Abstract: Heterogeneous discontinuities with different properties are widely distributed in rock masses. The unconformity is usually beyond consideration; however the deformation and strength characteristics of heterogeneous discontinuities yield the basis for the stability evaluation of rock mass. In this study, samples from the typical sandstone-mudstone interbeds in the Jurassic in Three Gorges Reservoir Region are tested. The joint roughness coefficient (JRC) values of flat discontinuity and four different irregularly shaped discontinuities are calculated in a quantitative approach using the fractal geometry theory. Then the numerical shear tests on these five kinds of samples are carried out by the PFC2D software. The shear stress-displacement curves are acquired for heterogeneous discontinuities with different roughnesses under various normal stresses. According to numerical results, the strength characteristics of heterogeneous discontinuities are analyzed using Barton’s JRC-JCS model and compared with the homogeneous discontinuities. At last, on the basis of shear failure mechanisms of heterogeneous discontinuities, two types of modified Barton criteria (i.e., type I and type II) are proposed by introducing a concept of strength factor, which are applicable to strength evaluation of heterogeneous discontinuities. The results indicate that shear strength of heterogeneous discontinuity falls between those of two homogeneous rock discontinuities with the same joint roughness, and is close to strength of soft rock discontinuity at low normal stress, which follows the modified type I Barton criterion. At high normal stress, the strength of heterogeneous discontinuity is close to that of the homogeneous discontinuities of hard rock, which follows the modified type II Barton criterion. In practical project, the evaluation of rock mass stability can employ the modified criteria in combination with the stress state of heterogeneous discontinuities, which improves on some of the shortcomings of previous studies.

Key words: heterogeneous discontinuity, shear strength, PFC2D, numerical tests, JRC-JCS model