Figure 1

Axial stress, radial stress, pore pressure (left axis), and axial and radial strains (right axis), versus time, plotted for a complete test following the UPPD/M protocol. Sample ZRP-3A_123D is here first loaded to in-situ stress and pore pressure conditions (0–70 hr), then stabilized (70–105 hr), and finally depleted under uniaxial-strain boundary conditions (105–180 hr, with zero change in radial strain). The core of the UPPD/M protocol consists of three pore pressure steps (35 MPa to 25 MPa, 25 MPa to 15 MPa, and 15 MPa to 3 MPa) that are each followed by three inflation-deflation steps (note the spikes in pore pressure, radial stress, and responding axial strain). All inflation and deflation steps were sequentially numbered, e.g. the inflation steps number from #1 to #9, whereby steps #3, #6, and #9 were parameterized as purely elastic response. The tangent of the strain development with pore pressure change is taken as the uniaxial-strain compressibility C_m, either as total C_m, or elastic C_m, depending on the loading direction. The inelastic uniaxial-strain compressibility C_m is then obtained by subtracting the elastic C_m from the total C_m per pore pressure step.