Experimental Study on Behavior of Saturated Loose Sands Under Complex Shear Loading

摘要

Various stress-controlled monotonic and cyclic shear tests of loose sand under various complex initial consolidation conditions and different variation pattern of shear stress are conducted by using the soil static and dynamic universal triaxial and torsional shear apparatus. Influences of such factors as coefficient of intermediate principal stress and orientation of principal stress on strength and deformation behavior are examined on the basis of monotonic shear tests. The variation patterns used in cyclic shear tests are able to reproduce the stress conditions in seabed under wave loading. Therefore the deformation behavior of loose sand under complex consolidation condition subjected to cyclic loading is discussed. Based on the experimental data the following conclusions can be given. (1) Under the application of monotonic shear stress, orientation of principal stress obviously influence the effective stress path and stress-strain relation of sand. When orientation of major principal stress approaches the vertical, loose sand behaves hardening with dilatancy. When the orientation angle between major principal stress and the vertical become larger, loose sand displays noticeable strain softening with shear contraction and then strain hardening with dilatancy etc. To compare with the effect of orientation of principal stress, influence of intermediate principal stress on shearing deformation behavior seems not obvious, but its effect on friction angles mobilized both at phase-transformation state and at steady state cannot be overlooked. Coefficient of intermediate principal stress has effects of amplification and regionalization on the friction angle. The friction angle under triaxial tension condition is considerably larger than that under triaxial compression condition. (2) Under the condition of cyclic loading, the influence of orientation of principal stress of initial consolidation is remarkable. When pre-shear stress is imposed on the application plane of dynamic stress, shearing deformation of loose sand results mainly from the accumulated deformation. When no initial shear stress is applied on the application plane of dynamic stress, most of shearing deformation is of cyclic nature, and the generalized shear strain arises essentially from the axial strain caused by deviatoric stress induced by the difference of normal stresses in different directions. Based on comparison between the test results of monotonic and cyclic shear obtained from torsional shear tests, it is manifested that for the same initial stress state, the effective deviatoric stress ratio at steady state and the effective deviatoric stress ratio at phase-transformation state are equal to each other respectively for both monotonic shear and cyclic shear. Therefore, the effective deviatoric stress ratio at steady state and the effective deviatoric stress ratio at phase-transformation state can be used as two characteristic parameters which are able to represent the comprehensive influence of complex stress condition including orientation of initial principal stress and coefficient of intermediate principal stress for monotonic and cyclic shear conditions.