Rock mass consists of two components: Discontinuities and intac rock, and each of which has a important effect on the rock mass properties. engineering are frequently challenged by the difficulties of handling the inherent variability of soil and rock mass properties from initial stage of site characterization to the final design of engineering structure. to determine the rock mass properties such as the strength and deformability, normally empirical or experimental methods are utilized. Experimental methods such as in-situ tests are expensive and they require a test drift which may not be available at the preliminary design stage. Therefore rock mass ltr" In this paper, the author teied to apply probabilistic approach to consider the uncertainties and variability in rock properties. for this aim, the GSI System developed by hoek et. al. and monte-carlo method was used for Probabilistic Estimate of rock mass properties in pir-taghi dam site. Then, The Monte carlo simulation results used for numerical modelling with 3DEC. at the end, total Displacement distribution around the water Conveyance tunnel Was obtained. The assassed value can be used in Preliminary design stage and considered as basis for further numerical modeling calibration studies
