SLBC tunnel crosses dangerous ‘shear zone’ in major breakthrough

Engineers working on the Srisailam Left Bank Canal tunnel in Telangana’s Nagarkurnool district have reached a significant milestone, successfully crossing a hazardous geological shear zone that had halted work for over a year. The breakthrough comes after authorities revised the tunnel’s alignment following a fatal collapse in February 2025 that killed eight workers and forced a reassessment of excavation methods.

The shear zone, a stretch of unstable fractured rock measuring about 10 to 15 meters, posed severe risks due to its loose and shifting conditions. Officials from the Telangana State Irrigation Department confirmed that it took nearly a month to safely navigate the section using a new drilling-and-blasting approach with real-time geological monitoring. This method replaced the previously used tunnel boring machines, which proved unsuitable for the complex geology of the Nallamala Forest Range.

The ₹4,600 crore project aims to deliver Krishna River water to irrigate 400,000 acres and supply drinking water to 516 villages in the former Nalgonda district. As of now, over 34.5 kilometers of the 43.93-kilometer tunnel have been completed from both ends—20.5 km from Devarakonda and 14 km from Domalapenta. Only 9.6 km remain to be excavated from the inlet side within the Nallamala forest.

Following the February 2025 disaster, an investigation found that heavy seepage led to an underground water reservoir-like buildup. When the tunnel boring machine advanced into the zone, the ground collapsed, trapping eight workers. In response, the state government ordered a full alignment review and shifted the route 100 meters away from the accident site. A bypass tunnel was constructed along the new path.

To enhance safety, the National Geophysical Research Institute (NGRI) conducted a helicopter-borne geophysical survey using VTEM technology, mapping underground formations up to 1,000 meters deep. This data helped engineers avoid additional shear zones and fault lines along the revised route.

Colonel Parikshit Mehra, a tunneling expert with experience in the Himalayas and former Border Roads Organisation engineer, has been brought in as the project’s tunneling secretary. He confirmed that the most dangerous shear zone has been safely crossed and that the bypass tunnel has advanced 141 meters from the inlet side. The revised alignment is expected to rejoin the outlet alignment after another 240 meters of excavation.

Despite this progress, engineers remain cautious. NGRI has identified four more vulnerable zones along the remaining stretch, primarily due to the Nallavagu stream flowing above the tunnel. Real-time sensor monitoring is being used to track ground stability at every stage.

The state government now aims to complete the project by May 2028. Upon completion, the 43.93-kilometer SLBC tunnel will be India’s longest tunnel constructed from just two portals, marking a major advancement in Telangana’s irrigation infrastructure.

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