The experimental tests of loose sands subjected undrained monotonic shearing with the relative density of 30 % are performed by using the soil static and dynamic universal triaxial and torsional shear apparatus which is newly developed in Dalian University of Technology. A series of tests are conducted for loose sand under complex stress conditions with different coefficient of intermediate principal stress

orientation of principal stress and mean principal stress. Based on the experimental results

the effects of coefficient of intermediate principal stresses on the friction angles at phase-transformation state (PTS) and peak stress state as well as effective stress paths are respectively examined. It is found that the coefficient of intermediate principal stress will play an important role in controlling the undrained shear strength parameters of loose sands while its effects on both effective stress paths and the variation patterns of the relation between stress and strain is relatively minor. The dependency of soil strength parameter on the coefficient of intermediate principal stress is illustrated theoretically by using the generalized double shear criterion and can agree well with the experimental data.