Zheng Yong-lai

Strength model and mesoscopic mechanism of intermediate principal stress effect on rock strength

Intermediate principal stress effect on rock strength, strain, energy dissipation and micro crack distribution are studied by using particle discrete element numerical simulation software PFC3D. Numerical simulation reproduces the typical phenomenon that rock strength has an initial increase and a subsequent decrease with the increase of intermediate principal stress. On micro level, the increase of intermediate principal stress makes the micro cracks distribution in horizontal plane from symmetrical to asymmetrical. When intermediate principal stress ratio b is large than 0.5, micro cracks have a substantial increase at yield stage and energy dissipated by frictional sliding also has a substantial increase. Intermediate principal stress restrains the expansion of micro cracks normal to intermediate principal stress direction (or with component in this direction), which makes rock strength increases. On the other side, intermediate principal stress promotes the expansion of micro cracks normal to minimum principal stress direction (or with component in this direction), which makes rock strength decreases. The restraint and promotion of micro cracks in different directions lead to the intermediate principal stress effect. Based on Weibull theory, a failure probability model is developed to describe the effect of the intermediate principal stress on rock strength. Using shear stress to express the computed strength of each micro-unit, a strength criterion is proposed to quantitatively reflect the intermediate principal stress effect on rock strength. New criterion can be regarded as a modified Mohr-Coulomb criterion with a uniformity coefficient and it becomes Mohr-Coulomb criterion when materials are absolutely homogeneous. New criterion matches previously published experimental results and numerical simulation results better than common strength criterion.

Design of a shaking table test box for a subway station structure in soft soil

A reasonable choice of structure of a model box is significant for a shaking table test to be successful in geotechnical engineering. A model box has been designed for the shaking table test of a subway station structure in the soft soil of Shanghai in the paper. The reasonable geometric similarity scale of the subway station structure has been determined by a 3-D dynamic analysis under the action of lateral equivalent static loading. The shape, size and structure of the model box are chosen by considering all the involved factors comprehensively. The shape of the box is similar to that of a typical station structure, and the ratio between the plane dimension of the model ground and that of the model structure is big enough to reduce the influence of boundary condition effectively. The structure is strong enough to avoid being demolished by shaking during a test. The contact conditions between the model soil and box are clear to help the data gained from the test well fit that from numerical analysis. The total weight of the model soil and box is less than the bearing capacity of the shaking table apparatus and there is no resonance between the model soil and box. The results show that the model box can be used to simulate the dynamic response of a subway station structure very well, so it provides a firm foundation for the success of the shaking table test of a subway station structure.