The deformation modulus of rock masses is very important and useful in the designing of surface and underground structures in rock. This parameter is calculated from empirical relations, in-situ tests, and also numerical modeling methods. The results of in-situ tests are more accurate than other methods because more volume of rock is placed under loading. The plate Load Test (PLT) is the famous method used in the site investigation of the geotechnical project. Although the test theory is simple, the interpretation of its results always is accompanied by complexity and error. In this research, the assumptions of ISRM relations to calculating the deformation modulus of rock masses were studied. The results of PLT of Azad pump storage power plant project in the west of Iran was analyzed and the tests were modeled with 3DEC software. The discontinuities characterizations like spacing, persistence, shear and normal stiffness were changed to get the best fit between the experimental and numerical results in two cases, parallel and perpendicular to the bedding. After that, the deformation modulus of sandstone rock masses around underground structures in Azad project was calculated for different conditions (depth, gallery dimensions, bedding direction, joint stiffness, and stress ratio). The results show the deformation modulus is increased with increasing depth of extensometers installation. Then a mathematical relation was proposed based on the numerical results with changing the size of the gallery to adjust the results of PLT with depth. As the depth increased, the deformation modulus increased due to the increase in shear and normal stiffness. The results also show the deformation modulus changes with stress ratio and bedding orientations
