The coarse-grained materials are normally in three-dimensional (3D) stress state which changes with time and location. To investigate the strength characteristic of coarse-grained materials at different coefficients of intermediate principal stress

large-scale true triaxial tests were performed under isotropic consolidation and proportional loading conditions. The results show that the intermediate principal stress has an important effect on the strength of coarse-grained materials. The strength of coarse-grained materials in 3D stress state increases greatly compared to the strength in conventional triaxial stress state. The relationship of the major principal strain and the difference between the maximum and the minimum principal stresses in 3D stress state is steeper than that in conventional triaxial stress state. The difference between the maximum and the minimum principal stresses increases from 39% to 50% when the coefficient of intermediate principal stress increases from 0 to 0.25. Both the interlocking force c and internal friction angle ? increase with the increase of the coefficient of intermediate principal stress b. Especially

the interlocking force c increases significantly when b increases from 0 to 0.25. With the same minimum principal stress

the ratio of deviatoric stress to spheric stress at the failure state is the smallest with b=0

whereas the failure stress ratio reaches the maximum with b=0.25. However

the failure stress ratio decreases with the increase of b from 0.25 to 0.75. The stress ratio at failure state decreases with the increase of the minimum principal stress at the same coefficient of intermediate principal stress.