A new approach to evaluate effective stress coefficient for strength in Kimachi sandstone

• Modified Failure Envelope Method (MFEM) for rock is developed.
• Effective stress coefficient for peak and residual strengths for rock is evaluated.
• Multistage triaxial compression test is utilized.
• The effective stress coefficients are found to be dependent on confining pressure.
• The effective stress coefficients are compared with Biot׳s effective stress coefficients.

This paper is devoted to the experimental investigation of effective stress coefficient for peak and residual strengths of saturated Kimachi sandstone. Authors have described the Modified Failure Envelope Method (MFEM), which can be used to obtain the effective stress coefficients for peak and residual strengths (α-Peak and α-Residual). The effective stress coefficients for intact and fractured Kimachi sandstone (α-Biot׳s and α-Fractured) were also evaluated using conventional methods, and the data were compared with the coefficient values obtained by MFEM for the peak and residual strengths. The effective stress coefficient for intact rock, α-Biot׳s decreased with increasing confining pressure, and was in the range 1>α-Biot׳s>0.8. The effective stress coefficient for fractured rock, α-Fractured, was larger than that for intact rock and was close to unity. The effective stress coefficient calculated for peak strengths, α-Peak, using both the single and multistage MFEMs decreased with increasing effective confining pressure and was in the range 0.8>α-Peak>0.4. For residual strength states, effective stress coefficient, α-Residual, was between the peak strength value and that for intact rock. Based on the results, multistage MFEM is suitable for obtaining an effective stress coefficient for the peak strength, α-Peak. An equation to obtain the effective stress coefficient from total confining pressure and pore pressure, and a method to choose the coefficients for elastic stress analyses and failure evaluations for intact rock structures or structures in rock mass were proposed.