Abstract: The intermediate principal stress has an important effect on the strength of coarse granular materials. However, the large-scale triaxial tests commonly used in strength study does not consider the effect of intermediate principal stress on the strength of coarse granular materials. Large-scale true triaxial tests under isotropic consolidation and proportional loading conditions were conducted to investigate the strength properties of coarse granular materials under three-dimensional stress states. Test results show that the peak value of the difference between major and minor principal stress of coarse granular material increases with the increasing of the coefficient (b) of intermediate principal stress, especially when b increases from 0 to 0.25 the peak value of the difference between major and minor principal stress increases nearly 50% compared to strength under axisymmetric triaxial stress state. Lade-Duncan failure criterion well describes the variation trend of internal friction angle for coarse granular materials; but overestimates the internal friction angle. SMP failure criterion commendably depicts the numerical value of internal friction angle when b increases from 0 to 0.25; when b is larger than 0.25, however, the results of SMP criterion decrease with the further increasing of b, which underestimates the internal friction angle. In view of this, a one-parameter composite strength criterion combining Lade-Duncan criterion and SMP criterion was proposed based on test results. The proposed criterion well reflects the numerical value and variation trend of the strength of coarse granular materials. The proposed strength criterion is shown as a triangular conical surface in three-dimensional principal stress space. The strength and triangular conical surface are small and the cone angle tends to be circle when the internal friction angle under axisymmetric stress state is small; with the increasing of internal friction angle, the strength and triangular conical surface gradually become larger and the cone angle tends to be triangular.

Key words: coarse granular materials, three-dimensional stress state, strength criterion, intermediate principal stress, stress-strain relation, large-scale true triaxial test