The three-dimensional (3D) morphology of a rock joint has a great impact on its shear behavior. To study the relationship between the 3D morphological characteristics and the peak shear strength, several tilt tests were conducted on four groups of tensile fractures and direct shear tests were carried out under different constant normal loads (CNL). The normal load ranges from 0.325 to 8.0 MPa. In this study, fresh tensile fractures which were splitted from granite and sandstone samples were used. The morphology of each tensile fracture was measured before direct shear tests. A new peak shear strength criterion for rock joints is proposed using two 3D morphological parameters which are termed as the maximum apparent dip angle h à max and the roughness parameter C. The calculated peak strengths using the proposed criterion match well with the observed values. In addition, a comparison of the proposed model with the Grasselli's model (2003) and Xia's model (2014) shows that the proposed model is easier in the form and gives a rational improvement. At last, direct shear test data of tensile fractures which are collected from Grasselli (2003) are used to verify the proposed model. It is seen that the proposed model has a reliable estimate of the peak shear strength of tensile fractures and presumably for rock joints.

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Since rocks are aggregates of mineral particles, the effect of mineral microstructure on macroscopic mechanical behaviors of rocks is inneglectable. Rock samples of four different particle shapes are established in this study based on clumped particle model, and a sphericity index is used to quantify particle shape. Model parameters for simulation in PFC are obtained by triaxial compression test of quartz sandstone, and simulation of triaxial compression test is then conducted on four rock samples with different particle shapes. It is seen from the results that stress thresholds of rock samples such as crack initiation stress, crack damage stress, and peak stress decrease with the increasing of the sphericity index. The increase of sphericity leads to a drop of elastic modulus and a rise in Poisson ratio, while the decreasing sphericity usually results in the increase of cohesion and internal friction angle. Based on volume change of rock samples during simulation of triaxial compression test, variation of dilation angle with plastic strain is also studied.

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