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In civil and mining, the design of some structures, such as pillars in mines and bridges, requires the knowledge of the effect of the relative size of dimensions (shape effect) on the strength of the structure. Therefore, to assess the effect of shape on the uni-axial strength of rocks, laboratory tests were conducted on samples of 3 different types of rocks (namely; Isfahan dolomite, Bookan limestone and Gadouk sandstone) with various sample relative dimensions. Analyzing the data of these tests resulted in an overall relationship as where D, L and are the diameter, length and the uni-axial compressive strength of a sample with
and K was obtained to be 1.14, 1.21 and 0.96 for the 3 rock types relatively. Also, as and confining pressure affect uni-axial compressive strength and tri-axial compressive strength similarly, 3 axial tests, in
various confining pressures, were conducted on the samples of the same rocks with . Graphs of and - were compared. After omitting the term from these formulas the overall relationship
of was obtained. Finally, for the 3 type of rocks, a single formula as… …with coefficient of determination of 0.9534 was introduced. By applying the simulation proposed in this study, within the limit of the results, the costly time consuming tri-axial tests can be replaced with uni-axial ones.