Abstract: Based on a project of hydro-power plant in Yunnan Province, we analyzed the selection of location, depth and direction of the test points via a modified borehole-jack elastic-modulus gauge. The test points should be in critical locations of the project, and the test direction should conform with the actual stress direction after the project is in operation. We then obtained the elastic modulus of rock mass at the abutment and the foundation of the dam, through which we analyzed the characteristics of pressure-deformation curves, and discussed how to select parts of the curves to estimate elastic modulus. There are two kinds of pressure-deformation curves displaying different deformation characteristics in borehole-jack test. In the case of curve A, in the presence of low pressure, the pressure and deformation displayed elastic-plastic relation, while in the presence of high pressure, it showed linear relation and non-linear relation in the rebound section. In the case of curve B, the pressure and deformation was in a linear-elastic relation when the pressure increased, and in a non-linear relation in the rebound section. Besides, we compared the results of borehole-jack test with those of lab test and bearing plate test. The borehole-jack test result is slightly larger than that of bearing-plate test, whereas lab test results are much bigger than the other two. The test results are referential to the determination of elastic modulus and deformation modulus, and this modified testing technique provides scientific basis for selecting the foundation base of the dam.

Key words: in-situ test, rock deformation, borehole elastic modulus, deformation modulus