The effective pressure law for permeability in Chelmsford granite and Barre granite

The permeability was measured of three samples of Chelmsford granite cored in mutually perpendicular directions, while varying the conditions of confining pressure Pc and pore pressure Pp. At intervals along the prssure path “local” values of the coefficient α of the effective pressure law (Peff = Pc−αPp) were calculated. The values of α ranged between 0.6 and 0.7 for the three samples, showing almost no directional effect in Chelmsford granite. Similar procedures were applied on two samples of Barre granite. The measurements were made while decreasing Pc (unloading) as well as increasing Pc (loading). Large hysteresis in permeability was observed, and α was found strongly stress-history dependent (or path dependent). A simple model based on frictional sliding inside the rock seems to explain well these observations. Also, the data suggest a decrease of α with increasing confining pressure in both rocks. This can be interpreted in terms of changes in the geometry of the cracks during closure.