Holmestrand Underground Railway Station - Analysis of Groundwater Inflow and Methods for Water Sealing
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The planned new railway station in Holmestrand whose location will be inside a basalt plateau poses challenges concerning the grouting and water and frost protection. Based on grouting works in adjacent tunnels Snekkestad and Sjøskogen as well as the entrance tunnels of the station hall, a loosely attempt to predict the grouting sitatuation for the station hall will be made. Pre-construction investigations including CPTU, oedometer, piezometers and ERT tomography have hardly revealed any signs of inflows. The precipiation pattern causes seasonal fluctuations in the groundwater. Hydrologically, the Holmestrand plateau is naturally drained by the existing Holmestrand road tunnel. The Holmestrand plateau consists of column basalt and various soil masses cover its top. There exists a weakness zone that may cause problems for the station hall, meaning that grouting will have to be extensive to maintain the inflow criterion. By the aid of a formula of calculating the expected inflow for the station hall, a strict requirement emerges when comparing this to a standard double-lane railway. Grouting works will take place at an inflow criterion of 5 L/(min/(100 m)) by pre-grouting fans. Should difficult geological conditions occur, control fans, extra fans and post-grouting fans may be applied. Measuring water loss and inflow may happen, both by construction of dams in the tunnel and directly measuring the inflow from boreholes. Alternatives to grouting include concrete lining, etc. Likely water and frost protection solutions are PE foam plates, concrete elements, sprayable membranes such as the BASF masterseal 345, etc. The latter has been tested in a frost laboratory and the Gevingåsen tunnel, where the most important conclusions are that the freezing index has no effect of how deep the zero-degree isotherm penetrates into the rock mass and that the temperature pattern is not cumulative. The tunnel results show no significant leakages. Also, the BASF masterseal 345 shows high deformation load capacities.Results from the grouting show highly varying trends according to the difference in geology, both when studying the distribution of grout amounts and the pressure per borehole per grouting fan and the total amounts of grout mass and the average pressure per grouting fan. In areas of the cross-section with low-quality rock the pressure seems to decrease and the amounts of grout mass increase. Also, the inflow criterion does not always control the amounts of injected mass, as the overburden and surface installations, as well as the rock mass and joint system situation play an important role. 3D measurements giving the rock stress distribution may also serve as guidelines for the grouting works, as will the vertical joint situation of the station hall. The water and frost protection solution is likely to be either the PE foam plates or the concrete elements. The author argues that the BASF masterseal 345 shows decent qualities but requires more testing before being applied at such a project.