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Assessment of Rock-mass Groutability in High-stress Metamorphics; Seepage Control and Resilience Intervention – the Upper Tamakoshi Headrace Tunnel Case Study, Nepal
 

Authors

Eirinaios Christakis, Sanjib Sapkota  


 

DOI

Abstract

Underestimation of geomechanical properties and a possible misinterpretation of the anticipated rock-mass behaviour during dynamic hydraulic operation of a pressurised underground structure, can either influence the performance of a hydropower scheme or even undermine the resilience and stability of the structure. This becomes apparent, especially where tunnels are constructed in complex geological environments; influenced by active tectonics, subjected to high external stresses, or intersected by adverse geological features and lineaments. A broadly known historical record of hydropower tunnel failures underline the necessity for proactive measures, in order to mitigate geological risk, control tunnels’ performance, and optimise their resilience.
High pressure barrier grouting constitutes a key intervention method for sections of low-quality rock-mass, in order to control internal seepage, mitigate out-leakage, address blast induced damages to the surrounding profile, and achieve synergy with the concrete lining segments.
Aiming to develop efficient and optimised grouting schemes, the authors discuss the role of barrier grouting as an intervention technique, and analyse the groutability of metamorphic formations (Orthogneiss) under high-stress environment, in the Higher Himalayas. The technique is applicable in unlined sections (i.e. shotcrete sprayed) as well as in transitions between concrete lined and unlined stretches. The analysis focuses on correlating the intrinsic rock-mass properties with the rock-mass response, based on real-time field testing and injection output (i.e. permeability testing and grouting data). The objectives of the analysis is to develop a supplementary tool for the pre-estimation of grouting requirements and the subsequent optimisation of grouting programme, tailored to the needs of the project.

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