The values of the effective stress coefficients (α) for intact and fractured rocks were evaluated for three different rocks by conventional method and compared for peak and residual strengths evaluated by Modified Failure Envelope Method (MFEM), which was developed by the authors.
For Kimachi Sandstone, intact rock α values decreased with confining pressure and varied between 1 and 0.8. α value for fractured rock of the same sandstone was higher than the intact rock, reaching nearly one. α value for peak strength decreased from 0.8 to 0.4 with effective confining pressure under both single and multi stage MFEMs. For residual strength state, α value was between that for the peak strength and that for the intact rock.
For Inada Granite, α value for intact rock decreased with confining pressure from 0.9 to 0.7. α value for fractured rock was higher than that for intact rock and nearly one. Value for peak strength was obtained only by multistage test and decreased from 0.8 to 0.2 with confining pressure. The value for residual strength could not be obtained.
For Shikotsu Welded Tuff, the α value for intact rock slightly decreased from 0.95 to 0.90 with increasing confining pressure. The value for fractured rock was higher than α for intact rock and nearly one. MFEM could not be effectively used to determine the values for peak and residual strengths possibly due to pore collapse.
From the above results, it can be concluded that the multistage MFEM is very effective to obtain the value for peak strength. The choice of coefficient values was proposed for stress analysis and failure evaluation, in intact rock structures or structures in rock mass.