A NEW EMPIRICAL APPROACH TO ESTIMATE THE RATIO OF HORIZONTAL TO VERTICAL IN-SITU STRESS AND EVALUATION OF ITS EFFECT ON THE STABILITY ANALYSIS OF UNDERGROUND SPACES

Abstract

In-situ stress is one of the most important input data to study stability analysis of underground and surface geomechanical projects. The measured vertical stress has a linear relation with depth. The average value of unit weight (g) was obtained 0.026 MN/m3 (2.56 ton/m3) using 1041 test results of different rocks with 0.001 difference with 0.027 MN/m3 which is a reliable coefficient for estimating vertical stress. The ratio of horizontal (sh) to vertical (sv) stress (K =sh/sv) is estimated by theoretical and empirical methods. The results showed that the estimating ratio of horizontal to vertical stress (K) by a theoretical method such as Terzaghi and Richard is much smaller than 1, and the estimation of the K value utilizing empirical methods such as Hoek and Brady is much greater than 1, with its value even approaching 4 in the near ground surface. To overcome the lack of an applicable comprehensive relation for the estimation of the K ratio and improve the shortcomings of previous methods, a new empirical relation was developed to estimate the K ratio utilizing a significant number of in-situ test results. Stability analysis of Masjed Soleyman powerhouse caverns was carried out by numerical modelling for five values of the K ratio obtained by previous stress estimation methods and this study. The in-situ stress estimation method (K ratio changes) showed a significant effect on stresses, displacements, strains, depth of the plastic zone and significantly affect the stability analysis and support system design of the powerhouse and transformer caverns.