Design criteria for permeation grouting in hard rock at great depths

Grouting at tunnelling in hard rock at great depths below the groundwater table, when constructing a nuclear waste repository for example, makes special demands on the strength of the grout in order to resist groundwater forces. The aim of this study is to distinguish the most important parameters to ensure that suitable grouting can be performed at large depths. Initially, laboratory studies were conducted in order to determine the most critical parameters. The results from the laboratory tests were used in a field study at the Äspö Hard Rock Laboratory in Sweden. The results indicate that the most important characteristics for the rock are the fracture aperture and the groundwater gradient and for the grout: a resolute grouting pressure and sufficient early strength. This implies that for cementitious grouts the yield stress is important and for non-cementitious grouts it is the shear strength prior to gelling. The results emphasise the need to perform hydraulic characterisation of the rock mass in advance. The results also show the importance of performing a proper grouting, especially at large depths where a decreased penetration length of the grout increases the risk of a high gradient and hence erosion of the grout