Influences of principal stress direction and intermediate principal stress on the stress–strain–strength behaviour of completely decomposed granite

The measurement and study of the stress–strain–strength behaviour of soils in general stress states involving the change of magnitudes and direction of the principal stresses are necessary and important. To investigate the strength behaviour under such conditions, consolidated undrained tests on remoulded hollow cylinder specimens of completely decomposed granite (CDG) were carried out using a hollow cylinder apparatus. Tests were conducted by maintaining a fixed principal stress direction with angle α from the vertical direction together with a fixed value of intermediate principal stress coefficient b. It is observed that the value of the friction angle decreases with an increase in α and the failure surface is anisotropic. There is an increase in friction angle with an increase in b value up to b = 0.25, and the friction angles are almost the same for b > 0.25. In addition, the behaviour of the soil in an undrained simple shear condition was examined. The simple shear condition is very near to the condition of α = 45° and b = 0.25. After having analyzed the test results of all hollow cylinder specimens, it was found that the strength anisotropy is very strong and is dependent on the principal stress direction and intermediate principal stress coefficient.