Ms. Saloni Pandya Dr. Ajanta Sachan

Department of Civil EngineeringIndian Institute of Technology – Gandhinagar, India

Dynamic Behaviour of Unsaturated CH soil under Cyclic Loading in

Unconsolidated Undrained Conditions

5th Tongji-UBC Symposium on Earthquake Engineering, May 4 th-8th, 2015 Tongji University Shanghai, China

Introduction

Geotechnical properties of Ahmedabad (India) soil

Cyclic Triaxial Test

Results and Discussion

Conclusion

Objective

5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China 2

Introduction

Source : www.gov.pe.ca

Unsaturated Soil Mechanics

τff = c �+(σf -ua)f*tanφ �+ (ua-uw)f*tanφb

In the present study, effect of initial static axial strain application followed by strain controlled cyclic loading on dynamic parameters of unsaturated CH soil were evaluated under unconsolidated undrained (UU) conditions.

Static axial stress induced by static axial strain application, represented embankment load on the soil and passing vehicles like cyclic loading on soil

Source : Fredlund, D.G. and Rahardjo, H. (1993). Soil Mechanics For Unsaturated Soils. John Wiley & Sons.

35th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

Specific Gravity

(Gs)2.7 Insitu Water

Content 22 % Insitu Bulk Density 2.0 gm/cc

Grain size Distribution Gravel = 1 % Sand = 15% Silt = 52% Clay = 32%

Atterberg’s Limits LL = 51% PL = 20% PI = 31% SL = 14 %

Cc = 0.12 Cr = 0.02

c = 20.3 kPa = 18.8 cu = 114 kPa = 0.35Shear Strength

Compressibility

Soil Classification

Basic Soil Properties

Soil Classification = CH

Geotechnical Properties of Ahmedabad Soil

45th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

Sample Size : Diameter- 50mm andHeight -100mm using Moist tamping method

A series of cyclic triaxial tests under UU conditions were performed on these soil specimens by applying different initial axial static strain (S) followed by strain controlled cyclic loading (C).

Test was carried out at 100kPa confining pressure, 0.5Hz frequency and 0.2 % axial strain amplitude (Strain Controlled cyclic Loading ) 50 Cycles

Initial axial Static Strain was applied in following S/C ratios : 1, 2.5, 5, 7.5 and 10

Shear Modulus and damping ratio were determined for first 5 cycles and then averaged.

Cyclic Triaxial Test

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Cyclic Triaxial Setup, Geotechnical Laboratory , Indian Institute of Technology -Gandhinagar, India

65th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

G of the CH soil increased with increase in S/C Ratio.

Comparison of hysteresis loop showed an upward shifting with increase in static axial strain (S/C ratio).

Stiffness degradation in extension phase (unloading phase) was negligible as compared to compression phase for lower S/C ratios.

The hysteresis loops in compression phase (loading phase) could reach the constant stress value after certain number of cycles.

The loops in extension phase (unloading phase) showed almost same stress values in all the cycles.

Results and Discussion

75th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

Contd.

0 2 4 6 8 10 126

7

8

9

10

11

12

13

14

15

S/C

G (M

Pa)

-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25

-45

-25

-5

15

35

55

75

95

115

135

S/C=1 S/C=2.5 S/C=5 S/C =7.5 S/C =10

εa (%)

σd (k

Pa)Variation of Shear Modulus with S/C ratio

Comparison of Stress strain curves of all tests at different S/C ratio

85th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

Contd.

-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25

-45

-25

-5

15

35

55

75

95

115

135

S/C=1 S/C=2.5 S/C=5 S/C =7.5 S/C =10

εa (%)

σd (k

Pa)

95th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

-1.5 -1 -0.5 0 0.5 1 1.5

-50

0

50

100

150

200

250

Strain (%)

Stre

ss (k

Pa)

Observed typical Stress strain curve for S=0%

Comparison of Stress strain curves of all tests at different S/C ratio

No of Cycles 1 2 3 4 5Gavg

(kPa)Gavg

(Mpa)Davg

(%)Static

Displacement (S)mm

Cyclic Amplitude

(C) mmS/C G1

(kPa)G2

(kPa)G3

(kPa)G4

(kPa)G5

(kPa)

0.2 0.2 1 8633 8126 8042 7864 7925 8118 8.12 16

0.5 0.2 2.5 9886 9224 8989 9008 8982 9218 9.22 17

1 0.2 5 11150 10152 9771 9825 9723 10124 10.12 17

1.5 0.2 7.5 12322 10553 9920 9602 9531 10386 10.39 17

2 0.2 10 16704 14832 14382 13979 13259 14631 14.63 13

Contd.

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The hysteresis loop for higher S/C ratio (S/C =10) depicted similar trend in compression (loading ) and extension (unloading) phases

Effect of S/C ratio and loading cycles on damping ratio was observed to be negligible.

Degradation of shear modulus with number of loading cycles was evident in the present study.

11

Evaluation of soil behaviour in unsaturated state is extremely necessary for the its strength/stability analysis

Application of 2 % – initial static axial strain is observed to be required for the soil under consideration, in order to ensure an improved and standardized behaviour of strength degradation with number of cycles, in extension (unloading ) phase of cyclic loading under UU (Unsaturated) condition.

Effect of degree of saturation, matric suction and dry density on the cyclic strength of soil can also be considered as a part of future study.

Conclusion

5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

Thammathiwat, A., and Chim-oye, W. (2004). “Behavior of Strength and Pore Pressure of Soft Bangkok Clay under Cyclic Loading”, Thammasat Int. J. Sc. Tech, 9 (4), pp. 21-28.

Boulanger, R .W, Arunathan, R., Harder, L. F.Jr., Torres, R. A., and Driller, M. W. (1998). “Dynamic Properties of Sherman Island Peat”, Journal of Geotechnical and Geoenviromental Engineering, ASCE, 124 (1), pp. 12-20.

Fredlund, D.G. and Rahardjo, H. (1993). “Soil Mechanics For Unsaturated Soils” John Wiley & Sons.

References

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Thank You