What caused the tunnel disaster in Sikkim’s Namchi?, Pg14

Key Highlights:

• On July 20, 25 workers died in an explosion within an under-construction Head Race Tunnel (HRT) of the NHPC’s 500 MW Teesta Stage VI Hydropower Project in Samardung, Namchi district, Sikkim.
• The incident was caused by the sudden explosive release of suspected methane gas trapped in rocks.
• Both NHPC and the Sikkim government have initiated high-level investigations to determine the exact cause.
• This is the first recorded incident of a methane gas explosion in a tunnel project in Sikkim.

Detailed Insights:

• The explosion occurred in a Head Race Tunnel (HRT), a critical component of hydropower plants designed to transport large volumes of water to power generation turbines.
• Preliminary reports suggest that workers encountered methane-trapping rocks during drilling or construction, leading to the gas release and subsequent explosion.
• Standard surveys for tunnel projects in the Himalayas, including topographical and geotechnical tests, cannot detect the presence of methane.
• Methane is a colorless, odorless, and highly flammable gas that can explode when mixed with air in specific ratios and exposed to an ignition source, such as sparks from drilling.
• Methane deposits in hilly terrains can be found as natural gas in porous rocks, localized pockets in young geological formations, or from anaerobic decomposition in waterlogged soils and waste dumps.
• Sikkim is located in the Rangit Tectonic Window zone, and the Geological Survey of India has identified coal reserves in southern Sikkim, particularly in the Namchi area.
• The coal in Namchi has been analyzed to contain 53.40% fixed carbon, 39.5% ash, 3.4% volatile matter, and 3.7% moisture.
• The Himalayan region is geologically fragile due to its young age, active tectonic instability from the collision of the Indian and Eurasian plates, and steep, erosion-prone terrain.
• Human activities like heavy blasting, tunnelling, unregulated urban expansion, and deforestation exacerbate the geological instability, leading to frequent landslides and mudslides.
• Sikkim has previously experienced extreme weather events, including a Glacial Lake Outburst Flood (GLOF) in October 2023, which destroyed Sikkim Urja Limited’s 1,200 MW dam and damaged NHPC’s Teesta-V Hydropower Project.

Scientific/Technical Concepts Involved:

• Head Race Tunnel (HRT): A pressurized conduit or tunnel that carries water from a reservoir or intake structure to the power generation turbines in a hydropower plant.
• Methane: A colorless, odorless, flammable hydrocarbon gas (CH4), often found in natural gas deposits and produced by anaerobic decomposition of organic matter.
• Rangit Tectonic Window zone: A geological feature in the Himalayas where older rock formations are exposed through younger overlying thrust sheets, indicating complex tectonic activity.
• Glacial Lake Outburst Flood (GLOF): A sudden release of a significant volume of water from a glacial lake, often caused by the failure of the natural dam containing it.