Grouting of fractures using oscillating pressure

In connection with the storage of spent nuclear fuel in the Swedish crystalline rock a new technology for the grouting of-very narrow — 100 )µm — fractures is being developed., The method is based on bentonite or cement with a low water content as grouting fluid and a very high static pressure — order of 2 MPa — superimposed by a high-frequency oscillating pressure (with a very large amplitude), the latter for facilitating the entrainment process as oscillations of the grouting particles will significantly decrease the viscosity of the grouting fluid. This paper discusses the grouting methodology, some initial laboratory and prototype studies and some initial efforts in describing the entrainment process using pressure transient theory for fluids. This theoretical approach makes it possible to calculate the variation of the vibrational amplitudes of the grouting particles along the fracture and thus the changing viscosity. A special emphasis is put on the determination and description of the viscosity properties of the grouting fluids as the viscosity will strongly affect the entrainment efficiency due to the very small fracture aperture and the high viscosity of the grouting fluids. Field tests performed in the granite in the Stripa mine yield very good results and it is expected that this technology could be used in a number of special applications where a very low hydraulic conductivity is required but also as an appealing alternative to conventional methods in rocks with large-aperture fractures.