Evaluation of in-situ stress state along the shotcrete lined high-pressure headrace tunnel at a complex Himalayan geological condition
Peer reviewed, Journal article
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Original versionGeosystem Engineering. 2021, 24 (1), 1-17. 10.1080/12269328.2018.1548306
The Himalayan region is renowned with its complex topography and active tectonic movement, which causes accumulation and sudden release of strain energy instigating changes in the in-situ stress state. This paper evaluates in-situ stress state at the Upper Tamakoshi Hydroelectric Project where shotcrete lined headrace tunnel with considerable hydrostatic head is being implemented. Start of the pressure tunnel at the downstream-end was proposed inside slope topography of about 300 m above valley bottom based on in-situ stresses measured by 3D overcoring at the test tunnel located at valley level. It was believed that pressure tunnel start location with a hydrostatic head of 420 m was inferred to have almost similar in-situ stress state measured at the test tunnel. However, minimum principal stresses measured by hydraulic fracturing were found insufficient to avoid hydraulic jacking. This manuscript evaluates in-situ stress state of the area and explores reasons behind much lower stresses than expected. Detailed assessment of the stress state using both measured data and 3D numerical analysis is performed, which revealed that the stress state has high degree of spatial variation even at similar overburden conditions. The analysis further demonstrates that presence of local shear zone has considerable de-stressing effect.