Experimental Research On Non-coaxial Deformation Behavior And Modeling Of Soft Clay Under Principal Stress Rotation

Principal stress rotation is common in geotechnical engineering, when the ground subjected to dynamic loadings as traffic loading, wave loading and multi-direction earthquake and static loadings as excavation, and high-rise building construction, the direction of stress on the soil element will rotate. While the widely used soil constitutive models in present are all based on the test results from fixed principal stress direction triaxiat tests, and they are fail to consider the effect of principal stress rotation on the deformation behavior of soil. As the hollow cylinder apparatus is used in the geotechnical testing, the deformation behavior of soil and constitutive modeling under principal stress rotation are studied in the worldwide. And this is also one of the hottest and most difficult problem in the geotechnical research field. Till now days, the study of soft clay response to pure principal stress rotation has not been paid sufficient attention yet, neither at experimental research or theoretical research. In order to provide guidelines to the geotechnical design and construction on the soft clay ground in the southeast China, it is urgently to study the deformation behavior of soft clay considering the effect of principal stress rotation. Thus, the deformation behavior of soft clay subjected to principal stress rotation was studied from experimental research, physical interpretation and constitutive modeling in this study. A soft clay constitutive considering the effect of principal stress rotation was proposed based on the experimental results. The main achievements of this study are summarized as below:1. Based on the fixed principal stress direction and pure principal stress rotation tests on the intact and reconstituted clay. The non-coaxial behavior of soft clay subjected to continuous principal stress rotation was studied in particular. The deformation development, pore water pressure accumulation as well as stiffness degradation of soft clay under principal stress rotation were also analyzed. And the mechanism of the influence of principal stress rotation was explained from the perspective of microstructure. The following tests results can be concluded:(1) The non-coaxial behavior of soft clay was not obvious under fixed principal stress direction tests, especially soft clay behaved coaxially well the shear tress increased.(2) Obvious deviation between the directions of plastic strain increment and principal stress of soft clay was observed under continuous principal stress rotation. The non-coaxial angles fluctuated with principal stress rotation. The influence of intermediate principal stress parameter, shear stress level, number of cycles and inherent anisotropy on the non-coaxial behavior of soft clay was unobvious.(3) Plastic strain accumulated in the soft clay even with the pure principal stress rotation, train components fluctuated with principal stress rotation. Specimens could failed with continuous strain accumulation. The intermediate principal stress parameter has a significant effect on the deformation behavior and failure form. The development of strain was similar to that of stresses, while the strain lagged20behind the stress.(4) Pore water pressure was all induced by the pure principal stress rotation, the rate of pore water pressure accumulation was also influenced by the intermediate principal stress parameter. Significant pore water pressure was induced even under low shear stress level of5kPa.(5) Even when the stress level was lower than the failure stress, soil could failure with pure principal stress rotation, so the design did not take the influence of principal stress rotation into consideration may be unsafe.(6) From the perspective of microstructure, the mechanism of-soil deformation induced by the principal stress rotation can be attributed to the disturbance of the microstructure, the particles were crushed or reoriented by the rotational shearing.2. According to the soil constitutive models considering the effect of principal stress rotation, it can be concluded that the calculated results matched well to the experimental ones only when the non-coaxial behavior of soil was well considered. Thus the non-coaxial behavior of soft clay subjected to principal stress rotation was studied deeply. And failure boundary surface was taken based on the test results from fixed principal stress direction tests. The effect of tan plastic stress increment was taken into consideration, a non-coaxial plastic flow model was proposed based on the boundary surface theory. When compare with the test results, it can be found that the model can predict the non-coaxial behavior of soft clay properly.3. A constitutive model of soft clay considering the influence of principal stress rotation was proposed in the framework of general plastic theory based on the analysis of test results and the non-coaxial plastic flow rule. The Rowe’s stress-dilatancy was corrected by the influence of non-coaxial. When the principal stress increment was transformed into general physical space, the direction of principal strain increment considering the influence of non-coaxial behavior was used instede of stress direction. The calculated results were compared with the experimental results and other models, it was found that the model consider the change of non-coaxial angels given better simulation results.