1. Dynamic Behaviour of Unsaturated CH soil under Cyclic Loading in Unconsolidated Undrained Conditions
5th Tongji-UBC Symposium on Earthquake Engineering, May 4th-8th, 2015 Tongji University Shanghai, China
Ms. Saloni Pandya Dr. Ajanta Sachan Department of Civil Engineering Indian Institute of Technology – Gandhinagar, India

2. Objective Introduction
Geotechnical properties of Ahmedabad (India) soil
Cyclic Triaxial Test
Results and Discussion
Conclusion
5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

3. Unsaturated Soil Mechanics
Introduction
Unsaturated Soil Mechanics
τff = c ̍+(σf -ua)f*tanφ ̍+ (ua-uw)f*tanφb
Source : Fredlund, D.G. and Rahardjo, H. (1993). Soil Mechanics For Unsaturated Soils. John Wiley & Sons.
Source :
Static axial stress induced by static axial strain application, represented embankment load on the soil and passing vehicles like cyclic loading on soil
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.
5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

4. Geotechnical Properties of Ahmedabad Soil
Basic Soil Properties
Specific Gravity (Gs)
2.7
Insitu Water Content
22 %
Insitu Bulk Density
2.0 gm/cc
Soil Classification
Grain size Distribution
Gravel = 1 %
Sand = 15%
Silt = 52%
Clay = 32%
Atterberg’s Limits
LL = 51%
PL = 20%
PI = 31%
SL = 14 %
Soil Classification = CH
Compressibility
Cc = 0.12
Cr = 0.02
Shear Strength
c = 20.3 kPa
 = 18.8
cu = 114 kPa
 = 0.35
5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

5. Cyclic Triaxial Test Sample Size : Diameter- 50mm and
Height -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.
5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

6. Cyclic Triaxial Setup, Geotechnical Laboratory , Indian Institute of Technology -Gandhinagar, India
5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

7. Results and Discussion
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.
5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

8. Contd.
Comparison of Stress strain curves of all tests at different S/C ratio
Variation of Shear Modulus with S/C ratio
5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

9. Contd.
Comparison of Stress strain curves of all tests at different S/C ratio
Observed typical Stress strain curve for S=0%
5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

10. Static Displacement (S)mm Cyclic Amplitude (C) mm
Contd.
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.
No of Cycles 1 2 3 4 5
Gavg (kPa)
Gavg (Mpa)
Davg
(%)
Static Displacement (S)mm
Cyclic Amplitude (C) mm
S/C
G1 (kPa) G2
(kPa) G3 G4 G5
0.2
8633
8126
8042
7864
7925
8118
8.12 16
0.5
2.5
9886
9224
8989
9008
8982
9218
9.22 17
11150
10152
9771
9825
9723
10124
10.12
1.5
7.5
12322
10553
9920
9602
9531
10386
10.39 10
16704
14832
14382
13979
13259
14631
14.63 13
5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

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

12. References
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
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
Fredlund, D.G. and Rahardjo, H. (1993). “Soil Mechanics For Unsaturated Soils” John Wiley & Sons.
5th Tongji-UBC Symposium on Earthquake Engineering, May 4-8, 2015 Tongji University Shanghai, China

13. Thank You