Hindustan Construction Company Ltd (HCC) achieved a major breakthrough by day lighting the Maroshi-Vakola tunnel stretch.


Pic: Rons Bantwal

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Mumbai: Hindustan Construction Company Ltd (HCC) achieved a major breakthrough by day lighting the Maroshi-Vakola tunnel stretch, which is the longest section of the Maroshi to Ruparel College water tunnel.

This section is now being concrete lined and tunnel excavation has commenced on the Vakola-Mahim section. The entire project is scheduled to be commissioned by Aug-2012.

HCC has constructed 21 kms of existing 26 kms water tunnels that are designed to supply drinking water to Mumbai. The contractor is currently constructing 12 kms of tunnels of the 23 kms of water tunnels under construction.

The 12 km long Bhandup Complex to Charkop tunnel was the last tunnel completed by HCC. Despite extremely difficult geological conditions, the contractors completed this particular tunnel 5 months ahead of schedule.

 

HCC was also awarded the contract for the Maroshi-Ruparel College Tunnel Project by the Municipal Corporation of Brihan Mumbai with a Contract value of over US$90 million. This is a Water Supply Project executed under Jawaharlal Nehru National Urban Renewal Mission (JNNURM) and funded by The Government of India, Government of Maharashtra & Municipal Corporation of Greater Mumbai.

The Maroshi-Ruparel tunnel work consists of construction of tunnel having 3.60 m diameter and 12.24 km long. Excavation is with TBM (tunnel boring machine) from Maroshi connecting to Ruparel College. The tunnel is connected to vertical shafts at Maroshi, Vakola and Mahim. The shafts are 12 m in diameter and typically 80.00 m deep.

Two, 100 m long, tunnel boring machines, weighing 150 t each, were lowered in the shaft piece-by-piece and assembled at the bottom before commencement of tunnelling work. Progress was, on average, 20 m of tunnel completed in a single day. The record was 40 m tunnelling completed in a single day with one tunnel boring machine.

The length of tunnel is divided into three sections namely, Maroshi-Vakola (5.83 km long), Vakola-Mahim (4.55 km long) and Mahim-Ruparel College (1.86 km long). An interesting aspect of the Maroshi-Vakola tunnel section is that it passes through Mumbai Airport and crosses both the Runways of Mumbai Airport at around 70 meters below the ground level.

The tunnelling work, therefore, had to be monitored throughout this section to ensure that the work progressed without damaging the surface or causing disruption to any operations on the ground level.

The tunnel boring throughout was extremely challenging due to heavy seepages and varying rock strata. During the monsoon season, the tunnel seepage increased to as much as 25,000 m³ per day. HCC were, therefore, forced to do extensive grouting in order to eliminate seepage and possible contamination of the treated potable water the tunnels are designed to carry.

Another complication was weak sections of rock which necessitated widespread tunnel protection works to ensure safety. Despite the difficulties, the tunnel excavation in this section was completed within schedule.

Major Rehabilitation Programme

This project is part of the rehabilitation and improvement of the drinking water distribution system from Bhandup Treatment Works to the Western Suburbs and Southwest part of the Mumbai city. The distribution system comprises Vaitarana, Upper Vaitarana, Bhandup, Maroshi, Tansa (East) and Tansa (West) main pipelines.

Bhandup-Maroshi main pipeline runs up to Maroshi only and supplies water to the Veravali Group of Reservoirs. Other mains continue to carry water to Dharavi / Mahim and beyond.

The existing Vaitarana, Tansa (East) and Tansa (West) main pipelines run below runway and pass through airport land. As a result, for security reasons, maintenance has become impossible.

Furthermore, encroachment in other areas also necessitated rehabilitation of these mains. Under the circumstances, MCBM decided to construct a tunnel from Maroshi up to the existing line at Ruparel.

The tunnels will have many advantages over existing surface pipeline:

It is secured at a depth of 70 meters below ground level and cannot be tampered as it happens in case of surface pipelines. Hence less chances of leakages and pilferage of water. 

It will require minimum maintenance as the entire tunnel is made of concrete unlike surface pipeline which corrode over a period of time.

 

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