Application of Mohr-Coulomb failure theory to very porous sandy shales

摘要

Application of Mohr-Coulomb's failure theory to the studied sandy shales is impossible without the consideration of porosity. In this study, the slope (friction angle PHI) and the intercept (cohesion c) of the tangent to the Mohr-Coulomb failure envelopes were determined unequivocally only after these envelopes were constructed separately for each of eight sets of rock samples selected in such a way that these sets differ from one another by the value of their porosity. It is asserted that the Mohr-Coulomb failure theory can be applied to the studied sandy shales when porosity is taken into account. Observations show that the peak axial stress (sigma_(1p)), cohesion (c) and internal friction angle (PHI) are well correlated with porosity in each of eight selected sets of the studied rock samples. As a result, it is found that the increase in porosity value leads to a decrease in the values of sigma_(1p), c and PHI, and these relationships can be described by the following dependencies: Power dependence between the porosity (n) and cohe sion (c). Linear dependence between the porosity (n) and angle ofinternal friction (PHI). Simplified dependence of the peak axial stress (sigma_(1p) on the porosity (n) and confining pressure (sigma_3) based on theMohr-Coulomb failure theory.