The injection of TASS-tunnel. Design, implementation and results from the pre-injection

The Swedish repository facility for spent nuclear fuel will be placed in crystalline rock at a depth of 400-500 m. In order to limit groundwater inflow to the facility, grouting is planned. To comply with the stringent material, execution and inflow restrictions, a series of research and development projects concerned with rock characterization for grouting, grouting materials and grouting design have been carried out by the Swedish Nuclear Fuel and Waste Management Co (SKB). The understanding and methods developed were tested in the sealing project carried out at Aespoe Hard Rock Laboratory (Aespoe HRL), Sweden, and presented in this report. The project consisted of the construction of a short tunnel at 450 m depth, the TASS-tunnel. Grouting was conducted as pre-grouting using ordinary grouting fans outside the planned tunnel contour, as well as with grout holes entirely within the planned contour. A cement-based low-pH grout and a silica based grouting agent, silica sol, were used. The methodology used included determination of the fracture transmissivity distribution; identification of the smallest hydraulic aperture that needs to be sealed; grout selection based on fracture aperture and grout penetrability; design of grout hole geometry, grouting pressure and time in order to achieve the penetration length required; and monitoring of the actual execution based on inflow in control holes with subsequent design revision. Special concern was given to equipment and execution due to the high groundwater pressures, 3.5 MPa. Before starting the construction, inflow to core drilled holes along the tunnel position amounted to 45-90 liters/minute. The project showed that it was possible to limit the inflow to the target value 1 liter/minute per 60 m tunnel.