Rescue agencies suggest alternative small-diametre tunnels to skip unstable strata

Rescue operation underway to trace workers trapped inside the partially collapsed Telangana’s Srisailam Left Bank Canal tunnel, in Nagarkurnool, Telangana.

HYDERABAD

The team leaders of key rescue agencies involved in tracing the eight persons trapped in the Srisailam Left Bank Canal (SLBC) tunnel since February 22 morning have informed Chief Minister A. Revanth Reddy that debris removal in the last 20 metres of the tunnel is most problematic and challenging.

Since that portion of the tunnel, particularly its left flank and roof, is still loose and not stable with seepage of water in high volume going on, the government could consider alternative tunnel roots from the 13.45 km from the mouth to link it with the original path where the stable strata is available. It could be done by taking up two tunnels dug from either side with 5-metre diameter each to handle the designed discharge capacity of 4,000 cusecs.

The Chief Minister asked the agencies to make recommendations along with technical information so that the State government could take it forward. He also asked them to prepare a document about the entire rescue operation so that it could be useful as a guide for dealing with such difficult tasks, which was freaky and unique.

Col. Parikshit Mehra of the Border Roads Organisation (BRO) said such tunnels would generally have safe ducts, small alternative tunnels for use in emergency situations. But, in the case of SLBC there is no such arrangement and it is making the rescue work more difficult. There is over 5-metre deep muck/debris around the tunnel boring machine (TBM) now covered under the debris.

Another rescue agency team leader felt that the seepage water accumulated in the rock formation faults over the years had gushed out once the tunnel excavation was resumed after years of work shutdown, leading to its roof collapse and huge volume of water and mud filling the tunnel.

Pankaj Gaur of the work agency Jaiprakash Associates said the conveyor belt which had gone out of order is expected to become functional again by Monday evening as its non-availability was badly hampering the shifting of debris and muck from the tunnel accident area to outside the tunnel to help trace the trapped persons.

Some of the TBM parts were too heavy for dismantling/cutting them into parts and shifting out of the tunnel, the representatives of the rescue agencies explained to the Chief Minister. Some suggested availing the services of National Centre for Seismology to take up the strata study for depths up to 500 metres from the surface of the earth.

H.V.S. Satyanarayana of CSIR-NGRI said they had conducted ground probing radar (GPR) scanning studies at 47 locations in the tunnel so far but had not arrived at a pin-point conclusion about where the eight persons are trapped.