No repeat of Delhi fiasco here, Mumbai tunnels won’t flood’

By Prajakta Kasale, Ranjeet Jadhav and Rajendra B. Aklekar

The issues that plague the Pragati Maidan tunnel in New Delhi have raised concerns about the safety of similar projects in Mumbai like the Mumbai Metro Line 3 and Coastal Road, which are nearing completion, as well as the numerous infra works. The government authorities and agencies that are implementing these projects told mid-day that, taking into consideration the peculiarities of Mumbai, they have taken adequate measures to ensure water leakage and seepage do not happen.

Coastal Road

The 10.58-km Coastal Road is being constructed from the Princess Street flyover at Marine Drive to the southern end of Bandra Worli Sea Link. Twin tunnels have been built at a depth of 12 metres to 67 metres under Malabar Hill. Each tunnel is 12.19 metres in diameter and has 11 cross tunnels. The construction of the first tunnel started on January 11, 2021, at Priyadarshini Park.

Workers near the Worli connector of the Coastal Road on January 25, 2024. Pic/Satej Shinde

The excavation of the first tunnel was completed on January 10, 2022, and the work on the second tunnel began in April 2022 and was expected to be over by the end of 2022. After parts of the tunnel boring machine (TBM) got damaged, work was halted and finally completed in May 2023.

Engineers involved in the project said water ingress and the load of the Malabar Hill were the two major issues they faced while excavating the three-lane tunnels. A BMC official said that they dealt with leakage by using four-layer barriers. "Water ingress through tunnel lining was a big concern, especially for subsea tunnelling. In dealing with such risks, multiple barriers have been considered for tunnel design," said an official.

To make the tunnels leakage-free, workers carried out primary grouting or filling the gap between the segmental lining and the excavation surface with cement material that is injected simultaneously with TBM driving, secondary grouting or filling the cavity where the primary grouting is incomplete, used EPDM (ethylene propylene diene monomer, a synthetic rubber) gaskets to create a watertight system between segment rings and hydrophilic cord, which is expandable).

A civic official said, "One of the concerns when the TBM was moving underneath the Malabar Hill, was the stability of the tunnel. During the design stage, concrete segment tunnel lining had been designed to take huge loads sufficiently. It was also revealed that the rocks were basalt, so no major issues came up during tunnelling." The officer added that the water reservoir in Malabar Hill was a critical structure and it featured on the list of possible challenges. To overcome this, the possible impact on the structure had been investigated thoroughly through computer simulation it. Moreover, extensive instrumentation and monitoring had also been carried out.

Orange Gate-Marine Drive tunnel

The MMRDA has approved the appointment of Larson & Toubro (L&T) as the contractor for the construction of this vehicular underground tunnel, which will provide a traffic-free route between the Orange Gate at Eastern Freeway and Marine Drive, enhancing connectivity and reducing travel time.

The Eastern Freeway has significantly improved transportation to south Mumbai. However, the increasing amount of traffic at Orange Gate has become a challenge for the Mumbai Port Trust and the Main Post Office square on P D'Mello Road. To address these issues and create a seamless road-based transit system, the MMRDA plans to construct a 9.23-km corridor that will link the Orange Gate to the Coastal Road at Marine Drive.

The project includes a 6.51 km twin-tube underground tunnel, with each tunnel accommodating 2+2 traffic lanes, 1+1 emergency lanes, and pedestrian walkways. The project entails constructing a viaduct for vehicle access at the Eastern Freeway, open cuts and access roads on both sides of the corridor. The left-side tunnel will pass beneath the seaway and resurface after B D Somani Chowk.

This project has been planned considering the safety of underground Mumbai Metro Line 3. The depth of the tunnel will be about 40 metres from the ground level, ensuring it doesn't obstruct railway or high-rise buildings' foundations. Safety measures are paramount, given the project's proximity to heritage sites and densely populated areas in south Mumbai.

