Abstract: Interfaces with different strengths are usually existed in rock mass, the shear strength of these interfaces plays a critical role on the stability of rock mass, and it is thus highly important to accurately evaluate their shear strength. With the help of 3D-printed resin molds, the soft-hard jointed cement specimens with different joint roughness coefficients (JRC) and joint wall compressive strengths (JCS) were fabricated to conduct laboratory tests and numerical simulation of direct shear under constant normal stress. The test curves of shear stress-displacement agree with those by numerical simulation. Based on the test results, a coefficient of proportionality T was proposed to characterize combined compressive strength CCS of soft-hard joint, and meanwhile an influence factor ? was proposed to characterize the effects of JRC, JCS and normal stress ?n on the peak shear strength of soft-hard joint by calculation and statistics method , thus a method for estimating the peak shear strength for soft-hard join surface was derived. The residual shear strength of soft-hard joint is mainly controlled by the soft joint side, and this control effect increases with the increase of JRC. The shear stress-displacement curves of soft-hard joint with smaller CCS earlier reach the peak and enter the residual stage. The study indicates that the proposed model may be used to evaluate the peak shear strength of soft-hard joint by integrating the influence mechanism of joint surface morphology, joint compressive strength and normal stress.

Key words: soft-hard joint, direct shear test, joint roughness coefficient, combined joint wall compressive strength, peak shear strength model