
• Soil layering
• input motion intensity
• pile casing termination
• Boundary conditions

2

Modelling

• The soil and pile were modelled using 4-noded linear tetrahedron

elements with three degrees of freedom in each node.

• Transmitting boundaries (Wolf and Song, 1996) are used to allow for

wave propagation.

3

Time(sec)

0.0 0.5 1.0 1.5 2.0

Displacement (m)

• 0.007
• 0.006
• 0.005
• 0.004
• 0.003
• 0.002
• 0.001

0.000 Peak Displacement=3 m/s2 Peak Displacement=1 m/s2 Peak Displacement=5 m/s2

Predominant Period = 0.16 sec. Time(sec)

0.0 0.5 1.0 1.5 2.0

Acceleration (m/s2)

• 2

2 4 Peak Acceleration=3 m/s2 Peak Acceleration=1 m/s2 Peak Acceleration=5 m/s2

Predominant Period = 0.16 sec.

A Ricker wavelet acceleration with 3 m/s2 peak amplitude and 0.16 sec period is used.

4

Verification of Model

Pstatic=20 KN

Boundary in static

D= 8m H=12 m Ricker Wavelet (Base Acceleration) Static Dynamic Es=Varies Es=4.69 × 102 MN/m2 Ep= 2.3×10 MN/m Ep= 2.3×104 MN/m2 νs=0.48 νs=0.48 νp=0.30 νp=0.30 Pile diameter d=0.20 m L=50d νp=0.30

Boundary in dynamic validation validation

4 2

Static loading

Young M odulus (M N/m 2)

10 100 1000

Pile Head Deflection (mm)

0.0 0.5 1.0 1.5 2.0 2.5 Elastic solution by Davies and Budhu (1986) 3D Finite Elem ent Solution 2D Finite Elem ent Solution

5

Model Verification

Lateral Pile Head Deflection (m)

• 0.0 08
• 0.0 06
• 0.0 04
• 0.0 02

0.0 00 0.0 02 0.0 04 0.0 06 P resent S tudy

W o n g (20 04)

Dynamic loading

6

Results and Discussion

Effect of Boundary Conditions:

Transmitting boundaries by Wolf and Song (1996) performs better in absorbing energy in comparison with rigid, infinite and transmitting boundaries

Time(sec)

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Lateral Displacement of Pile Head (m)

• 0.008
• 0.006
• 0.004
• 0.002

0.000 0.002 0.004 0.006

Quite Boundary ( Lysmer and Kuhlemeyer, 1969)- Silent Boundary ( Wolf and Song, 1996)- ζ=2% Rigid Boundary ( at 240d distance)- ζ=2%

Linear Elastic Soil and Pile No hysteretic damping

7

Results and Discussion

Soil Plasticity: The pile head response decreased due to soil plasticity, particularly evident for strong input motion

Frequency (Hz)

0.1 1 10 100

Spectral Acceleration (m/s2)

2 4 6 8 10 12 14

Linear Elastic - Peak Acc=0.3g Mohr - Coulomb - Acc=0.3g Linear Elastic - Peak Acc=0.5g Mohr - Coulomb - Acc=0.5g

8

Results and Discussion

Soil layering: Three different soil profiles are considered

Bending Moment (KNm)

1 2 3 4 5 6

Normalized Pile Depth

0.0 0.2 0.4 0.6 0.8 1.0

Case 1 Case 2 Case 3

Peak Acceleration=0.5g

9

Results and Discussion

Casing Termination During Construction: Significantly higher bending moments can occur at the point where casing is terminated.

Bending Moment (KNm)

• 0.5

0.0 0.5 1.0 1.5 2.0 2.5

Normalized Pile Depth

0.0 0.2 0.4 0.6 0.8 1.0

No Casing Termination Casing Termination at 1/3 of the Pile

Peak Acceleration=0.5g

10

CONCLUSIONS

Transmitting boundaries should be used in modelling problems involving dynamic loading. However, consistent boundaries, which simulate the stiffness of the soil extending to the infinity, should be used for more realistic analyses. Considering soil nonlinearity leads to more realistic response, resulting in 40 % reduction of the spectral acceleration of the micro-pile head. This is due to the degradation of soil stiffness and increased material damping. The change in the frequency content, however, was insignificant. With the input motion intensity increasing from 0.3g to 0.5g, the peak spectral acceleration of the pile head increases 1.67 times for elastic soil and only 1.09 times for plastic soil. The effects of the soil plasticity are significant and they should be accounted for in the analyses.

11

CONCLUSIONS

The bending moments decreased with increasing depth. The variation of bending moments remained insignificant to a depth 0.4 pile length. Bending moment is larger for piles in stiffer soil stiffness. Thus, to

• btain a realistic response a realistic soil profile with increasing

stiffness with depth should be considered in the analyses. When the pile toe penetrates through two layers with significant contrast in stiffness, high bending moments develop at the interface of the two layers, and should be considered in design of micropiles. The casing termination resulted in significant increase in the bending moments at the point of termination. The bending moment at this point was 7.3 times higher than the no-casing termination case.

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