The effective pressure law for permeability during pore pressure and confining pressure cycling of several crystalline rocks

The permeability of samples of Pottsville sandstone, Pigeon Cove granite, and Westerly granite was measured while simultaneously cycling P c and P p . From these data, “tangent” values of α, the coefficient of the effective pressure law, were calculated at intervals during the cycles. Tangent α's were strongly affected by the order in which the increments in P c and P p were applied to the samples. When the pore pressure was changed before the confining pressure, α took on values near 1.0, whereas it was significantly lower in the other case. These results confirm observations previously made on two other granites and published elsewhere. However, this dependency on stress path decreased rapidly with the number of cycles. After a few cycles, α approached 1.0, which apparently favors the use of the simple difference, P c ‐P p , for evaluating the effective pressure.