Design of unlined pressure tunnels in Norway – limitations of empirical overburden criteria and significance of in-situ rock stress measurements

A key requirement for unlined pressure tunnels is confinement, i.e. that the minimum principal stress must exceed the internal water pressure. Failure to meet this requirement can cause hydraulic jacking of the rock mass, leading to excessive leakages. Before reliable methods for measuring the minimum principal stress were available, engineers designing unlined pressure tunnels used various overburden criteria to assess rock stresses based on an assumed link between the weight of overburden and the minimum principal rock stress. Despite the usefulness of overburden criteria during a preliminary project phase, their simplifications and assumptions strongly limit their reliability for final design of pressure tunnels. Rock stress data from 15 modern Norwegian Hydropower plants presented in this paper show how the minimum principal stress magnitude, as estimated from overburden criteria, can differ significantly from the stress magnitude originating from in-situ measurements. The recognition that overburden is no reliable indicator of in-situ stress has emphasized the need for performing rock stress measurements. In Norway, hydraulic methods have been preferred for measuring stress related to pressure tunnel design since the early 1980s. Due to budgetary limitations testing is typically limited, however, to one or two critical locations. This is an unsatisfactory situation given the potentially large variability of stresses over relatively short distances. It is therefore proposed to investigate alternative methodologies to establish a more “continuous” stress log along the pressure tunnel. To achieve this, we plan to further develop the “simplified jacking test”, a cost-effective method occasionally applied in Norway to get a crude estimate of the jacking pressure. The aim is to develop this method into more standardized forms and to investigate a potential correlation to the more established ISRM hydraulic fracturing and jacking tests. The research presented in this paper is part of the Norwegian hydropower research centre HydroCen, based at NTNU in Trondheim, Norway.