Predicting the Critical Grout Pressure in Anisotropic Rocks Based on the Maximum Energy Release Rate Criterion

Abstract

The grout pressure is one of the most significant parameters during grout injection into a jointed rock mass. Applying a low injection pressure reduces the penetration length and causes poor injection, while applying a high injection pressure leads to unwanted deformations and instability of joints during grouting operations. In this study a new analytical approach was developed and examined and is proposed to predict the critical grout pressure by assessing the stability of cross-joints with grout boreholes in an anisotropic rock mass. Joint stability was measured based on the maximum energy release rate (MERR) criterion. The critical injection pressure is the pressure at which the joint approaches the instability threshold. The study area for validation of the proposed method was the anisotropic rock mass of Sanandaj Azad Dam. To collect the required data, a series of laboratory tests were performed, including the uniaxial compression test to determine the mechanical properties of rock and the Brazilian disk test to determine the fracture toughness of rock on specimens with anisotropy directions of 0°, 30°, 45°, 60°, and 90° relative to the loading direction. The results show a good agreement between injection pressures predicted using the proposed method and data recorded during grouting operations at different depths. Moreover, to study the anisotropy impact on the predicted grout pressure, a horizontal joint was evaluated at the prescribed depths with different anisotropy angles relative to its direction. The results suggest that increasing the anisotropy angle is directly related to changes of the critical injection pressure. Accordingly, a higher anisotropy angle relative to the crack growth direction leads to a greater critical grout pressure.