Abstract: With the help of 3D scanning technology and 3D printing technology, several specimens with the same natural joint but different joint wall compressive strengths were first made, and shearing tests under different constant normal stresses were conducted on them. The direct shear test results show that the strength characteristics on both sides of soft-hard joints affect the shear expansion and shear strength, that is, with the increase of wall strength coefficient , the shear dilatation increases but the shear strength decreases. Moreover, the study also indicates that the damages in the joints surfaces occur on both sides of the soft-hard joints during the shearing process, and the equivalent joint wall strength JCSeq is affected by the wall strength coefficient . Compared with the hard joint side, the equivalent wall strength JCSeq is improved to a greater extent by the soft joint side, and the trend is strengthened with the increase of the wall strength coefficient . That is why JRC-JCS strength formula has some limitations in predicting the strength of soft-hard joints. Based on the above analysis, JCSsoft is the basis for establishing the modified relational function f( ) of soft-hard joints equivalent wall strength JCSeq with respect to wall strength coefficient . And an improved JRC-JCS shear strength formula considering different wall compressive strength combination types of soft-hard joints is proposed, which is the complement of current JRC-JCS strength formula.

Key words: soft-hard joint, joint wall compressive strength JCS, shear strength formula, failure characters of joints