Ahmad Ghassemi, Xejun Zhou

In this work, poroelastic properties of Sierra White granite are determined, including elastic moduli, Biot's effective stress coefficient, α, and Skempton’s pore pressure coefficient, B. The Biot’s coefficient of this rock was determined using two different approaches. It is found that overall, the Biot’s coefficient decreases from 0.77 at low effective stress (3~4 MPa) to 0.45~0.55 at high effective stress levels (30~40 MPa). Unlike the Biot’s effective stress coefficient, which is related only to the solid rock, Skempton’s B is a property related to both solid rock and pore fluid. Although a single undrained hydrostatic compression test can theoretically be used to measure B, two types of laboratory tests were performed to fully reveal this parameter’s behavior. In the first test, the back pressure and confining pressure are increased stepwise to maintain a constant effective stress (usually very low), and Skempton’s B is measured at different back pressure levels. This test reveals the Skempton B behavior related to the pore fluid compressibility, and consequently, this test can be used to evaluate if the pore fluid is free of air. In the second test, the back pressure is maintained at a constant level, and the confining pressure is increased stepwise, and then Skempton’s B is determined at different confining pressure levels. This test correlates Skempton’s B with the effective stress. Details of the laboratory test protocol for combining these two types of tests are described and the results for Sierra White granite are provided. It is found that for Sierra White granite, Skempton’s B falls in a narrow range of 0.8~0.86 in the first test after the pore fluid is depleted of air; while in the second test it falls in a large range (0.86 to lower values depending on the effective stresses).