Rock mass deformation modulus is one of the most important parameters in rock engineering studies. This parameter shows the amount of rock mass deformation which is happened due to loading or unloading cycles. Deformation modulus can be obtained directly and indirectly from in-situ tests or empirical and analytical equations, respectively. Plate loading test (PLT) is one of the most applicable test in order to determine rock mass deformation modulus. This test includes one or more hydrolic jacks that apply the load on the surface of rock mass. The displacements in rock mass can be obtained via extensometers that are placed on the plates and also behind the plates in excavated borehole. Back analysis methods generally can be devided into direct and reverse methods. In this study the direct method was applied. In this method according to the results of several numerical models, the amounts of difference between the obtained results from numerical modeling and the measured results from PLT were reduced to minimum. FLAC 3D was employed in this study in order to build a numerical model and simulate PLT. Several back analyses that were applied to the model point out loading cycles in 500, 600 and 750 steps had the best fit with the measured results from PLT. In order to analyse the results and determine the most suitable loading step, least square and minimax methods were used in this study to calculate error functions. The best results for each method with the least value of error function were 750 and 500 steps, respectively. The results of loading cycles with 500 steps were shown less values of difference between numerical, in-situ and theoretical results in comparision with 750 steps. Finally, as a result, the most similarities between numerical modeling and measured results from PLT were obtained with loading cycles in 500 steps. Determined rock mass deformation modulus based on these results shows appropriate similarity with the primary modulus that was applied to the model.