Thane-Borivli tunnel

The project will be constructed at a cost of over Rs 14,400 crore. It seeks to reduce travel time between Thane and Borivli faster and decongest Ghodbunder Road. The route will feature an 11.8-km-long connecting road and two 10.25-km-long three-lane tunnels built beneath SGNP, from Tikuji-Ni-Wadi in Thane to the WEH in Borivli.

The twin tube tunnels are expected to shave off 40 to 45 minutes of travel time and decongest Ghodbunder Road

Considering that the tunnel will pass through SGNP, the MMRDA will take precautions to avoid causing harm to the park's biodiversity such as using a TBM to avoid upsetting flora and fauna. The tunnel is expected to take five-and-a-half years to be completed from the day the tender is awarded.

There will be cross tunnels every 300 metres, and the design will allow vehicles to travel at a top speed of 80 kmph. The new route is expected to shave off 40 to 45 minutes of travel time and reduce the need for 10.5 lakh metric tonnes of fuel. The initiative will contribute to a 36 per cent reduction in carbon emissions. Special measures will be taken to ensure that the air within the constricted tunnel remains clean and fresh.

Mumbai Metro Line 3

The fully underground 33.5-km-long corridor from Colaba to SEEPZ in Andheri was envisaged to give respite from traffic to Mumbaikars and improve the transportation scenario in the financial capital. For phase one, between Bandra Kurla Complex (BKC) and Aarey, about 95 per cent work has been completed. The line is expected to be fully operational by May.

Workers look on as the MMRCL achieves the 35th breakthrough, at Dadar Metro station on December 16, 2020. Pic/Ashish Raje

Mumbai Metro Rail Corporation Limited (MMRCL) sources said that there is a 153-metre-long Metro tunnel below the Mithi river and the situation in this area is different from other areas due to the water above.

The New Austrian Tunnelling Method (NATM) has been utilised in three steps: top heading, benching and invert. "The first step includes creating a support system to stabilise rocks. The bottom of the river consists of soil and weak rock which could result in instabilities and water leakage due to high permeability. The MMRCL has installed an "umbrella" of steel bars above the excavation to support the ground for it and this will be strengthened by cementing (grouting) to reduce permeability," a source told mid-day.

Two more tunnels were built in the area and TBMs were deployed along with an earth pressure boring machine. The two TBMs dispense foam to form a paste. "The paste creates positive pressure on the face of the rock and as a water barrier and maintains pressure," the sources said. According to sources, when the water level is high so are hydrostatic forces and the tunnel design has accounted for these factors, making the line and the station watertight.

"Entry and exit points are above the highest flood level margin, keeping no space for water to seep in from anywhere. If some water does enter the station, a small network of the drains will carry it to the pump to discharge it out of the network," the source added. During the monsoon period, the river overflows, causing floods at times. "To ensure operations are conducted normally and the water pressure does not increase dramatically, the MMRCL is making the facility waterproof by installing pumps to drain water," a source said.

What happened to Pragati Maidan tunnel?

>> The Pragati Maidan complex, which includes a 1.3 km long tunnel and five underpasses, was inaugurated by Prime Minister Narendra Modi in June 2022 ahead of the G-20 summit. Facing waterlogging and seepage issues since it opened, the tunnel, which connects Mathura Road and India Gate C-Hexagon with Ring Road, has been shut several times to pump out water and repairs.

>> The Delhi Public Works Department (PWD) issued a show-cause notice to L&T on February 3 citing "serious technical and design deficiencies" and "design flaws" and seeking a response within 15 days. The notice said the company failed to complete the construction of underpass number five near Bhairon Marg under the railway tracks within the stipulated period.

>> The notice also highlighted the accumulation of water across various locations in the underpasses. The PWD has identified 12 issues, including continuous leakage, cracks in concrete, waterlogging, malfunctioning of drainage systems and non-uniformity of kerbstones. L&T, however, said it was making a counterclaim of Rs 500 crore against PWD.

Rs 14,400 cr
Cost of Thane-Borivli twin tunnels

9.23 km
Length of crridor that will link Orange Gate to Coastal Road