A National Record in Urban, Hard Rock Conditions

Project Overview

The Mumbai Water Supply Tunnel runs between the Kapurbawdi and Bhandup areas. The tunnel provides the city’s approximately 20.5 million residents with a reliable water supply, even during the seasonal monsoons that regularly contaminate Mumbai’s water resources. The basalt rock tunnel alleviates Mumbai’s current leakage problems from its aging lines and provide inhabitants with a consistent flow of clean drinking water.

Geology

Abrasive basalt rock and some fractured ground with potential for water inflows.

Onsite First Time Assembly

Onsite First Time Assembly (OFTA) was used to assemble the main bearing, lube system, back-up decks and horizontal, vertical and stacker conveyors. OFTA saved the contractor both time and money by assembling the parts at the jobsite and eliminating pre-assembly at the manufacturing facility in Shanghai, China. The OFTA process took place at the shaft bottom in a 100 m (328 ft) long starter chamber and a 50 m (164 ft) long tail tunnel. TBM components were lowered into the shaft using mobile and gantry cranes.

Excavation and Breakthrough

Due to the urban location of the tunnel, the TBM was launched from a 109 m (357 ft) deep shaft, and its launching sequence included an initial start-up excavation of 50 m (164 ft) with vital back-up decks connected to the TBM using cables. The first bore began March 30, 2012 and upon completion, the decks were lowered and a continuous conveyor system was installed for muck haulage and storage.

Robbins provided both the TBM and conveyor system for the project, as well as Field Service personnel to monitor the equipment and assist with daily upkeep and inspection.

Although the machine was ultimately a success, it did experience its fair share of challenges during the 21-month bore. Difficult ground, including basalt rock, fractured ground, and water inflows, was encountered throughout the tunnel. The tunnel team took all precautionary measures and advanced slowly. The crew maintained good ventilation throughout execution and utilized consistent dewatering to deal with water inflows.

Ground support also played a critical role in poor ground: The rock support system and ring beam erector reduced downtime and stabilized rock. Challenging ground conditions, combined with the sheer depth of the 109 m (357 ft) tunnel, made the machine’s excellent advance rates a particular achievement.

By the end of TBM tunneling, the Robbins machine had reached high rates of 870 m (2,855 ft) per month and 58 m (188 ft) per day, both records for TBM tunneling in India. The contractor stated that the good rates were achieved because of “good performance of the machine and a conveyor system for muck haulage in place of conventional methods.”