Multi-reversal direct shear test is conducted on the undisturbed and remolded infilled joint soils under in-situ stress where normal stress is as far as 10 MPa for test from a hydropower station. It is revealed that the peak shear strength is higher but decreases more rapidly for the undisturbed samples compared with the remolded ones; however

it seems the residual shear strengths tend to be the same. In addition

a large amount of particle crushing occurs

Br (relative particle crushing potential) is adopted to quantify the degree of particle crushing in the shear zone and the non-shear zone. The results show that the amount of particle crushing in the shear zone is larger than that in the non-shear zone; and the mechanisms of particle crushing are different for the two zones. Furthermore

particle crushing is considered as the main reason for the nonlinear relationship of the residual shear strength under high pressure; the decrease of shear strength (ratio of the residual shear strength to the peak shear strength) is linear with Br and S2 (the ratio of clay fraction before test to the one after test for the shear zone). It is indicated that the energy liberation and continuously generated clay fraction caused by particle crushing decrease the shear strength.