By rosebell
The Srisailam Left Bank Canal (SLBC) tunnel project in Telangana’s Nagarkurnool district has achieved a crucial breakthrough, with engineers successfully negotiating one of the most dangerous geological formations in the Nallamala forests along the revised alignment, officials familiar with the matter said.
The breakthrough comes more than a year after the deadly tunnel collapse on February 22, 2025, which claimed the lives of eight workers and forced authorities to abandon the original alignment in the affected stretch.
The latest achievement is significant as the shear zone encountered on the new alignment posed a major challenge to excavation because of unstable rock conditions and high geological risks.
A shear zone is a weak geological formation created by intense pressure and stress, causing rocks to shift, fracture and become loose and unstable. The shear zone along the revised SLBC alignment extended for about 10-15 metres. “It took nearly a month to cross the vulnerable stretch safely,” an official of the state irrigation department said.
The ₹4,600 crore tunnel project aims to transfer Krishna River water from the Srisailam reservoir to provide irrigation to four lakh acres in the erstwhile combined Nalgonda district and drinking water to 516 villages en route.
Of the total 44-km main tunnel, a 20.5-km stretch was completed from the Devarakonda end and another 14 km from the inlet end at Domalapenta. Two tunnel boring machines were deployed from either side to expedite the work.
With only 9.6 km left to be drilled from the inlet side deep in the Nallamala forest range, the tunnel collapsed on February 22, 2025, within four days of work commencing, trapping eight employees, including two project engineers.
Subsequent investigations revealed that the accident occurred due to a similar geological formation. Extensive seepage had been detected and grouting undertaken to control it. However, the intervention resulted in a huge reservoir-like accumulation beneath the surface. When the Tunnel Boring Machine (TBM) advanced into the area, the ground collapsed, killing eight workers.
