Widening and deepening adjacent large shafts in rock

The influent pumping station (IPS) at Bandra plays a key role in the Mumbai Sewerage Scheme. Bandra IPS will be one of the largest pumping stations of its type in the world, with a pumping capacity of nearly 22 m 3 /s. The rock support scheme for the widening and deepening of three existing shafts is described, with particular attention being given to the scheme for the narrow zone of rock between the two pump shafts, termed the ‘narrow neck’. Inherent site conditions and the demands of a new station on a prescribed station layout have placed unusual constraints on the customary rock support methodology. Several excavation and rock support alternatives are discussed and the limitations of routine design methods are highlighted. The finally agreed scheme was a progressive and carefully planned rock support and excavation sequence that considered the possibility of localised instability during the excavation works and provided contingencies for several possible events. A concrete bridge was constructed to restrain the existing retaining walls and through-going grouted bolts continuously supported the whole rock section, before, during and after excavation. Inspection and monitoring was completed before each stage of excavation. The rock face and retaining walls remained intact and no serious failures were experienced during excavation. A brief discussion of possible rock design alternatives, given a virgin site of similar rock, follows the conclusions. Rock excavations are now complete and the civil works at Bandra IPS are nearing completion.