Tunnel Excavation and Support in Exceptionally Poor Rock Mass (Shear Zone) Condition in Lesser Himalaya, Nepal: Case Study of Upper Kabeli Hydropower Project (28.1 Mw), Taplejung Nepal
DOI:
https://doi.org/10.64862/Keywords:
Excavation, Tunnel, Heading and benching, Shear zoneAbstract
Thin to thick (few cm to several m) shear bands and shear zone are commonly encounter during tunnel excavation in lesser Himalaya. 25.5 m thick shear zone is encountered (Ch: 4+075.75-4+032.25) during excavation of Headrace Tunnel from outlet of UKHPP. This shear zone (weak section) consists of light greenish gray silty clay with some weathered phyllite fragments. Groundwater conditions during excavation is dry and the Q-value is exceptionally low (<0.01). The attitude of shear zone is 220-240/40-50 (DD/DA) and makes oblique striking with tunnel direction (008) and foliation (70-100/20-30). Sequential heading and benching excavation and immediate support installation works are adopted for the safe excavation of the weak section and 50 more days were spent to excavate the 25.5 m stretches. Non-core recovery probe drilling is conducted for the direct investigation of the onwards rock mass condition.
References
Sunuwar, S. C. (2006). Engineering geological problems of shear and fault zones in Nepal Himalaya. Journal of Nepal Geological Society, 34, 47–52. https://doi.org/10.3126/jngs.v34i0.31878
Sunuwar, S. C. (2016). Geological mapping in the Nepal Himalaya: Importance and challenges for underground structures. Journal of Nepal Geological Society, 51, 47–52. https://doi.org/10.3126/jngs.v51i0.24096
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