The permeability of samples of Pottsville sandstone, Pigeon Cove granite, and Westerly granite was measured while simultaneously cycling Pc and Pp. 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 Pc and Pp 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, Pc‐Pp, for evaluating the effective pressure.
