CHARACTERISTICS OF UNLOADING MICRO CRACKS IN SHALLOW ROCK MASS AT LEFT BANK OF JINPING Ⅰ HYDROPOWER STATION DAM

In the mountain valley area, epigenetic deformation of the shallow rock mass is common. Directional samples are taken at left bank of Jinping Ⅰ hydropower station dam. Thin sections are made. Direction distribution of micro fracture zones and micro cracks are analyzed by polarization microscope. Three conclusions are drawn. Firstly, micro fracture zone in thin section which is perpendicular to σ₃ has no obvious directionality, but micro fracture zone in thin section which is perpendicular to σ₂ or σ₁ is parallel to bank surface. Secondly, micro cracks tend to occur along low index plane including cleavage plane, which accords with least energy principle. So intracrystalline cracks in calcite crystal show linear or step style. Thirdly, unloading stress induces and controls the direction of micro cracks, so micro cracks have obvious directionality.