Permeation grouting of sedimentary rocks is often used to reduce the permeability to control water inflows in relation to underground construction projects such as stations and shafts for railway tunnels. The method typically involves injecting cementitious grout which is perceived to fill pre-existing open fissures and joints. In practice these fissures are not uniformly continuous and the use of higher pressures often results in hydraulic jacking to open the pre-existing joints or weaknesses to permit the cement to penetrate further. The oil industry have for more than fifty years used the technique of hydraulic fracturing to increase permeability. The changeover in behaviour between placing cement particles in fissures to block them and using sand to prop them open is considered in this paper. It is postulated that some of the lower permeability grouting targets (around 10~(-7) m/s) can actually result in higher permeabilities. It is suggested that this is because of the propping effect of cement grouts. Case histories and detailed studies are described to support this hypothesis and illustrate how it may occur in practice.
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