Overlooking subsurface layer composition might have led to Bowbazar mishap: Experts

Kolkata: Kolkata Metro Rail Corporation Limited (KMRCL) overlooking the fact that along with clay there may be sand pockets in the subsurface layer of the city, might have led to the disaster in Bowbazar in which several buildings developed cracks and portions of some old structures collapsed.

KMRCL is currently constructing an underground tunnel between Esplanade and Sealdah, which is the most difficult stretch of East-West Metro railway.

Geologists maintained that after successfully constructing an underground tunnel under river Hooghly, the KMRCL engineers thought that there was only clay in the subsurface layer of Kolkata.

Their belief was further confirmed when they did not face any problem while constructing the underground tunnel at BBD Bag.

Before the construction of the underground tunnel between Esplanade and Sealdah, KMRCL had conducted soil test at some points and the engineers were convinced that there was only clay and the German boring machine would not face any problem in setting up the tunnel.

There are around 150 houses on this stretch that are between 100 and 150 years old.

The geologists maintained that the boring machine was working at Bowbazar without knowing the existence of the sand pocket there. Immediately, water started oozing out and the sand began sliding, weakening the foundation of the old structures.

"It is known to the engineering community that when a free space is created beneath the soil strata, a lot of water may seep and get accumulated within that free space. The amount of seepage depends on the level of groundwater. In such a case, a waterproof jacket needs to be provided, so that water may not percolate towards the free space. If the seepage is allowed, huge amount of unsettlement in the adjacent structures may occur. It has been reported that the tunnel was full of water, which indicates that seepage was released by the soil strata beneath the adjacent structures. We are also not sure whether the aquifer under the ground could be detected during soil test or not," said Gupinath Bhandari, associate professor of the Civil Engineering Department of Jadavpur University.