Effects of back pressure on shear strength of saturated sand in triaxial tests

The back pressure saturation method is commonly adopted in triaxial test s to increase the saturation degree of samples. But the value of the back pressure is not stipulated in Chinese standards nowadays. To investigate the back pressure effects, a series of triaxial tests are carried out on Fujian standard sand. The stress-strain relationship of saturated soils under different back pressures and confining pressures is analyzed, and the development of pore pressure is obtained. The effective friction angle obtained from different failure criteria are also deduced. The test results reveal that the back pressure plays an important role in the undrained shearing behaviors of dilative soils, however, it seems to produce no effect on drained condition. According to the test results, it is suggested that, the back pressure should be 300~500 kPa, and be equal in one series of tests. In undrained shear test the strength corresponding to the maximum effective principal stress ratio (σ'₁/σ'₃)_max is recommended as the maximum undrained shear stength. While the residual stress of soils needs to be considered, Δu=0 (excess pore pressure equal to zero) could be the failure criterion.