The effects of an axisymmetric fluid jet impinging on a rock half-space are examined. A recently developed constitutive model for porous elastic materials, which explicitly provides for the compressibilities of the solid grains and fluid that comprise the material, is reconciled to the model developed by M. A. Biot for soils. An increase in pore fluid pressure is shown to reduce the compression in the rock matrix, displacing the Mohr's circle in the direction of the Griffith failure surface. Shear stress due to the subsequent radial flow of fluid, though small and neglected in all previous work, is shown to have significant effect. Shear stress increases the difference between the maximum and minimum effective principal stresses, enlarging the Mohr's circle toward the failure surface. Accurate predictions of threshold pressures to cause failure were achieved when the component compressibilities and shear stress were taken into account.