Abstract: Based on the sandstone’s high geostress area from where a high slope in Jinping, a series of pre-peak unloading tests have been carried out; and the complete stress-strain curve, deformation characteristics and degradation effect of deformation parameters are studied. The test results show that compared to loading failure, the rock’s brittle characteristics of unloading failure is obvious; and unloading failure needs lower deviatoric stress under the same initial confining pressure. The lateral strain and volumetric strain increase sharply in the process of unloading; and the expansion characteristics in the unloading direction is obvious. When the initial confining pressure is less than a value(30 MPa), the rock’s failure unloading percentage would increase along with initial confining pressure’s enlargement. But when initial confining pressure grow up to a value(40 MPa), the brittle characteristics is very obvious; and a few unloading percentage can cause the rock failure. Taking unloading percentage H which expresses unloading level as variable quantity, and making fitting of deformation parameters when in high and low stress. We get the functions between deformation degradation parameters and unloading percentage under unloading process. The test results and calculation method could benefit excavation for the stability analysis of slope under high geostress.

Key words: unloading, high stress, deformation parameters, degradation effect