Topographic Effect in the Selection of Unlined Pressure Tunnels / Shafts - A Review of a case from Nepal Himalaya

The design principle behind unlined pressure tunnels/shafts is that the minimum principle stress should be higher than the hydrostatic head acting on the periphery of tunnel/shaft. So, reliable estimation of in-situ stresses is the key while implementing the unlined tunnels/shafts concept. In addition to the gravity induced stresses, the overall stress state in any location under consideration is influenced by the tectonic activity at the earth’s crust and topographic condition. Moreover, magnitude of tectonic stress level is largely influenced by the local tectonic and geological environment such as faulting and shearing activities. The Himalayan region is renowned with its active tectonic movement (earthquake activities), which causes accumulation and sudden release of strain energy instigating changes in the stress environment. This paper evaluates in-situ stress state at the Upper Tamakoshi Hydropower project in Nepal, where unlined/shotcrete lined headrace tunnel with considerable hydrostatic head is being implemented. A detailed assessment is made on the in-situ stress state by using both measured data and three-dimensional numerical analysis. The causes of the variation in the in-situ stress state are evaluated and discussed in detail.