Anisotropy of Shear Strength in Unsaturated Cohesive Soil Induced by Stress and Suction History

TOYOTA Hirofumi

Simple mechanical element tests such as the unconfined compression test are usually used for the conventional slope stability analysis in undisturbed samples obtained from the real ground. However, precise shear strength cannot be inferred from the simple tests because the stresses in the ground have acted from the various directions after the structures were constructed on the real ground. Bjerrum et al. proposed the experimental methods to estimate the real ground strength, and they observed the anisotropy of saturated undrained shear strength in real ground by combination of simple mechanical element tests. However, the tests for anisotropy of unsaturated soils have been rarely conducted in spite of existing of unsaturated soils in real ground.

The drained shear strength anisotropy of unsaturated cohesive soil was confirmed by the hollow cylinder torsional shear test in previous study. In this study, the change of the void ratio and the degree of saturation was examined to clarify mechanism of the anisotropy. The anisotropy was expressed using the difference of major principal stress direction (the difference between direction of major principal stress during K-constant consolidation and during shearing). Unsaturation of the specimens were carried out by the pressure plate method in the conditions of suction 200 kPa and 400 kPa.

The main results are summarized as follows:

1. There is the anisotropy of the elastic shear modulus in both saturated and unsaturated cohesive soils. The elastic shear modulus increases with the increase of suction (the decrease of the degree of saturation), and the elastic shear modulus decreases linearly with the increase of the difference of major principal stress direction. The rate of the decrease is approximately constant with independence of the degree of saturation.

2. There is the anisotropy of the drained shear strength of unsaturated cohesive soil. The relationship between the strength and the difference of major principal stress direction are expressed by the downwards convex curve, which takes the maximum strength in the difference of major principal stress direction of 0 and 90 degrees, and the minimum strength in the difference of major principal stress direction of 45 degrees. Although the difference of the strength induced by the anisotropy is only 8% of the maximum strength, it is confirmed the anisotropy of strength is significant even in consideration of the potential effect of measuring error

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