On July 23, rescuers recovered the final victims of a suspected methane gas explosion at India’s 500 MW Teesta Stage VI hydroelectric project in the Himalayan state of Sikkim, about 400 miles north of Kolkata, near the border with Bhutan and Tibet.
The four-day operation ended with the deaths of 25 workers and project officials as investigators began to examine what caused the underground construction disaster.
The explosion occurred on July 20 inside the project’s head tunnel. Civil contractor Patel Engineering Ltd., which is building this section of the tunnel, said a sudden release of suspected methane trapped in the surrounding rock sparked a gas explosion that filled the corridor with thick, toxic fumes.
Project documents issued 10 days before the explosion identified methane gas, water ingress, cavity formation and unstable soil as active construction hazards in the same section where the accident occurred. The documents called for improved gas monitoring, ventilation and operating procedures during excavation. There is no indication whether these measures were in place when the explosion occurred.
“We heard a loud sound and then debris flew by,” survivors told Namchi district officials, according to the Associated Press.
The precise sequence, including whether the gas explosion caused the tunnel collapse or was exacerbated by one, has not yet been publicly established.
According to the AP, 21 workers were working underground when the explosion occurred, while six project officials went in during the initial response. Two workers escaped.
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Federal and state rescue teams retrieved the final victims through toxic gases, stagnant water and poor visibility. Methane, carbon monoxide and hydrogen sulfide limited rescuers equipped with breathing apparatus to short underground rotations, the Associated Press reported.
Members of India’s National Disaster Response Force and other rescue agencies prepare for overnight operations at the Teesta Stage-VI Hydroelectric Project in Sikkim, where crews searched for trapped workers after the July 20 tunnel explosion.
Photo courtesy Government of Sikkim
NHPC Ltd., India’s state-owned hydropower developer, said senior management remained on site coordinating with federal and state rescue agencies throughout the recovery effort.
The Sikkim government has formed a high-level committee to determine the cause of the blast and recommend safety improvements. Patel Engineering said that “a detailed investigation will be conducted to ascertain the cause of the incident.”
According to the state order establishing the investigation, investigators will examine the source of the gas release, tunnel ventilation, atmospheric monitoring, emergency procedures and compliance with applicable safety requirements.
Sikkim Chief Minister Prem Singh Tamang said the state would act if the probe identified negligence.
“A thorough investigation will be done,” Tamang said in remarks reported by Indian media. “If any negligence is found on the part of the executing company, strict action will be taken.”
NHPC acquired the unfinished Teesta Stage-VI development in 2019 after the original developer Lanco Teesta Hydro Power Ltd. understood an insolvency procedure.
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The government-approved run-of-the-river project is being built on the Teesta River and includes two headwater tunnels that will carry water to an underground power station housing four 125 MW generating units. NHPC currently expects commercial operation to begin in September 2029.
Neither NHPC nor Patel Engineering had said when underground construction might resume or whether the accident would affect the project’s planned 2029 completion.
Investigators have not determined what caused the explosion or whether documented methane control measures were operating when it occurred.
