Experimental Study On The Anisotropy And Non-coaxiality Of Soft Clay Considering The Change Of Principal Stress Direction

In practical engineering situation such as slope engineering, traffic engineering and ocean engineering etc., not only does the magnitude of principal stress vary, but also the principal stress axes rotates in the process of consolidation and loading. The change of principal stress direction is related to the mechanical behaviors of soil in different directions. Another significant property is the non-coaxiality between directions of the plastic principal strain increment and the principal stress in the rotation of the principal stress. Ignoring of the non-coaxiality will lead to underestimate of soil deformation and unsafe design in practical engineering. The study of the anisotropy and the non-coaxiality of soil with the consideration of the change of principal stress direction can simulate the real stress state of soil and help us explore the work performance of soil in practical engineering.A large number of traffic engineering, ocean engineering and other large-scale engineering projects are distributed in the eastern coastal areas of China, where is widely distributed with deep soft clay layer. However, the study of the anisotropy and the non-coaxiality of soft clay with the consideration of the change of principal stress direction still remains insufficient. Furthermore, most experimental studie didn’t consider the effects of drain condition on non-coaxiality and the couplings of the magnitude of deviator stress and the direction of principal stress. So the study of the anisotropy and the non-coaxiality of soft clay with the consideration of the change (or rotation) of principal stress direction is important to predict the deformation of soft clay, explore the mechanism of soil disaster and control the soil disaster with the rotation of principal stress.In this study, a series of unit tests were carried out on Wenzhou soft clay. The main research contents and conclusions are as follows:1. A new kind of device and method is developed to prepare large diameter soft clay sample under Ko condition, involving slurry consolidation method. This device is able to prepare large diameter uniform soft clay sample in high pressure condition quickly. After the sample preparation, all the samples are tested with moisture content test, density test and consolidation test to check the uniformity and repeatability. Test results show that all the soil samples are of good uniformity and repeatability.2. Soft clay specimens are cut from different inclinations with the sedimentary direction. The results of consolidation test, permeability test and triaxial test conducted on these soil specimens indicate that the anisotropy of compressibility, permeability and strength etc. cannot be neglected. When the cutting direction is close to sedimentary direction, the compressibility is smaller and the strength is larger than other cutting directions. All the strength peak points lie on a straight line passing through origin, the slope of which is 1.17 and the effective friction angle for Wenzhou soft clay is 29.3°.3. By using the GDS hollow cylinder apparatus, a series shear tests are conducted on soft clay with a fixed inclination of principal stress with the vertical direction under drained and undrained conditions. According to the test results, the inclinations of principal stress with the vertical direction have significant effects on stress-strain relationship, effective stress path and pore water pressure accumulation. The non-coaxiality is not evident when principal stress angle a equals 0°,45°nd 90°. The direction of the major principal strain increment is always deviated to 45° relative to the direction of major principal stress. Compared with drain conditions, undrained condition can lead more significant non-coaxiality.4. By using the GDS hollow cylinder apparatus, a series principal stress rotation tests are conducted on soft clay under undrained condition. Tests results shows that pore water pressure would accumulate, and the axial strain, radial strain, tangential strain and shear strain would develop periodically under the rotation of principal stress. The angle of major principal strain increment is always larger than the angle of major principal stress under the rotation of principal stress.