The effective stress law is widely used to model the deformation of crustal rock. It states that effective pressure, the difference between confining pressure and pore fluid pressure, dictates rock strength. In low-porosity rock, previous work has shown that the stability of faulting is controlled by the magnitude of pore fluid pressure rather than the effective pressure. To understand the effect of pore fluid pressure on porous rocks, I deformed water-saturated samples of Darley Dale Sandstone (~13-15% initial porosity) at strain rates from 10-4 to 10-6 s-1 while using a constant effective pressure of 10 MPa and pore fluid pressures ranging from 2 to 180 MPa. The results show that the shear strength is well-predicted by the effective stress law, but fault growth at Pf = 180 MPa resulted in pervasive grain comminution and stabilized fault propagation compared to all other lower pressure experiments.