mumbai

Updated: Dec 20, 2017 16:07 IST

The legwork for India’s first undersea tunnel, part of the Prime Minister’s ambitious Mumbai-Ahmedabad bullet train corridor, has taken off in the city.

The train, which will cover a distance of 508km, has a 21-km underground corridor from BKC to Kalyan Shilphata. Of this, seven km is under the Thane creek, with 1.8km of it to be built under the sea bed, and the remaining under the mangroves marshland on either side of the creek.

While this is the first undersea tunnel being constructed in India, the first underwater tunnel was inaugurated for the Howrah-Kolkata Metro line earlier in 2017.

A team of engineers from National High Speed Rail Corporation Limited (NHSRCL), RITES Ltd, and Japan’s Kawasaki Geological Engineering are currently in Mumbai to initiate phase one of the work of the tunnel. The teams are venturing daily into the Thane creek to get data on the seabed, based on geo-technical surveys. Based on the report of this survey to understand the structure and depth of the seabed at Thane Creek, the team will design the precise alignment for the tunnel. The survey began on December 11 and will be completed by December 24.

Achal Khare, managing director of NHSRCL, said: “We intend to start construction work of this tunnel by the second half of 2018, and complete it by December 2021. We have been surveying this 1.8-km stretch for more than a week, but are deterred constantly because of natural hurdles. However, we have completed 80% of the survey work.”

The team is using static refraction method to study the seabed structure. This involves firing a high-energy sound wave towards the seabed from below the surface of the water, and mapping the refracted sound wave to determine the density of the rock under the sea bed. Only a very dense rock can safely support a tunnel of this nature, Khare said.

Meanwhile, NHSRCL and RITES have completed surveying the mangroves around the creek using Light Distance and Ranging (LIDAR) -- an aerial survey of the forest using a 100 megapixel camera. This is the first time LIDAR was used as opposed to a manual survey of the forest on foot, which cut short the survey time by more than 7 months. The aerial survey for the entire corridor was completed in two months.

Currently, a team of more than 10 officials from Japan and five officials from RITES and NHSRCL are working for eight to nine hours a day. However, of this time, they can spend only 3 to 4 hours in the creek water. Work can only be done between two high tides, which act as the entry and exit window into the creek. After high tide, water begins to recede from the land, cutting access to the main creek.

S Kunar, who has been working on this project on behalf of RITES, told HT, “A team of officials was once stuck on the boat in the middle of the creek overnight. They failed to come back to land during high tide, so their boat had to remain in main water. It takes about 45 minutes to travel to the survey spot, and an hour to set up machinery. Usually, the duration between two high tides is five hours. Sometimes the high tides are at odd hours such as 4am.”