Experimental Study On The Deformation And Microstructure Of Intact Soft Clay Under Principal Stress Rotation

Principal stress rotation is one of main characteristics of the stress paths of vehicular or wave load. The influence on soil foundation is one of the key issues in infrastructures, and is very important in economy and safety assessment. Soft clay is widely spread in Yangtze River Delta, the most prosperous area in China. So the study on the effect of vehicular or wave load on soft clay foundation has significant importance on rational design. However, few achievements have been reported. Based on the former research, a series of experimental studies on principal stress rotation on Hangzhou intact soft clay were carried out by Hollow Cylinder Apparatus, Scanning Electron Microscopy and other instruments. The study on soil deformation and microstructure were highly focused and the main achievements were as follows:1. A series of undrained tests had been conducted, in which the pricipal stress direction were fixed and rotated with different coefficient of intermediate principal stress. Experimental results revealed that (1) soil deformation were hardly affected by principal stress direction, however it greatly affected by the coefficient of intermediate principal stress. (2) Non-coaxiality, namely the direction of the incremental pricipal strain does not follow the same direction of the incremental pricipal stress, is very common in all the tests. This phenomenon would be gradually faded away in fixed pricipal stress direction tests when the applied stresses increased, while in principal stress rotation tests it always existed. (3)When the coefficient of intermediate principal stress equal to 0.5 and regardless the effect of the shear stress, the axial strain and torsional shear strain in pure principal stress rotation tests could be roughly expressed as 2 times of the sine function of the principal stress angle. And the circumferential strain was equal to the axial strain in magnitude, but opposite in the direction. The radial strain slightly changed and could be taken as zero.2. A semi-theoretical calculation of incremental strains was deduced by analyzing the test results, in which the magnitude and the direction of stress could be both considered. 3. A series of experimental studies, including fixed principal stress undrained tests with different coefficient of intermediate principal stress, and undrained principal stress rotation tests, on the microstructure of clay samples had been finished. It was found that (1) the intact clay particles had thin-sliced shape. Point-plane contact and plane-plane contact were two main contact patterns. The whole particle combinations had a honeycomb, or petal structure. The microstructure of samples was quite complicated, and along each direction the soil pores were poorly arranged. Due to deposition, the orderliness along the vertical direction was better than that along the horizontal.(2) Among the observed three planes, the microstructrue of depositional plane had the weakest effect on the deformation, while the microstructures of zθ-plane and zr -plane (two orthogonal planes) had stronger influence. In addition, the void ratio of zθ-plane had strong effect on the features of deformation.