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India Project Tunnel Borer Damaged

Thursday, September 12, 2019

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Since the tail-end of August, Kolkata Metro Rail Corporation personnel have been working on the East-West Metro in Bowbazar, India, where a tunnel-boring machine reportedly struck an aquifer, causing flooding within the tunnel.

The incident is reported to have happened at 5 p.m. on Aug. 31.

What Happened?

As reported in The Telegraph, while a tunnel-boring machine was conducting excavation work, the machine struck through a sand aquifer that contained a massive amount of water. According to a project engineer: “As the tunnel-boring machine moves forward, concrete rings are placed along the tunnel. The diameter of the rings is smaller than that of the tunnel and the gap is filled with grout—a mixture of water, cement and sand.

“Apart from water seepage, the tail skin brush, made of steel and fibre, prevents the grout from spilling into the tunnel-boring machine.”

However, because the water pressure was so intense, the tail brush gave way, causing water to flood through it.

Another KMRC official explained, “The sand aquifer, from which water gushed, was 600 meters in length and the machine had crossed about 250 meters of it. It is possible that throughout this stretch the seal of the tail skin brush was under tremendous pressure from water outside. It finally gave way on Saturday evening.”

As a result of the flooding, huge settlements of soil also started to break way, and by 6:45 p.m., residents living over Durga Pithuri Lane felt a quake as buildings began to develop cracks. In an attempt to save the buildings, personnel erected steel props to support roofs and various structures, while grouting was also inserted to reinforce the base. Unfortunately, these safety measures failed, and many buildings continued to develop deep cracks and fissures.

The following day, during the early hours of the morning, the first set of residents were evacuated out of the area. By 9 a.m., portions of buildings were reported to be caving in and more people were evacuated to nearby hotels. At noon, police erected barricades around the affected area.

On Monday (Sept. 2), KMRC workers drilled a hole in the tunnel’s surface and began additional grouting procedures. The Times of India reports that more than 450 cubic meters of a concrete mixture was injected into the opening; however, it still wasn’t enough to fill the vacuum created as a result of the flooding. At 7 a.m., residents were evacuated from their homes on Shyakra Para Lane after chunks of concrete started falling off buildings.

By 11 a.m., the KMRC brought in South African engineer Paul Verrall and Kiril Shramko to inspect the damage. Verrall is reported to have decades of experience and has worked on tunnels under heritage buildings in Greece, Turkey and Egypt and even supervised borings of the East-West Metro tunnels under the Hooghly and dilapidated buildings in Burrabazar.

“We brought him in because the construction supervised by him had emerged unscathed even as it went through dilapidated buildings of Brabourne Road and heritage landmarks like Writers’ Buildings,” said KMRC MD Manas Sarkar.

“We are here on KMRC invitation and we are doing our best to help them,” said Shramko.

Verrall later suggested that grout be inserted from all sides in order to stop the seepage of water. Yet, KMRC reported that they would need more time—roughly two more days—to stop the water and fill the vacuum.

What’s Happening Now

Since the incident occurred, 53 buildings have been identified as affected and more than 500 people have been evacuated from their homes. In some cases, entire structures have caved in on themselves, while others have been instructed to be demolished due to dangerous titling.

"It is a disaster," said Manas Sarkar, Managing Director of KMRC. "We never anticipated this. It is unprecedented. It has never happened in India and only in rare cases elsewhere in the world. I am sorry. We are trying to get things under control."

Following inspections, geo-technical analysts and engineers discovered that the subsidence zone of the affected tunnel formed an ellipse shape. To prevent spreading, the team is planning to build a concrete underground wall around the affected zone.

According to officials, once spreading has halted in the zone, measures can then be taken to stop the settlement of soil. Additionally, a four-member team has been formed under the leadership of John Endicott, a Cambridge-educated British engineer, to take stock of the progress and suggest next steps moving forward.

“We have found 10mm subsidence at the point farthest from the centre of the affected zone, where the subsidence was found to be around 200mm,” an official said. “It is a sign of hope that the subsidence readings have come down but that doesn’t mean that it will not go up again.”

Once the project is back on track and completed, the 16.6-kilometer-long (10.3-mile-long) East-West Metro will run from Howrah to Salt Lake Sector V; 10.8 kilometers of this will be underground and 520 meters of that distance will be underwater.


Tagged categories: Accidents; AS; concrete; Grout; Health and safety; India; Infrastructure; Inspection; Port Infrastructure; Program/Project Management; Rail; Safety; Tunnel

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