The much awaited tunnelling work on Bangaluru’s Underground Metro Corridor has started this week Karnataka CM flagging off the work. However, the challenging rocky terrain of Bengaluru, which is a mixture of both hard rock and soil, makes the underground tunnelling work an extremely complex affair. Though massive breakdowns that delayed the project for over a year during Phase-I is not expected to happen, tunnelling is still a delicate and cautious task, say project experts.
According to a tunnelling expert, “The cutter head attached to the TBM, which plays the pivotal role in boring through the rock and soil, operates in a circular fashion to bore through hard rock. Due to the nature of soil in Bengaluru, the rotations need to be done three to five times more than what was being done in the case of underground sections elsewhere in the country such as Delhi, Mumbai or even Chennai Metro.”
Another peculiar aspect is the presence of soil. “If the drilling is rigorous, the soil on top will crumble. So a very careful balance needs to be maintained on both fronts. You can neither undercut nor overcut,” he said. The TBM can drill an average of 6 to 7 metres a day. “On rare occasions, it can do much more. In Phase-I, a TBM drilled up to 21 metres a day,” he added.
‘Urja’ will first drill 895 metres from the upcoming Cantonment Metro Station to Shivajinagar Metro Station. Another expert said the geology of the underground tunnel section is about 250m of hard rock, 350m of mixed ground conditions and 255 m of soil.