Experimental and Numerical Investigations on the Effect of In Situ Stress and Discontinuities Orientation on the Deformation Modulus of Rock Masses

Abstract

Deformation modulus is one of the geomechanical parameters of rock masses; it is widely used in the design and numerical modeling of rock structures. This parameter can be calculated using empirical methods and in situ tests. Considering that the results of in situ tests are more reliable than those obtained by the other methods, the plate Jacking test (PJT) is more widely used than other in situ methods, because it affects a larger volume of rock. Investigations demonstrate that the difference between the actual conditions of the PJT and the assumptions governing the theoretical relationship suggested by the International Society of Rock Mechanics (ISRM) causes some errors in the obtained results, and the interpretation of the results is always complicated. In this study, by performing six PJTs in the Azad pump storage power plant (PSPP) project, in the west of Iran, two parallel and two perpendicular to the bedding were selected and the deformation modulus was calculated by using the proposed ISRM equation. The effect of changes on normal and shear stiffness, discontinuity spacing, and the number of discontinuities was investigated through numerical modeling of these tests using the 3DEC software (version 5.20). Subsequently, the results were compared with those of in situ tests and the model that best matched the in situ results was presented. In the first step, the appropriate depth according to the depth of the implementation of the in situ tests was chosen to calculate the deformation modulus parallel and perpendicular to the discontinuity using the numerical modeling. In the second step, the equivalent deformation modulus was estimated based on the analytical formulas and compared with the numerical and experimental results. Furthermore, the influence of various parameters such as the ratio of the dimension of the gallery (W) to the dimension of the loading plate (a), overburden (in situ stresses), and stress ratio (K0) was considered and sensitivity analysis was performed. The results revealed that when the W/a ratio was greater than 10, the gallery dimension effect disappeared. Increasing the K0 ratio and maximum horizontal stress at a constant depth increased the stiffness of discontinuities and decreased displacement, leading to an increased modulus of deformation for the rock mass. The results also showed that, for this site, with increasing depth, the value of the modulus of deformation changed in the range of 12–26 GPa. In addition, the variation in the discontinuity angle showed that the maximum value of the modulus in the state parallel to the bedding (0°) was around 24.3 GPa and the minimum modulus at the angle of 45° was equal to 10.4 GPa, which was in agreement with the in situ test results. Finally, by analyzing the experimental and numerical results, a correction factor was provided to verify the in situ results calculated from the ISRM relation at different depths of the borehole.