Srisailam canal tunnel work in fast pace

Hyderabad: Every day, round the clock, a 1,700-tonne steel behemoth is literally butting heads with some of the hardest rock known to man as it grinds its way through granite in a bid to ensure that the Srisailam Left Bank Canal Tunnel (SLBC) is ready for use by October next year.

Of the 43.9-km-long Tunnel I of the SLBC project, engineers have so far dug through about 31 km working with two tunnel boring machines (TBMs) working simultaneously from the two ends of the tunnel. While the TBM from the inlet side near the Srisailam dam has made its way through some 12.2 km, the one digging in from the outlet end near Dindi reservoir in Nalgonda district has so far tunnelled through 18.8 km.

The SLBC project’s tunnelling works are divided into two parts, the 7.2 km Tunnel II from Pendlipaka to Teldeverapalli which has been completed while Tunnel I extends from Srisailam reservoir to Teldeverapalli which is upstream of the Dindi reservoir.

“There is about 4.5 km of granite which is very hard, to be dug through from the outlet end. We monitor the progress continuously. Sometimes, the TBM manages to carve through 1.2m and sometimes it is 4m an hour,” Anil A Kamat, president of Jaiprakash Associates, which has been contracted the work, told Telangana Today.

The TBM, he said, is designed to excavate up to 32 metres of rock a day as well as complete all related lining works and put the finishing touches to the section of the tunnel that has been excavated.

It is not the granite itself that has been slowing things down, explained S Suneel, the Chief Engineer of the Irrigation Department and in-charge of the SLBC tunnel project. “The logistics associated with the maintenance of the TBMs and mechanical issues are the ones that are also taking time,” he said.

For instance, when the main bearing of one of the TBMs needed to be replaced last year, it took a crew of about 50 persons, working in three shifts each, seven months to replace. Considering that the bearing itself weighed a whopping 40 tonnes, the crew also had to dig an extra niche in the tunnel walls up to a depth of 30 metres to set up the infrastructure to remove and replace the bearing. “This alone took seven months to complete,” he said.

Considering that the machines operate in an “aggressive environment” that includes very high levels of humidity, they require regular maintenance, from something that sounds as simples as changing of wires to replacing the cutter heads on the TBM’s cutter head. “The TBM, with each ‘stroke’ can go through 1.6 metres after which it stops and retracts the cutter head. The crew then examine each of the 67 cutters fixed to the head and replace those that exceed the wear and tear limits,” Kamat said.

He, however, said while several indecisions till 2013 posed challenges for the project’s progress, after “this government came in, there was a lot of prioritisation for this project. We have been given full support by this government to complete the project by October 2019.”

Each TBM was designed to finish 500 metres of tunnelling and associated work each month which meant one km of work completed each month, he said. “The granite encountered from the outlet end has a strength ranging between 210 and 800 UCS. This is very hard granite,” Kamat said.