Quantitative estimation of scale effect is a complex problem which contained many uncertainties and should be solved using probability calculus and statistical approach. This paper aims to derive the structural factor according statistical strength theory involving discontinuity surface conditions account to estimate the design rock mass strength. A short review of scale effect estimation techniques based on statistical strength theory is given. A new method of structural factor evaluation is proposed. This technique allows accounting discontinuity conditions by changing the variation of tested specimen random sample. A function that describes the decreasing of strength due to poor discontinuity surface quality is introduced to correct the initial and central statistical moments of strength random distribution. The evaluation of the joints condition function based on analysis of the results of uniaxial compressive strength tests and petrographic structure of specimens is shown. Improving the statistical approach of structural factor evaluating increase the accuracy of the rock mass strength assessment and allow avoiding costly modifications of the mining excavation support design. A case of rock mass strength estimation under conditions of coal mine “Komsomolets Donbassa” according to proposed statistical method is studied.
