Major Cavity Formation and its Rehabilitation Measures in Head Race Tunnel of RHEP-412 MW – A Case Study

Hydropower projects developed in mountainous regions throughout the world are generally associated with adverse and challenging geological conditions which in some cases manifest themselves into formation of large underground cavities. Formation of large underground cavities is broadly associated with either weak rock conditions which are very common in complex geological environment or structural aspects of rock mass i.e. the orientation of major prevailing foliation trend. Rampur Hydroelectric Project (RHEP) is located in northwestern part of Lesser Himalayas and is also associated with complex geological formations having considerable anisotropy associated with different types of rock formations. Most of the head race tunnel of RHEP was excavated in poor to good rock conditions in correspondence to occurrences of different rock formations. The present paper mainly describes weak/extremely poor rock conditions and their associated cavities that were encountered during heading excavation of downstream part of head race tunnel from Kasholi which also forms a critical part of 15.177 km long HRT at RHEP. These weak rock conditions and cavities finally manifest themselves into formation of a major cavity (25m*30m) which was later rehabilitated successfully. The case study also brings out different methodologies adopted during rehabilitation of this large cavity. Moreover, insight is also presented regarding excavation of a parallel Bye-pass adit form the left wall of main adit at Kasholi. This paper also emphasizes application of geotechnical projections which play vital role in decision making such as introduction of Bye-Pass adit, which not only ascertain scheduled commissioning of the project but also act as another access face for managing the major cavity from other end. Conclusions are also presented referring the structural association of medium scale cavities with the final major collapse.