Shear mechanical properties of rock mass with discontinuous joints under unloading normal stress

In practice, the rock masses in a state of compression-shear stress are subjected to the normal stress unloading path after excavation. In view of this, the specimens of discontinuous joints with different roughnesses JRC are prepared by using the similar materials, and then the unloading shear tests with gradually decreasing normal stress σ are carried out, and compared with the conventional direct shear tests. The test results show that the change laws of failure and strength characteristics of the specimens under two stress paths are different: with the increase of JRC, the specimens under unloading normal stress exhibit stronger tensile failure characteristics, while those under conventional direct shear show stronger shear failure characteristics. The strength of the specimens increases with the increase of JRC under both the normal stress unloading and the conventional direct shear, but the normal stress unloading condition is mainly reflected in the increase of φ, while the conventional direct shear condition is mainly expressed in the increase of c. The main reason is that the normal stress unloading condition exhibits a gradual decrease in σ, resulting in a more obvious climbing effect. The conventional direct shear condition shows a gradual increase in shear stress τ, resulting in a more obvious gnawing effect. If the strength of the specimens under normal stress unloading is deduced according to the strength parameters of the conventional direct shear tests, it will be much greater than the actual strength, which may lead to unsafe results. Finally, a modified model considering the weakening of rock bridge and the influences of JRC is proposed based on the Jennings criterion. The errors between the theoretical prediction and the experimental results are less than 10%, which shows that the proposed model can be better used to predict the strength of discontinuous jointed rockmass under unloading normal stress.