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

Rescue operations are underway after a collapse inside the under-construction NHPC Teesta Stage VI Hydroelectric Project tunnel at Namchi on July 21, 2026. | Photo credit: PTI

Flammable gas was repeatedly encountered and caught fire more than once in the main tunnels of the Teesta Stage-VI hydroelectric project, suspected of a methane leak that killed at least 15 workers on Monday (July 20, 2026), according to a technical study written by National Hydroelectric Power Corporation Limited (NHPC) engineers.204.

The paper, authored by five NHPC engineers and presented at EUROENGEO 2024, the 4th European Regional Conference of the International Association for Engineering Geology, in Dubrovnik, Croatia, describes the geological difficulties faced in completing the project’s two main tunnels of 500 MW, each about 13.7 km long. The document was written long before the accident and does not describe the recent explosion. The head tunnel is the component that delivers water from the source to the power plant’s turbines.

According to the paper, “flammable gas” was first detected in November 2009, during excavation by the project’s original developer, at the head of the tunnel, where it “ignited”. After NHPC resumed work, a gas leak was again reported from the roof of the tunnel on a separate face, where a hand-held detector recorded readings of up to 30% of the minimum concentration at which methane in air can ignite. The two “faces” (the working end of the tunnel) named in the paper – designated 4A and 4B – were reached through Tunnel-3, the same access tunnel in which the July 20 explosion occurred. Tunnels are additional passages carved into the mountains that provide access to the main tunnel.

The paper attributes the gas to the rock itself, saying it was detected at the contact between the hard quartzite and the softer phyllite. The project tunnels through young, fractured Himalayan rock in Seismic Zone IV, including carbon-rich phyllite of the type associated with trapped gas.

The NHPC said the July 20 explosion followed a “sudden explosion of suspected methane” during work about 1.5 km inside the tunnel at Samardung locality in Sikkim’s Namchi district and announced a detailed investigation. The Sikkim government has set up 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.

In addition to the gas, the paper catalogs extensive instability along the tunnels. When the NHPC resumed excavation in June 2021, after the project had been dormant for about nine years, it found that several sections built by the previous developer had already collapsed, with depressions in places reaching the surface, and access tunnels flooded as drainage was not maintained. During fresh excavation, crews encountered shear zones compressing the soil and flowing water, including high-pressure inflows of 700 liters to 1,000 liters per minute on two faces that caused ground subsidence. The paper, available online, is titled “Planning and Methodology Adopted to Address Adverse Geological Issues Encountered During HRTS Excavation of Teesta VI Hydroelectric Project (500 MW), Sikkim, India”.

The project was originally built by Lanco Teesta Hydropower Limited, part of the Lanco Group, which failed to complete it and stopped work in December 2012 amid a financial crisis. NHPC acquired the company through insolvency proceedings in 2019 and resumed construction in 2021. In an email to The Hindu, NHPC said the designers were aware of the presence of methane in the tunnel. The July 20 incident is likely to have occurred due to an isolated spot near the face of Tunnel 4B that was sudden and did not give any indication or time for safe evacuation of people working inside the tunnel, NHPC said.

A number of measures have already been put in place to protect workers from explosions, including the use of “explosive-proof” excavation techniques; regular and continuous monitoring of “obnoxious/flammable gases”; rapid application of shotcrete (a construction technique) on exposed rock face to control methane release and vent tunnels, NHPC said.

In a post on the X social media platform on Wednesday (July 22, 2026), the company said ₹5 lakh will be paid to the next of kin of each deceased.

The paper did not say what caused the July 20 explosion, which remains under investigation. The Teesta-VI project received its initial environmental clearance on 21 September 2006. Deforestation was permitted from 2008-2021.

Published – 23 Jul 2026 04:30 IST