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In the afternoon of July 20, a blast resounded through a tunnel being constructed in Sikkim’s Namchi district. Workers said that the force of the blast threw them around two metres, and that almost immediately, they could smell gas.
The explosion took place in the head-race tunnel of the 500-MW Teesta VI hydropower project being constructed by NHPC, a public enterprise. Such tunnels are the main channels that route water from a river, and lead it to turbines to generate power, before channeling the water back to the river.
The explosion occurred after a “sudden burst of suspected methane gas trapped inside the rocks which led to an explosion generating dense fumes and toxic gases”, the company said in a press release.
While some workers on site escaped the explosion, 25 others were trapped after it triggered a rockfall and blocked the tunnel’s opening. Rescue operations began immediately and concluded on the third day. There were no survivors.
The company’s press release stated that it was carrying out a “detailed investigation” to “ascertain the cause of the incident”.
But while findings of the investigation are yet to be released, company documents that Scroll has examined show that NHPC had been aware of the presence of methane on the site since at least January.
A document prepared by the company shows that in January, the site saw a “sudden leakage of flammable methane gas” during excavation work, as a result of which several workers “sustained burn injuries”.
Despite this, another document prepared by the company indicates that in July, the company did not have adequate infrastructure to manage the presence of methane in the tunnel. Just ten days before the accident, NHPC had floated a tender for firms and experts to provide a range of services, including management of methane through measures such as detecting and monitoring the gas, and providing ventilation.
Moreover, in June, it also issued a tender for two “methanometers-cum-toximeters” for the same tunnel. According to the company’s own safety manual, the devices are used to detect methane levels in the atmosphere, and thus allows personnel to ensure that there is no risk of explosion.
The same month they also invited bids for the service of calibrating this equipment “on a quarterly basis for 2 years at Teesta 6”. The total value of these two tenders was estimated at Rs 3 lakh.
Experts said that these documents indicated that the company was aware of the risks of methane leaks, and yet continued with work at the site. “If they are issuing tenders just ten days before the accident happened, it means that they knew that there were methane pockets,” said Soumya Dutta, a visiting senior fellow at Delhi’s Impact and Policy Research Institute, who works on energy, climate and hydropower projects. “But after issuing the tender, they carried on the work carelessly. It should have paused till they made the tunnel safe.”
He added that even if the company had procured methanometers, “these need to be procured and put in operation and workers need to be trained before the tunnelling work is done”.
Pravin Sharma, who stood as an independent candidate from the Yangthang assembly constituency in Namchi district in the 2024 state election, echoed these criticisms. “If this infrastructure was present, then the possibility of all those trapped surviving could have been high,” said Sharma, who is also a social worker. “Was there early gas detection facilities or not? Was there constant monitoring and ventilation and the staff trained for such incidents?”
A geological challenge for two decades
Construction for the Teesta VI hydropower project began in 2005, and was at first carried out by Lanco Teesta Hydro Power.
It was originally envisioned to be operational by 2012, but the eastern Himalayas threw up several challenges for the project, hampering the timeline. In 2009, for instance, a “flammable gas” was detected while the tunnel was being excavated, which then “burst into flame”, The Hindu reported. In 2011, an earthquake blocked ongoing work.
“The eastern Himalaya geography is conducive for methane pockets,” said Vikram Gupta, a professor of engineering geology in Sikkim University. He explained that the region was abundant with “the Gondwana sequence of rock”, which contain hale and coal seams – these seams naturally lead to the creation and trapping of methane. He added “We do not find many traces of them in western Himalayas.”. Dutta noted that the young Himalayas also have weak, sheared and highly fractured rocks, and that construction or excavation of rocks here can expose these methane pockets.
Due to constant delays in construction and mounting expenses, Lanco failed to repay debts to banks. By 2018, the company went bankrupt and became insolvent. Construction had also been stalled for four years at this point. In 2019, NHPC took over the partially constructed project.
In 2023, the project also suffered immense damage when the South Lohnak lake burst and caused a glacial lake outburst flood downstream.
Since then, geological challenges have persisted, delaying the construction further. It is now expected to be completed in 2028.
