Tunnel Grouting: Engineering Methods for Characterization of Fracture Systems in Hard Rock and Implications for Tunnel Inflow

Abstract: In tunnel construction in hard rock, a common method to reduce water inflow is to perform pre-grouting. The grouting design should be adapted to the current rock mass conditions as well as the stated inflow requirement for the tunnel project. The aims of the research project have been to increase the understanding of water-bearing fracture systems in three dimensions for grouting; to suggest methods for characterization of fracture systems (tests and their interpretations) so that data relevant for tunnel grouting are obtained; and to improve the method of inflow prediction so that important geological aspects, such as three-dimensional fracture networks, can be incorporated. This has been done by developing a conceptual model for water-bearing fracture systems in hard rock for grouting purposes. A study of parameters of central importance for this has been done, followed by a study of tests suitable to supply this information. Consequences of different fracture systems on grouting results have been discussed, and ways to adapt grouting design are suggested. As integrated parts of this a number of case studies have been analyzed. The major conclusions are: A relevant conceptual model of the rock mass is a good basis for grouting design and inflow prediction. Awareness of anisotropy is important: identification of the orientations of water-bearing fracture sets is essential both for inflow prediction and for grouting design. A statistically based decision method is suggested for the construction stage, to determine whether or not a tunnel section needs grouting.