The desilting chamber with a dimension three times that of the head race tunnel was constructed through CZ 1 . This zone experienced falling of small broken rock pieces from the roof and the walls, immediately after blasting. The CZ 2 is located in the core of a syncline with high strength gneissic rock mass occurring outwardly (Figure 2). Therefore, it is more fractured and sheared resulting in poor rock mass quality. Water seepage is reported from this zone. A tributary channel of the Teesta River is flowing above the surface of CZ 3 and affects the rock mass quality due
to high water seepage condition. Special precautions like pre-grouting, water sealing and immediate support installations were taken during the excavation period. The detail support parameters for these zones are analyzed in Section 4.