Rock characterisation for a road tunnel in Mumbai (India)

The Mumbai Coastal Road is a freeway currently being constructed along the western coast of Mumbai. A key element of this route is a twin-bore tunnel with total length of about 3km (a 2km TBM-excavated central section, plus cut-and-cover ramps at both ends). The tunnel alignment underpasses Malabar Hill and then runs under Chowpatty Beach for most of its length. The central section is located below the sea level with ground covers ranging from a maximum of 70m at Malabar Hill to less than 15m under Chowpatty Beach. Each bore, excavated by Tunnel Boring Machine (TBM), has an internal dimeter of 11m and includes a section with a horizontal radius of curvature tighter than 400m. This paper describes (i) the desk studies and the geological survey which provided the basis for an additional ground investigation plan, (ii) the results of the additional boreholes, in situ test and laboratory test, (iii) the overall characterisation of the rock mass (and of the overlying soils) based on the entire set of old and new data. The rock characterisation provided the fundamental information for the TBM type selection, and for the design of all the underground works. The launching chamber and the first part of the TBM drive are located within a Basaltic lava flow, while the second part of the alignment is set in Volcanic Breccia underlying the fine-grained soils and made ground of Chowpatty Beach.

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