The Shocking Behavior of Fluid Flow in Deformable Joints

Abstract

If fluid is injected into joints in rock masses, several types of joint deformations can take place. At first the pressure rise in the joint is small enough that the joint does not actually open. Nevertheless the effective closure stress, i.e. the difference between the total earth stress acting normal to the joint plane and the fluid pressure, is reduced. Consequently the tightness of joint closure is lessened, resulting in a small increase of the effective open space, or aperture, of the joint. If the fluid pressure rise is small enough, the aperture can still be treated as nearly constant, and the preserve response therefore follows the usual laws of linear diffusion. But if the pressure increase is large, aperture increases must be accounted for, and the flow will be affected by nonlinear diffusion due to pressure dependant aperture, as well as a new storativity term due to joint compressibility. Eventually the fluid pressure may attain a value equal to, and even slightly greater than, the original total earth stress, and the opposing surfaces of the rock that meet at the joint can actually part. We refer to this behavior as joint "lift off", or "jacking." During lift off, the change in joint aperture and compressibility are very large compared to changes while the joint is still in roughness to roughness contact and the flow equation becomes so highly nonlinear that pressure pulses are no longer transmitted in a smooth, diffusive manner, but more like a propagating shock wave.