Telangana's SLBC tunnel clears shear zone, resets timeline

Critical Shear Zone Cleared

The Telangana government has pushed its Srisailam Left Bank Canal (SLBC) tunnel past the most hazardous stretch of the revised alignment—a 10–15 metre shear zone in Nagarkurnool district crossed over nearly a month of controlled excavation.

The real significance is not the milestone itself but the delivery model now visible. After the 22 February 2025 collapse that killed eight workers, the ₹4,600 crore, 43.93 km irrigation tunnel is being built on a different technical and commercial basis.

A New Delivery Model

The state irrigation department has dropped tunnel boring machines (TBMs) for the bypass stretch and switched to drill-and-blast with immediate ground support installed section by section:

- Rock bolts
- Shotcrete
- Steel girders

Colonel Parikshit Mehra, a Border Roads Organisation tunnelling specialist engaged as tunnelling secretary, is the central delivery figure.

The National Geophysical Research Institute (NGRI) now supplies the geological intelligence the original alignment lacked, mapping faultlines to 1,000 metres depth through a helicopter-borne VTEM survey.

Geological Risk and Remaining Work

That pairing is the story. Mechanised tunnelling was the wrong risk transfer for the Nallamala geology.

The 2025 collapse occurred within four days of TBM advance after grouting masked a reservoir-like accumulation.

NGRI has since flagged four further danger zones linked to the Nallavagu stream.

Roughly 240 metres of bypass excavation remains before the revised alignment rejoins the outlet side, with completion now targeted for May 2028.

Structural Implications for Contractors and Industry

For contractors and consultants, the implications are hard-edged:

- Geophysical pre-investigation is becoming a mandatory deliverable.
- TBM deployment on faulted, water-bearing ground now carries effectively uninsurable execution risk.
- Owners are willing to pull delivery back into direct departmental control when mechanised methods fail.

Telangana has converted a contractor-led mechanised job into an owner-directed, instrumented drill-and-blast operation, shifting where schedule, cost and accountability sit for the remaining works.

The industry signal is structural: geology is being surveyed and priced before excavation, not discovered during it.

Long-tunnel delivery in India is moving toward geophysics-led design, instrumented excavation and owner-held risk.