Rescue operation underway after a collapse inside the under-construction NHPC Teesta Stage VI Hydroelectric Project tunnel, in Namchi, on, July 21, 2026.
| Photo Credit: PTI
Flammable gas had been encountered repeatedly, and had caught fire more than once inside the headrace tunnels of the Teesta Stage-VI hydroelectric project in the years before the explosion, suspected to be from release of methane, which killed at least 15 workers on Monday (July 20, 2026), according to a technical paper written by the National Hydroelectric Power Corporation Limited’s (NHPC) own engineers in 2024.
The paper, authored by five NHPC engineers and presented at EUROENGEO 2024, the 4th European Regional Conference of the International Association for Engineering Geology, at Dubrovnik, Croatia, sets out the geological difficulties faced in completing the 500 MW project’s two headrace tunnels, each about 13.7 km long. The paper was written well before the accident and does not describe the recent blast. A headrace tunnel is the component that purveys water from a source to an electric plant’s turbines.
According to the paper, “flammable gas” was first detected in November 2009, during excavation by the project’s original developer, at a tunnel face where it “burst into flame”. After the NHPC resumed work, gas was again reported seeping from the tunnel roof at a separate face, where a handheld detector recorded readings of up to 30% of the minimum concentration at which methane in the air can combust. Both ‘faces’ (the working end of a tunnel) named in the paper — designated 4A and 4B — were reached through Adit-3, the same access tunnel in which the July 20 blast occurred. Adits are supplementary passages carved in mountains that allow access to the main tunnel.
The paper attributes the gas to the rock itself, noting that it was detected at the contact between hard quartzite and softer phyllite. The project’s tunnels pass through young, fractured Himalayan rock in Seismic Zone IV, including carbon-rich phyllite of the kind associated with trapped gas.
The NHPC has said the July 20 explosion followed a “sudden burst of suspected methane gas” struck during work about 1.5 km inside the tunnel at the Samardung site in Sikkim’s Namchi district, and has announced a detailed investigation. The Sikkim government has constituted a Special Investigation Team. About 25 workers were inside when the blast triggered a landslide that sealed the tunnel, and rescue operations are continuing this week.
Beyond the gas, the paper catalogues extensive instability along the tunnels. When the NHPC restarted excavation in June 2021, after the project had stood idle for roughly nine years, it found several stretches built by the previous developer had already collapsed, with cave-ins in places reaching the surface, and the access adits flooded because no drainage had been maintained. During fresh excavation, the crews met shear zones squeezing ground and running water conditions, including high pressure inflows of 700 litres to 1,000 litres a minute at two faces that caused the ground above to subside. The paper, available online, is titled ‘Planning and Methodology Adopted to Deal with Adverse Geological Problems Encountered During HRTS Excavation of Teesta VI Hydroelectric Project (500MW), Sikkim, India’.
The project was originally built by Lanco Teesta Hydropower Limited, part of the Lanco Group, which was unable to complete it and halted work in December 2012 amid a financial crunch. NHPC acquired the company through insolvency proceedings in 2019 and resumed construction in 2021. In an email to The Hindu, theNHPC said that project engineers were aware of the presence of methane in the tunnel. The July 20 incident was likely to have occurred due to an isolated patch near the face of tunnel 4B, which was sudden and gave no indication or time for safe evacuation of the people working inside the tunnel, the NHPC said.
A slew of measures were already in place to protect workers against explosions, including the use of “non-explosive” excavation techniques; regular and continuous monitoring of “obnoxious/flammable gases”; quickly applying shotcrete (a construction technique) on exposed rock face to check methane release, and ventilating the tunnels, the NHPC said.
In a post on social media platform X on Wednesday (July 22, 2026), the company said ₹5 lakh would be paid to the next of kin of each deceased person.
The paper does not establish what caused the July 20 explosion, which remains under investigation. The Teesta-VI project received its initial Environmental Clearance on September 21, 2006. Forest clearances were accorded between 2008-2021.
Published – July 23, 2026 04:30 am IST
