HYDERABAD: The Telangana government finds itself in a quandary over how to proceed with the construction of the Srisailam Left Bank Canal (SLBC) tunnel, following a fatal collapse and mounting technical complications.
Experts have ruled out the idea of a bypass tunnel to avoid the dangerous section, calling it unfeasible. Although the government considered setting up an audit point to aid excavation, experts highlighted that this would significantly increase both cost and duration. The only viable method to expedite the work, they said, is the Drill and Blast Method (DBM). However, even that presents major challenges, from acquiring necessary permissions to carrying out work under hazardous conditions.
Authorities have reportedly decided to continue excavation from the outlet side using the DBM technique, though this may further delay completion.
Bypass Tunnel Not Feasible
The tunnel, stretching 43.5 km from the Srisailam reservoir to the Dindi reservoir, is being excavated simultaneously from both the inlet at Domalapenta in Achampet mandal and the outlet at Manyavaripalle in Mahabubnagar district. So far, 34.372 km have been completed, leaving 9.560 km of tunnelling still to go.
The collapse occurred at the 13.93 km mark on the inlet side, within a highly sensitive shear zone comprising loose, fractured rock layers. A surge of water led to the roof caving in, killing eight workers. Rescue operations are still underway to retrieve the bodies.
In light of the tragedy, the government held consultations with multiple experts and agency representatives to determine a safer path forward. One proposal was to bypass the shear zone and reconnect to the main tunnel line at a later point. However, foreign consultants engaged by the agency informed the government that such a bypass is practically impossible. They also revealed that four more shear zones lie along the current alignment, any of which could pose similar risks.
The SLBC tunnel still lacks a single audit point. The dense forest cover on the surface has made this impossible so far. An audit point, typically used to transport excavated material, provide oxygen, and allow access for machinery, was considered from Tirmalapur in Amrabad mandal. However, experts rejected this plan, citing the 5 km distance from Tirmalapur to the main tunnel line, the additional cost, and the large-scale land acquisition needed for dumping excavated soil and rocks.
Government in a Dilemma
The original tunnel boring machine (TBM), in use for over 20 years, has severely deteriorated. It now drills only about 5 metres per day, far below its intended 2.5 metres per hour capacity. The machine was also reportedly damaged in the recent flooding incident.
Procuring a new TBM could cost ₹300–₹400 crore and may take up to a year to arrive from abroad. Experts have advised that DBM is the only realistic option left. However, since the tunnel passes through a wildlife sanctuary, work must be carried out without causing seismic disturbance or exceeding 50 decibels of noise, as mandated by central guidelines.
Unlike TBM technology, which automatically removes muck via conveyor belts, the DBM method requires manual removal of debris. Experts noted that an audit point is essential for DBM to work effectively – without one, even this method becomes impractical.
In the end, the government may suspend all work on the inlet side and continue solely from the outlet. This approach, too, is expected to extend the timeline for tunnel completion. With conflicting expert advice and mounting safety concerns, the government remains mired in indecision over how best to proceed with the SLBC project.
Rescue Efforts Continue
Search operations are still underway for the six workers trapped in the SLBC tunnel at Domalapenta in Achampet constituency, Nagar Kurnool district. Mini-JCBs have been deployed to remove broken components of the loco train and TBM. On Sunday, Tunnel Special Officer Shiva Shankar held another review meeting with rescue team representatives.
