“We have to go slower between Central College and Vidhana Soudha because of hard rock formations. Along some underground stretches, it was easy to execute work as the terrain had just soil, while other stretches posed problems. Blasts at the Central College site started during March-April 2011. It is granite rock formation, which is hard to break,” said Chief Engineer for underground stretch, N.P. Sharma. TBMs all set to reach Vidhana Soudha: Helen, the first TBM, is expected to reach under Vidhana Soudha. An underground Metro station has been proposed here and the work will start in two months. From the Central College site, Helen has dug through 400 metres, while the second TBM, Margarita, running parallel has progressed 270 metres. The total length to be covered from Central College to Vidhana Soudha by these TBMs is 750 metres.
Rocky Terrain: The stretch between Central College and Vidhana Soudha is rocky, which is giving a tough time to BMRCL workers. Each TBM has 44 rock cutters and each cutter, which gets damaged frequently, costs close to Rs 3 lakh. Replacing damaged cutters consumes a lot of times, especially because each of them weighs close to 600 kgs.
Inside the tunnel: The TBMs works on a slurry mechanism. Slurry is a liquid that helps excavated materials thrown out of the tunnel. There are two slurry pipes inside the tunnel. One pipe takes the soil, rock, clay and all the excavated material, while the other pipe brings fresh slurry. The pipe that removes excavated materials from the tunnel is attached to the slurry plant in Majestic area which separates the excavated material into clay, soil and rock. This is later dumped in a quarry at MS Palya.
“At the Central College site, around 80,000 cubic metres of rock has been excavated which comes to around 8,000 trucks. TBMs are monitored from the driver’s cabin, while the boring machines themselves are automated,” said Deputy Chief Engineer, Underground, Subrahmanya Gudge.