International Geologiical Congress - Oslo 2008

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GSS-01 General contributions to soil science

 

Study on strain-softening behavior of over-consolidated clay by Ring Shear Tests

 

Yong Hong, Geotechnical Engineering Institute, School of Civil & Hydraulic engineering, Dalian University of Technology (China)
 

 

The shear behavior of over-consolidated clay is essential to the slope stability analysis and the understanding of progressive failure mechanism of landslide risk. A lot of efforts have been made to study the shear resistance characteristics of over-consolidated clay under drained condition by triaxial tests and shear tests. The test results revealed that the shear stress paths of over-consolidated clays usually present feature of strain softening. That is, the shear strength of clay goes through an increase to peak value and falls to constant residual value with shear strain or displacement. The post-peak drop in drained strength of over-consolidated clay is considered as being due, firstly, to an increase in water content and, secondly, to reorientation of clay particles parallel to the direction of shearing (Skempton, 1985). And the strain softening of clay is accompanied by the development of structural discontinuities near slip surface which concern various factors. Although the nature of shear resistance of over-consolidated clay had been made relatively clear, the effect of factors like stress history, stress level et al., have not quantitatively analyzed and assessed due to the limitation of the testing ability of geotechnical apparatus.
In this study, by employing a sophisticated ring shear apparatus, which can do long shear displacement shear tests, a series of ring shear tests were conducted to study the shear behavior of a kind of silty clay under over-consolidated and drained condition. Based on the test results of samples with different preconsolidation pressure (PCP), shear speed (SS) and normal pressure (NP), the effects of stress history, shear speed and stress state on the shear behavior were examined and discussed. Moreover, the influence of cyclic loads on residual strength was also investigated.
In the all tests, strain softening was observed under different stress state. By investigating the shear resistance variation with shear displacement under several of PCP and SS, it was proved that the stress history and shear speed are two main factors that can influence the shear strength properties of the overconsolidated silty clay. It was also revealed that the overconsolidated silty clays with higher PCP have greater shear strength. The higher shear speed tests were usually accompanied with greater peak strength and higher post-peak drop in strength. And with shear speed becoming faster, shear displacement reaching to resident state increase. The effects of PCP and shear speed on shear behavior can be contributed to the damage of soil structure and water-soil coupling action during the strain softening process. After applying cyclic loads, residual strengths of most samples decreased, the largest decrease even up to 12%. But when shear surface (band) present irregular form after suffered cyclic loads, the residual strengths increased.

 

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