A statistical grouting decision method based on water pressure tests for the tunnel construction stage – A case study

In tunnel construction, a common method to reduce water inflow is pre-grouting. There is a need for a practical method, based on information about the rock mass, to assess whether inflow will remain below the set inflow requirements q(A), to form the basis for deciding at each tunnel front whether to grout or to proceed with excavation. The aim of this paper is to present a method for use at the tunnel construction stage to decide whether to grout or not. On the one hand, the aim is for it to be practical to use whilst on the other hand it must be scientifically founded and independent of the experience of the person responsible for making the decision. The input for the decision method consists of evaluated results from water pressure tests in a small number of tunnel front boreholes in combination with the specified inflow requirement for the tunnel. The method is statistically supported and is based on an equation for inflow prediction. To illustrate the decision method, it was applied to a 1 km section of the Onkalo Tunnel (built for Finland's spent nuclear fuel repository). The decision method results were compared with the results of decisions taken during the construction of the tunnel section and agreement between the two was investigated. The main findings of the paper are: For the case study tunnel section the same decisions would for the most part have been reached using the suggested decision method compared to the method that was actually used. The tunnel achieves the inflow requirement, indicating that the decision method would have led to satisfactory results. Further, a key advantage of the method is that it is transparent and scientifically based. Finally, one constraint on the method is that it is not recommended if a large part of the flow takes place in flow channels or if the probe holes are unlikely to intersect the water-bearing fractures. To increase the understanding of grouting in the Onkalo Tunnel, the grouting pressures were compared to the rock stress and a study was made of grout take in relation to the estimated hydraulic apertures of each grouting fan and to the expected grout penetrability.