Abstract：The deformation modulus of rock mass is an important input parameter in analyzing deformation，which should be determined by field tests. However，field tests are time consuming，expensive and the reliability is sometimes questionable. Based on data from in-situ measurements，E. Hoek proposed a new empirical formula for estimating values of deformation modulus of rock mass based on geological strength index(GSI). In order to provide more quantitative basis for evaluating GSI values，quantitative GSI system has been proposed by introducing measurable parameters with ratings or intervals which define structure and surface conditions of discontinuities. Especially，the methods to determine the parameter volumetric joint count(Jv) are studied in detail Finally，the proposed methods have been applied to practical projects and its rationality is further verified by comparing with field tests results.
