Abstract:Based on unloading tests of rock samples,the deformation,parameters and fracture features of rocks under unloading are studied. The test results show that:(1) The characteristics of intense rebound deformation,distinct dilatancy and brittleness failure of rocks are all shown in the process of unloading. (2) Deformation modulus E gradually decreases and Poisson ratio m increases in the process of unloading. The deceasing amount of E is 5%–27%,while m increasing 50%–335%. The variables all increase with initial confining pressure increasing and unloading intensity,which are related with volumetric strain. (3) The cohesion c decreases but the internal friction angle increases relatively with the loading tests,and both are less with unloading intensity. The peak value of c decreases 33.2%–47.8%,and remains normal value of 65.3%–77.6%;peak value of increases 14.7%–33.2%,and remains normal value of 5.9%–9.4%. (4) The fracture is characterized by strong tensile,and all tensile cracks are fully developed,showing hypo-unloading when the rock samples are unloaded in two directions;and the shear fractures expand following tensile cracks.