Design of grouting with silica sol in hard rock - New design criteria tested in the field, Part II

Journal article, 2007

An extensive field test was conducted in spring 2005 in the Törnskog Tunnel and consisted of design, execution and evaluation of a grouting campaign for 100 m of tunnel. The field test was part of the normal construction of the tunnel. This paper describes how the design of the grouting was coupled with the actual grouting procedure. A preliminary investigation of the tunnel showed that the location where this field test was conducted had a large fracture zone. A drill-core was taken in this zone and hydraulic tests were performed. From the evaluation of the rock core a fracture aperture distribution was assessed and the grouting design was focused on this part of the tunnel. The evaluation showed that apertures down to 14 μm needed to be sealed to cope with stipulations set for leakage into the tunnel (2 l/min and 100 m of tunnel). A design was made based on silica sol, where a critical penetration length was decided and the layout of the grouting fan could be determined. A new design was chosen, with a specific pumping time of 30 min at a grouting pressure of 1.1 MPa. The design worked well and the water ingress was reduced. A drip characterisation in both tunnel tubes was made. One tube was grouted with silica sol and the other with cement following a more traditional approach. The drips were both larger and more frequent in the cement-grouted tube than in the silica sol-grouted tunnel. Eight out of nine fans grouted with silica sol showed a significant sealing effect. For one fan the design was not followed. Instead, the workers used the more traditional method, i.e. only pumping until the design pressure was reached, which produced a poor result. This paper demonstrates the efficiency of design methodology that takes into account the hydraulic apertures and that the required minimum penetration length can be coupled to the apertures when formulating the grouting criteria.