EFFECTS OF STRESS PATH ON BIAXIAL STRENGTHS OF SANDSTONES

Abstract

The effects of stress paths on compressive strengths of sandstones have been studied through the biaxial compression tests.  The specimens are prepared from three different sandstones: Phu Phan (PP), Phra Wihan (PW) and Phu Kradung (PK) formations.  A true biaxial loading device is developed to test rock specimens under biaxial stress states. The specimens have the nominal sizes of 5.5×5.5×5.5 cm³.  Three different stress paths have been implemented under σ₃ = 0: path I σ₁ increases while σ₂ is maintained constant; path II σ₁ and σ₂ equally increase; and path III σ₁ increases while σ₂ decreases.  For the first stress path, the intermediate principal stress (σ₂) is varied from 0 to 70 MPa and for the other two, the mean stress (σ_m) ranges from 10-45 MPa.  The results indicate that different stress paths show different strength results.  The intermediate principal stress (σ₂) notably decreases the rock strengths.  The modified Wiebols and Cook strength criterion can predict the biaxial compressive strengths well.  The regression analysis results are 0.95, 0.93 and 0.96 for PP, PW and PK sandstones.  These findings are useful to extrapolate the conventional laboratory test results (σ₁>σ₂, σ₃=0) to some actual in-situ condition where σ₁≠σ₂≠σ₃.