Case study of crosshole GPR tomography for grouting distribution in rock fractures
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The Nordic method for tunnelling consists of drill and blast, pre-grouting and rock support adapted to the geologic conditions. Extensive use of pre-grouting is normally applied to limit water leakage. The injection is stopped at a predetermined pressure or total volume of injected grout (Stille, 2015). However, little is known about the dissemination of the grout within the host rock. The aim of this project is to better understand the grouting distribution in the fractures. The research is associated with the research program True Improvement in Grouting High pressure Technology for tunnelling (TIGHT) funded by the Norwegian research council. The objective of TIGHT is to increase understanding and subsequently implement efficient grouting control in tunnelling. The ultimate purpose is to reduce material consumption and time spent for grouting. Geophysical instrumentation was developed to carry a crosshole Ground Penetration Radar (GPR) investigation in a pre-grouted tunnel wall. The GPR tomography results have been compared to the data recorded in the drilled cores. Preliminary results indicate that crosshole GPR can provide valuable information on the distribution of fractures filled with grout or filled with water.