A hard rock tunnel case study: Characterization of the water-bearing fracture system for tunnel grouting

Grouting is a common method for reducing water inflow in tunneling projects in hard rock. A good grouting design should be adapted to the water-bearing fracture system in the rock mass. The aim of this work is to present a case study using relevant parameters, to suggest a combination of tests to provide them, and to show how a conceptual model of the water-bearing fracture system can be chosen based on them. The case study, to which the proposed methodology has been applied, is a section of the TASS tunnel at 450 m depth in crystalline rock in Aspo Hard Rock Laboratory in Sweden. In this tunnel project an extensive number of tests provided a fine opportunity for analysis and discussions. A set of functional parameters to describe the rock mass for grouting purposes has been suggested. These are: the hydraulic head h; the hydraulic apertures b of the fractures; fracture frequency P-10; the number and orientation of the major fracture sets; and the flow dimension D-q. A set of investigation methods is suggested, aimed at providing the necessary information for high-precision tunnel projects with strict inflow requirements such as the tunnel project studied. Based on these parameters, it is suggested that a conceptual model of the water-bearing fracture system in the rock mass should be chosen. It is shown how this was done in the case study, first in the investigation stage and then an update in the construction stage. Possible ways to adapt the grouting design to the conceptual model are suggested. Selective grouting may be considered for a rock mass with a 2D fracture system, one dominating water-bearing fracture set, when the tunnel orientation is close to perpendicular to this set. For rock mass with a 3D fracture system, two or more water-bearing fracture sets, systematic grouting may be preferable to avoid "moving the water leakage" from one tunnel section to a neighboring section. When the tunnel intersects a fracture zone the focus should be on sealing this zone