No More TBM for SLBC Tunneling, Second Machine to Be Pulled Out
Hyderabad: The tunnel boring machine (TBM) will not be used at the Srisailam Left Bank Canal, where a section of the tunnel had collapsed on February 22, resulting in the death of eight workers, amid fears that a similar incident could recur if the equipment was used again.
While one TBM was damaged in the collapse at the inlet end of the tunne, a functional TBM is awaiting repairs at the outlet end.
Instead, the method decided for continuing work from the inlet portion — that of adopting the traditional drill-and-blast method in Nagarkurnool district —will now be followed to continue digging from the outlet side in Nalgonda district.
It is reliably learnt that this decision was taken as the second TBM, stuck 20.43 km inside the outlet section of the tunnel since January 23 awaiting repairs, faces the prospect of negotiating a weak shear zone in the rock that is similar to the one that collapsed on the machine which was put back into use in January, after a two-year gap.
The period of inactivity had resulted in accumulation of enormous quantities of water in the weak zone where the machine was parked for two years. The work was resumed after repairs, but ended up in the February 22 disaster.
The SLBC tunnel is being dug using two TBMs simultaneously from its inlet and outlet ends. While the one in the inlet side was not only destroyed in the February 22 collapse but had to be cut into pieces during efforts to rescue the workers, who were later declared dead, the second machine in the outlet end was awaiting repairs for two years, needing a giant-sized bearing to make it operational again. Though the bearing arrived at the worksite some time ago, it has now been decided not to use the TBM but instead continue the work using the drill and blast method.
The shear zone just ahead of the TBM in Nalgonda district is feared to be similar to the one in Nagarkurnool district. If the TBM is repaired and begins working, the authorities are learnt to have concluded that a disaster similar to the one in the tunnel’s inlet side cannot be ruled out with fears of not just encountering softer rock, but possibly water accumulated in the zone for the past two years.
It is reliably learnt that work on dismantling the TBM in the outlet end has begun. Once it is fully removed and the tunnel cleared of the ten-metre diameter machine, work is likely to be resumed from this side of the tunnel using the drill and blast method.
“The only way to remove the TBM is by dismantling it as the cutter head diameter is much larger than the tunnel diameter of nine metres because of the cement concrete lining rings installed in the completed section of the tunnel. Under these conditions, the machine cannot just be driven back and needs to be removed in pieces. This work is going on,” sources said.
It may be recalled that the government had decided that the drill and blast method will be used for continuing tunneling from the inlet end and a decision on the alignment to bypass the shear zone where the collapse occurred will be taken after the National Geophysical Research Institute submits its report on the stability of the rock structures along the tunnel’s existing alignment and along its sides ahead of the collapsed site.
With the latest development involving the decision not to use the second TBM, work on restarting the tunnelling is expected to take some more time, the sources said.
The government, following the February 22 disaster, had said it was committed to completing the rest of the work, just around 9.5 km of the total 43.94 km long tunnel, in two years and put the project to work. The SLBC tunnel was designed to carry 33 TMCft of water to Nalgonda district from the foreshore of the Srisailam dam reservoir.
