Effective Stress Coefficient through MFEM and Confining Pressure Dependency of Bibai Sandstone

Fluid in porous rocks affect the performance of rock structures and thus it is important to determine the effective stress in wide range of rock engineering applications. In this study, Bibai Sandstone, a hard-clastic rock, was subjected to multistage and single stage triaxial tests to determine the coefficients of effective stress corresponding to peak and residual strength states ($\alpha_{-\text{Peak}}$ and $\alpha_{-\text{Residual}}$) through, Modified Failure Envelope Method (MFEM). The coefficients of effective stress of intact and fractured Bibai Sandstone specimens ($\alpha_{-\text{Biot}^{\prime}\mathrm{s}}$ and $\alpha_{-\text{Fractured}}$) were determined using hydrostatic compression test and compared with the values obtained for $\alpha_{-\text{Peak}}$ and $\alpha_{-\text{Residual}}$ by MFEM. Microstructure analysis was carried out on the images of blue resin impregnated thin sections using Imagej software and a post-compression macrostructure analysis was performed using micro-focus xray computed tomography (CT). $\alpha_{-\text{Biot}^{\prime}\mathrm{s}}$ and $\alpha_{-\text{Peak}}$ decreased with increasing confining pressure which was related to the microstructure of the rocks. Further, the applicability of MFEM were discussed.