Stability and potential failure mode of tunnels and underground rock caverns are directly related to the magnitude and orientation of the in-situ rock stress. In some cases, the high horizontal in-situ stress is essential in maintaining cavern stability, whilst in other cases the high rock stress may bring forth additional difficulties in tunnel construction and rock support design. It is crucial to take into account the in-situ rock stress in designing of the shape and orientation of underground works and selecting of excavation methods and rock support. With a number of examples of real projects the paper describes the impact of the in-situ rock stress on the tunnel/cavern stability and corresponding rock support design. The hazardous effects resulting from spalling and rock burst associated with very high in-situ rock stress are addressed with the example of the world longest road tunnel – the Lærdal tunnel.
Stress-induced instability is one of major concerns for the safe construction and operation of tunnels and caverns. This is true for both soft rocks and jointed hard rocks. Jointing controlled rock falls are also controlled by the stress condition in addition to the jointing geometry since the sufficiently high normal stress will prevent the rock block from falling even the geometry is not favourable.
The high horizontal stress may play a crucial role in maintaining stability of tunnels and caverns, which is particularly true for underground openings situated close to the ground surface. The Gjøvik Mountain Hall, which is 61m wide, 25m high and 95m long with the lowest rock cover of only 25 m, is an excellent example of using in-situ rock stress to maintain the cavern roof stability. Numerical analysis has demonstrated that it is the high horizontal stress that makes it possible to excavate such a large span cavern at such a shallow depth.