NHPC’s own 2024 paper recorded gas fires in Teesta-VI tunnels before fatal blast

According to a technical paper written by National Hydroelectric Power Corporation Limited (NHPC)’s own engineers in 2024, in the years before the explosion, flammable gas had been repeatedly encountered in the tunnels of the Teesta Stage-VI hydroelectric project and had caught fire more than once. This gas was suspected to be caused by the release of methane that killed at least 15 workers on Monday, July 20, 2026.

The paper, written by five NHPC engineers and presented at the 4th European Regional Conference EUROENGEO 2024 of the International Society for Engineering Geology in Dubrovnik, Croatia, reveals the geological challenges encountered in completing the 500 MW project’s two tunnel tunnels, each approximately 13.7 km long. The article was written long before the accident and does not describe the final explosion. The headrace tunnel is the component that delivers water from a source to the turbines of the power plant.

According to the article, the “flammable gas” was first detected “flaming” at the surface of the tunnel during excavation by the project’s original developer in November 2009. After the NHPC resumed operation, gas was reported to be leaking from a surface separate from the roof of the tunnel; here, a handheld detector recorded readings up to 30% of the minimum concentration at which airborne methane could burn. Both ‘faces’ (the working end of a tunnel) mentioned in the article, called 4A and 4B, were accessed via Adit-3, the same access tunnel where the explosion on 20 July occurred. Adits are additional passages carved into the mountains that provide access to the main tunnel.

The article attributes the gas to the rock itself, stating that the gas was detected in contact between hard quartzite and softer phyllite. The project’s tunnels pass through young, fractured Himalayan rocks in Seismic Zone IV, including carbon-rich phyllite, the type associated with trapped gas.

NHPC said the explosion on July 20 followed a “suspicious methane gas explosion” that occurred during work about 1.5 km inside the tunnel at the Samardung field in Sikkim’s Namchi district and announced a detailed investigation. The Sikkim government has constituted a Special Investigation Team. About 25 workers were inside when the explosion triggered a landslide that closed the tunnel, and rescue efforts continue this week.

Beyond gas, the paper also lists widespread instability in tunnels. When the NHPC resumed excavation in June 2021 after the project lay dormant for nearly nine years, it found that many areas built by the previous developer had already collapsed, with subsidence reaching the surface in places and access openings being flooded as drainage was not provided. During the new excavation, crews encountered shear zones with flowing water conditions compressing the ground, including high-pressure flows of 700 liters to 1,000 liters per minute on two surfaces, causing the ground to collapse. The document, available online, is titled ‘Planning and Methodology Adopted to Deal with Adverse Geological Problems Encountered During HRTS Excavation of Teesta VI Hydropower Project (500 MW), Sikkim, India’.

The project was originally built by Lanco Teesta Hydropower Limited, part of the Lanco Group, but it was unable to complete the project and stopped work in December 2012 due to the financial crisis. NHPC acquired the company through bankruptcy proceedings in 2019 and resumed construction in 2021. Hindu, theNHPC said project engineers were aware of the presence of methane in the tunnel. NHPC said the incident on July 20 most likely occurred due to an isolated area near the face of the 4B tunnel, which was sudden and gave no indication or time for the safe evacuation of people working inside the tunnel.