TWO TYPES OF UNLOADING DEFORMATION OF ROCK SLOPES UNDER HIGH GEOSTRESS CONDITION

The paper briefly introduces two types of commonly concerned engineering geological and rock mechani-cal problems of high slopes in hydropower projects under high geostress,1. e.,the baffling large scale relax deform-anon of rock slope at the Jinping No. 1 hydropower station and the fast unloading fractures caused by dam foundation excavation at the Xiaowan hydropower station. The following opinions are presented:①the large scale relax de-formation of the slope at Jinping is actually the result of unloading of the composite stress field of gravity and tecton-is elements,and the large scale,steep slope and strong horizontal tectonic stress are the main factors to enlarge the range of relax deformation.②The phenomena such as thin-tabulate fractures,arciform broken-off andshallot-skin like fractures of fresh hard rock at the excavated surface of dam foundation are caused by unloading of 3 which is almost perpendicular to the slope surface,1. e.,the increase of the difference of principal stresses leads to the rapid failure of rock. As a feature of this kind of rapid unloading relax deformation,the new cracks concentrativelv distributed in the depth of 5~6m near the surface.