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Abstract The mechanical properties of rocks are obviously affected by initial stress state and unloading rates that are caused by excavation. Based on triaxial unloading and scanning electron microscopy(SEM) analysis，the deformation，breakage，and strength features of marble samples of the Jinping First Hydropower Station under different unloading rates and geostress conditions are studied. With unloading rates vu and initial confining pressure increasing，the brittle rupture and tensile characteristics of rock are more obvious. There are annular tensile cracks in secondary unloading direction when the rock samples are unloaded in two directions；and the meso-shapes of the tensile fracture surface under SEM consequently are presented with branch-tension，and multilayer biscuit-lancinated and slippery plane-ejected shapes. The axial strain compression increment is gradually reduced，but the confining expansion stress is increased during the process of unloading. The influence of unloading rate vu on deformation modulus E is different at the different deformation stages during process of unloading. The deformation modulus E is gradually increased before peak strength，but it gradually increases firstly and rapidly decreases afterward with vu enhanced. The Poisson¢s ratio is gradually increased during process of unloading，and it is more distinct with vu and enhancing，especially after peak strength. Relative to data of traditional triaxial loading test，the rock cohesion c is remarkably reduced but inner friction angle is a little increased under unloading test. The faster the vu is，the more c is reduced，and the less the increases.
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Received: 22 May 2009
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