Experimental Study Of Reconstituted Clay And Intact Clay Under Principal Stress Rotation

Principal stress rotation, induced by wave or traffic load, is different from the stress path of conventional triaxial test, and leads to significant differences in soil responses. On the basis of previous research, undrained shear tests with principal stress rotation are carried out on both Hangzhou intact soft clay and reconstituted clay by using hollow cylinder apparatus (ZJU-HCA). The main results and achievements are as follows:First of all, two paralle tests are performed. The comparison results on stress-strain curves show that (1) during test, constant shear stress and constant rotation can be reliably achieved. Accurate data can be obtained. (2) The axial stress-strain and circumferential stress-strain of the two tests with same stress path are very similar. The amplitudes of radial strain and torsional strain are small with a mere difference of only 0.03%. (3) The changes of the strain components are consistent with the corresponding changes of stress components. All the above results prove the reliability of the testing apparatus, and the tests are repeatable, comparable and reliable.Consolidated-undrained shear tests of fixed principal stress direction and pure principal stress rotation are carried out on Hangzhou intact soft clay. In the tests, the mean principal stress is kept constant(150kPa) and the intermediate principal stress coefficient is set to 0.5. The influence of initial shear stress on different strain components, generalized shear strain, shear modulus and pore pressure under principal stress rotation are discussed. The test results show that (1) the characteristics of radial strain, circumferential strain, axial strain and torsional shear strain, as well as the influence of initial shear stress on different strain components, are consistent with previous research achievements. (2) The previous principal stress rotation has significant effect on the generalized shear strain of intact soft clay due to subsequent rotation of principal stress axes, and the shear modulus decreases. (3) The expression of pore pressure coefficient is deduced by double yield surface theory. Both the elastic modulus and the function of yield surface have influence on the pore pressure coefficient. The linear fitting results of pore pressure reveal that the volumetric yield surface function is not affected by the principal stress rotation when the initial shear stress smaller than the peak shear stress. However if the initial shear stress close to the peak shear stress, a relatively high pore pressure will be expected.Another purpose of this research is to discuss the consolidated-undrained shear tests of fixed principal stress direction on Hangzhou reconstituted clay. The author participated in the research of the vacuum suction devices. The influence of initial stress state on stress-strain, pore pressure and strength characteristics are discussed. The results show that (1) the relationship of axial stress-strain and circumferential stress-strain on reconstituted clay is mainly dominated by rotation angle. The corresponding axial strain and circumferential strain will have positive and negative development direction at the critical rotation angle, i.e.45 degree. In contrast, radial stress-strain and torsional stress-strain are less affected, and radial strain and torsional strain do not change from positive direction to negative direction. (2) The intermediate principal stress coefficient has a certain impact on pore pressure and strength characteristics. Comparatively the angle of principal stress has little effect on them. (3) It is found that the peak shear stress or pore pressure is not suitable to be used as the failure criteria for Hangzhou reconstituted clay. Instead the strength, corresponding to 8% generalized shear strain can be suggested as the failure criteria.The comparison between the results of reconstituted clay and intact soft clay under similar stress paths is conducted, which aimed to reveal the relationships between inherent anisotropy and induced anisotropy. The test results show that (1) the axial stress-strain, circumferential stress-strain and pore pressure characteristics are mainly dominated by the induced anisotropy. In contrast, the radial stress-strain, torsional shear stress-strain and generalized shear stress-strain are hardly affected by the induced anisotropy. (2) The magnitude and the range of the stress components on intact soft clay are larger than those of the reconstituted clay, and mainly controlled by the inherent anisotropy. The strength of intact soft clay is affected largely by inherent anisotropy, but the strength change of reconstituted clay varies little.The paper compares the results of both reconstituted clay and reconstituted sand under similar stress paths, aiming to reveal the responses of different soils. It is found that (1) as for pore pressure characteristics, reconstituted sand has an apparent phase transformation point but clay does not have. (2) There are obvious difference between stress-strain curves of reconstituted sand and clay. Sand has obvious strain softening and hardening behaviour, while only strain hardening can be found in reconstituted clay.