Young modulus is one of a basic geo-mechanical parameters used for the defining of the phenomena in rock mass. It is determined based on a uni-axial compressive test. According to the International Society of Rock Mechanics one may calculate it in three different ways as the tangent, secant and average modulus. This discretion causes, as the result that one can obtain results, which differ from each other significantly, even threefold or more.

The laboratory tests for Carboniferous rocks: claystones, mudstones, sandstones and coals are presented in the paper. The modulus value distributions for the recommended methodology were compared using statistical analysis. The typical range of the elastic linear deformability for the chosen rock types was determined. Thus the evaluation of the best Young modulus determination method was performed. Tangent Young modulus was recommended as the guiding one, studied with the range of 25–75% of ultimate stress. 503 rock samples have been investigated for this analysis.

The elastic behaviour of rocks and the range of their elasticity are of crucial importance in rock engineering. In solving problems in mining engineering or tunnelling, the change of stress fields forced by an excavation drivage first depends on the elastic and then the post-failure properties of rock. Hence, determining the rock mass properties appropriately is of crucial importance for a roadway stability evaluation and support design. Among elastic rock properties one may mention: Young's modulus, shear modulus, bulk modulus and Poisson's ratio. As the shear modulus and bulk modulus are functions of Young's modulus and Poisson ratio the last two parameters are the most important in solving geo-mechanical problems. Some authors even emphasise, that elastic modulus becomes a critical parameter to describe the rocks' behaviour under load because of their brittleness [1, 10]. All numerical models for stress and deformation analyses such as FLAC, PHASE or UDEC use Young's modulus for solving rock engineering problems [11]. Young's modulus is also a base to derive a deformation modulus of blocky and jointed rock masses, which are non-elastic.

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