Om tommelfingerregler og konusplassering i norske kraftverk

A key requirement for unlined pressure tunnels is to ensure that the rock stresses exceed the internal water pressure from within the tunnel. Failure to fulfil this requirement can cause hydraulic jacking of the tunnel with excessive leakages and associated large costs for reconstruction or rehabilitation. Decisions on where to place the transition between steel-lined and unlined tunnel were in the past made based on a crude evaluation of the weight of the overburden, under the assumption that this weight would correspond to the least principal stress in the rock mass. Such design criteria, chiefly developed in Norway, are internationally known as The Norwegian Criterion for Confinement. Despite their previous widespread use, there is strong evidence supporting that their use for final design of pressure tunnels should be avoided. A comparison of the minimum principal stress, as found from in-situ rock stress measurements, with that calculated from the Norwegian Criterion for Confinement has recently been performed for 15 Norwegian hydropower plants. This comparison shows that for the 15 investigated cases, seven showed measured values lower than expected from the overburden criteria, a clear reminder that actual measurements are absolutely required for safe location of the unlined parts of pressure tunnels. The work presented is part of the hydropower research being performed at HydroCen, based at NTNU in Trondheim, Norway.