Pore Pressure Effect on Compressive Strength of Low Porosity Rocks

Abstract

The objective of this study is to experimentally determine the effects of pore pressures on the compressive strengths of Tak granite, Lopburi marl and Lopburi marble.  Failure strengths are determined for various stress rates and confining pressures both under dry and saturated conditions.  The applied axial stresses are controlled at constant rates of 0.001, 0.01, 0.1, 1.0 and 10 MPa/s.  The confining stresses are maintained at 0, 3, 7, to 12 MPa using a polyaxial load frame.  The specimens are prepared to obtain rectangular blocks with nominal dimensions of 50×50×100 mm³.  The results indicate that the strengths of the saturated specimens are lower than those of the dry specimens, particularly under high confining pressures and high loading rates. The elastic modulus of dry specimens is higher than that of the saturated specimens. Based on the Coulomb criterion, the cohesions of the dry and saturated specimens are comparable. The dry specimens yield slightly higher friction angles than the saturated specimens. The distortional strain energy density required to fail the low porosity rocks under dry condition is slightly higher than that under saturated condition.