Safer, Secure Option Chosen by Govt Sees Progress in SLBC Work
Typically, as the TBM keeps going ahead, cement concrete linings are placed to cover the tunnel sides but there is always a gap between the cutter head and the linings.
Domalapenta:The February 22 accident in the SLBC tunnel’s inlet side that occurred all of a sudden had multiple causes, including how the tunnel boring machine (TBC) operates.
With a ten-metre diameter cutter head pushing through the rock as the TBC neared the 13.5 km point of the tunnel, a section of the roof gave in and thousands of tonnes of wet rock dust and huge boulders tumbled down, burying much of the TBM. The accident resulted in the loss of eight lives.
Typically, as the TBM keeps going ahead, cement concrete linings are placed to cover the tunnel sides but there is always a gap between the cutter head and the linings. That left an unsecured section of the tunnel which is where the collapse occurred due to accumulated water in the rocks above. “Though they had taken every precaution possible, including grouting the section ahead in which tunnelling was to be done ahead of the cutter head, a portion of the unlined section of the tunnel collapsed,” a senior engineer at the site explained.
With the irrigation department now opting for the drill-and-blast method, work has become safer. “We do not know what is ahead of the cutter head when a TBM is being used. In the method we are following, since we can check how the rock is changing with regulated and controlled blasts, there is greater control on how to move forward,” Col. Parikshit Mehra, tunnelling expert, who is now working with the state irrigation department said. “More than anything else, we can actually see the tunnel face that is being dug,”
He explained that in the revised alignment, when the workers reached the shear zone, steps were taken to strengthen the area surrounding the portion that was to be drilled through. And with small sections of the rock face removed through careful blasts, the exposed area was immediately strengthened using rock bolts, steel girders securing the surface and preventing any soil or rock fall. “We are also letting the water that is flowing down come down freely, the idea is to not let any pressure build up because of water accumulating,” he added.
