63. Successful Utilization of Double Shielded TBM in Low Cover Zone for Excavation of Inclined Pressure Shaft in Parbati H.E. Project – II (NHPC)

Tunnels are constructed under varying geological conditions ranging from very hard rock terrain to very soft ground conditions. Depending on the ground conditions the tunneling technique also changes. The Himalayan ranges adobe of glaciers and mighty rivers hold a great potential for development of hydroelectric projects. The 800 MW Parbati Project-II being a mega contribution of NHPC to the country in the field of hydroelectric power, is stretched over a vast area of lesser Himalayas. Himalaya one of the youngest tectonic mountain is considered to have a complex and diversified geology. The Parbati Project envisages to negotiate through different litho types of contrast characteristics. In addition to other tectonic imprints of Himalayan orogeny the structures are also laid across several minor and major thrusts. Keeping in view the versatility of ground conditions, different tunneling techniques have been adopted for timely and economic excavation of the underground structures. One of the components of Parbati Hydroelectric Project-II is the Twin inclined pressure shafts of 1546m each. The pressure shaft with finished dia of 3.5m are being lined with steel liners (Ferules) of thickness varying from 27mm on the top of shaft and 43mm in the bottom portion of the shaft sustainable to the high pressure generated. Out of 31.5 km of total HRT length, 9.05 km is being bored by Open Type TBM and the 1546m circular pressure shaft with inclination of 30o from horizontal has been successfully bored by Double shielded TBM. These two types of TBM were deployed keeping in view the rock mechanics of two different sites. In this paper we will discuss about the experiences gained while negotiating the inclined pressure shaft with Double shielded TBM. This is the first twin pressure shaft at 30o inclination with horizontal, bored successfully first time in the world.