Abstract：The layered marble samples are collected from the underground powerhouse of Jinping I hydropower station which is a large-scale national key project. To simulate the excavation unloading effect，a series of triaxial unloading tests are carried out on MTS815 Flex Test GT rock mechanics test system，and a series of triaxial loading tests for comparison are also performed. The results show that there are obvious differences among mechanical properties of layered marble with different axial compression directions. When the axial compression parallels bedding plane，the shear strength parameters c and φ decline but residual strength parameter φr increases under unloading condition. When the axial compression is perpendicular to bedding plane，the shear strength parameters c and φ increase but residual strength parameter φr declines compared with the results of axial compression paralleling bedding plane under the same unloading condition. The prompt lateral expansion causes the failures of these rock samples and confining pressure unloading after peak strength significantly weakens their bearing capacities. The elastic modulus increases approximately with initial confining pressure increasing in loading and unloading cases，but the rule of deformation modulus at peak strength is reverse. For a single sample under unloading condition，whether axial compression is perpendicular to or parallels the bedding plane，the deformation modulus always decreases with confining pressure declining；and the value of the former is 20%–51% higher than that of the latter；but the lateral expansion ratio(m) of the former is 3%–12% lower than that of the latter；and the rule is reverse. Under the same unloading condition，axial compression paralleling bedding plane is more easily to cause the failure of rock samples than axial compression perpendicular to bedding plane；but the latter samples are of more obvious brittleness. These conclusions reveal the unloading properties of layered marble and offer important reference to solve practical engineering problems.
