Geological Risk Assessment and Prevention Measures for Hydropower TBM Tunnelling Under High Cover

By virtue of its topography and snow fed peaks the lofty Himalayas naturally provide excellent sites for hydropower development. NHPC Limited, India’s largest hydropower development organization since its incorporation in year 1975 has constructed maximum number of hydropower projects in Himalayan region, which includes tunnel excavation through Drill and Blast method and by deployment of Tunnel Boring Machines for more than 500 km. Out of this the proportion of Head Race Tunnels (HRT) in particular is around 200 km. NHPC has successfully carried out TBM tunnelling under high cover for HRTs in its Kishanganga HE Project (MW) and Parbati HE Project (800 MW). Further, NHPC is constructing two parallel HRTs of 9.6 km each for its Pakal Dul HE Project (1000 MW), which would be the deepest tunnelling in country with TBM passing under cover of almost 2060 m. In this paper the authors will discuss about the preparedness to tackle the anticipated major geological issues and challenges during tunnelling with TBM under high cover in Pakal Dul HEP based on their experience in the above mentioned TBM tunnels. For long term sustainability of a hydroelectric project with deep tunnels for trans basin transfer of water and no provision of intermediate adit it is very important to carry out geological risk assessment in advance and ascertaining probable hazard scenarios prior to the deployment of the TBM, so that the TBM could be customized accordingly to handle adverse geological challenges. This paper includes the test results and interpretations on the basis of rock mechanic tests and TBM borability tests conducted on typical rock samples of all the lithounits to be encountered during TBM tunnelling in Pakal Dul HEP. The geological framework of the study area have been prepared on the basis of geological mapping carried out by author along the HRTs crossing an altitude of EL. ± 3700 M above MSL in the rugged Himalayan Mountains. Based on the experiences gained from TBM Tunneling in Dulhasti HEP, Kishanganga HEP, and Parbati HEP this paper not only highlights the probable Hazard Scenarios under high cover during excavation of HRTs with TBM in Himalayan Geology, it also discusses about the preventive measures to overcome such adverse geological conditions to minimizing time and cost overrun and construct a long-term sustainable project.