The retrieval shaft at Bowbazar which is mainly being constructed to bring out the two tunnel boring machines Chandi and Urvi is to play another crucial role. Apart from rescuing the two TBMs, the shaft at Durga Pithuri Lane is also to be used to pump out the stagnant water that was injected last year to create a counter-force against the aquifer.

On 31 August 2019, the TBM Chandi had accidentally hit an aquifer and water along with sludge started gushing into the tunnel.

Consequently, gaps developed in the soil layers above it and the surface.

The TBM was swamped by the gushing water of the aquifer flowing into the tunnel leading to subsidence of the ground.

The subsidence had assumed the shape of an ellipse resulting in several structures within the alignment collapsing like house of cards.

To combat the situation, the engineers of KMRCL, on the advice of the team including national and international experts, had injected around 1000 litres of water at various points from where the aquifer water was flowing into the tunnel.

The large volume of water was injected in a ‘coffer’ dam.

The dam had three layers of ‘coffer’ walls, at a distance of around 100 metres from the cutter head section of the TBM, Chandi. This was done to arrest the water that had flown inside the tunnel after passing through the cutter-head, main body, tail skin and concrete segments of the TBM turning it redundant.

In addition, holes were drilled into the ground and a mixture of cement, sodium silicate, bentonite and water was injected to fill the empty space.

Now, when the construction of the retrieval shaft has started, the stagnant water is also to be pumped out. “In the frontal portion of the TBM, there is water stored in the coffer dam,” informed a KMRCL official.

“When the big shaft is excavated, the defunct TBM and the linear walls inside which the water is still stored will be exposed. At that time, the water will be easily pumped out as the water pressure will no longer be required then,” he added.

According to the official, the concrete coffer walls will also be brought down later.