Abstract：The effects of fracture，joints and other inter-block gaps on rock mechanical properties can be very significant. A method based on site investigation and numerical test is conducted to determine and optimize the mechanical properties for engineering design. Contrasting with field experiment，it is convenient，fast and cheap. The method makes it possible to develop large sample rock mass in-situ tests，which will make a big push for the parameter risk prediction and reliability analysis of engineering rock masses. The research mainly includes two parts of work，i.e. the in-situ fine structure description and numerical test. Engineering geology information which is the input for numerical test is the base of the analysis. Detailed geological investigation is the soul of the fine structure description. Based on statistical window，geological sketch and digital photography，a new method for rock mass fine structure in-situ mapping is developed. It has already been taken into practice and has received good comments in Jinping First Stage Hydropower Station. After obtaining the basic geological information，the rock mass structure model is established through the in-situ mapping. In order to verify the validity and precision of the numerical analysis，a physical model direct shear test is carried out. According to the comparison，the relative error between the numerical test and the physical test is less than 10%. It is precise enough for rock mass mechanical parameter determination. Taking the Jinping First Stage Hydropower Station for example，large sample numerical experiments are carried out in the dam site. The complete stress-displacement curve，stress distribution，displacement distribution and plastic zone distribution are given by the numerical test. By comparison，the results obtained from numerical experiment and those from rock mass evaluation are marched well.
