Geology, water inflow prognosis and grout selection for tunnel sealing: Case studies from two tunnels in hard rock, Sweden

Successful sealing of tunnels by injection of grout into the fractured rock demands knowledge concerning the geological and hydrogeological conditions. This paper aims at presenting how a description of the geology and hydrogeology can be used as a basis for a grouting design. Important issues that are considered here are: identification of the extent of the grouting based on inflow requirements and a prognosis of tunnel inflow; and selection of grout based on the performance of grouting materials. Two tunnel sections from two different tunnels in crystalline rock were used as case studies. The first tunnel section (70 m) is found in Äspö Hard Rock Laboratory (south east Sweden) at 450 m depth. The rock is sparsely fractured and the main rock type is a diorite. The second tunnel section (36 m) is found in the Hallandsås tunnel (south west Sweden) excavated through a horst and at approximately 100 m depth. The fracture network is well connected. In this section the rock consists of gneiss and amphibolite. A conclusion based on the two case studies is that the principal descriptions of the geology and hydrogeology for the two tunnel sections identify general differences between the two tunnels. This explains in part the deviation found between early inflow prognoses and the measured inflows. Further, the estimated hydraulic aperture from hydraulic tests can be used as a basis for selection of grout. Finally, the fracture frequency or the variation in fracture frequency (e.g. between neighboring borehole- or tunnel sections) in combination with hydraulic tests indicate what areas to focus on when grouting