In the expression of interest that NHPC floated in July, the company said in the last few months, the progress of excavation had been “drastically hampered due to severe, adverse geological conditions”. It noted that apart from problems such as unstable rocks and soil, rock that easily fractured upon excavation, and the constant seepage of water, it also struggled because of “the presence of methane gas” at the site.
Because of these constraints, only about 44% of the head race tunnel work has been completed. Even the Central Electricity Authority, which oversees power projects in the country reiterated the company’s observations, noting that one of the reasons for this delay was “poor geology” in the head race tunnel.
Gupta recounted that he had visited the site around a month ago. “We could very easily smell that gas,” he said. “On the day of the accident, it is possible that a methane chamber suddenly leaked. If it had slowly oozed the way it was doing anyway, it would not have been an issue.”
Gupta said that at meetings he was present at, NHPC officials had spoken about the presence of methane at the site. “But, perhaps they did not expect that the gas would create a blast of this extent,” he said.
An inspection, and recommendations
After the January methane leak, work at the tunnel had temporarily stopped. In March, an inspection team from the CSIR-Central Institute of Mining and Fuel Research in Dhanbad visited the site, to determine how construction could be resumed.
After four days of monitoring, they concluded that unpressurised methane that was released from pockets could be safely managed under strict safety guidelines. The team recommended keeping methane levels below 1%, ensuring that the points at the site with active gas pockets had ventilation ducts within 15 metres and exhaust fans within 25 metres, and using suction pumps to extract the gas from the rock cavities from which they were released. They also suggested that every fresh excavation should be followed by a one-hour “gas dilution observation period”, during which any work that generated heat or flames was prohibited.
“Ventilation fans are crucial,” said Dutta. He explained that the presence of methane alone did not necessarily suggest that a blast was likely to occur – rather, the concentration of the gas in the air needed to cross its “lower flammability limit”, the point at which it can catch fire when it comes into contact with a flame or spark.
The NHPC safety document notes that “The explosibility of methane gas is between 5 to 15 percent” and that the “percentage of Methane required for maximum explosive violence is 10 percent”. It recommends the use of methanometers to continuously monitor these levels.
“The ventilation must be placed and designed in a way to ensure that methane and other flammable gases do not reach concentrations beyond the flammability limit,” explained Dutta. He noted that in tunneling activities, apart from methane, it was also crucial to measure levels of oxygen, carbon monoxide, methane and hydrogen sulphide.
He added, “The big question is also if there were enough ventilation fans, and were they working?”
Scroll emailed NHPC to ask whether the inspection team’s recommendations were implemented – we also sought responses to criticisms that the company did not have the necessary equipment and infrastructure and place to protect workers, despite knowing about the risks at the site. This story will be updated if the company responds.
Gupta, the geologist, explained that another hydropower project, Rammam Stage III, is being constructed by the National Thermal Power Corporation on the Sikkim-West Bengal border, just around 30 km from Teesta VI. He noted that the site had similar geological conditions, and that methane was present in it – but the company had stopped all welding and other fire-related activities to avoid such accidents. “They are not even using blasting to excavate the tunnel,” he said. “They are manually constructing it.”
Construction work during rainy season
A notification issued in May by the Namchi district collector’s office pointed to other safety risks that projects like Teesta VI faced. The notification stated that “all new earth excavation activities” including digging, levelling and quarrying were “prohibited with immediate effect” in the interest of “public safety and disaster risk reduction”. It stated that carrying out such work in the monsoons posed “serious risks including soil erosion, landslides, flooding, water logging, damage to public infrastructure, and threat to human life and property”.
The state has been issuing such notices during monsoons, which usually last up till September, for the past few years.
An official at the district office who asked not to be named told Scroll that this notice did not apply to construction for the hydropower project, since the notice stated that “only new construction cannot be taken up. Ongoing constructions can continue.”
The notice ordered that in case of ongoing public work, “adequate protective and safety measures” had to be ensured.
“There is no clarity what these protective and safety measures should be,” said the social worker and politician Sharma. “The possibility of disasters is not only in new excavations. Even ongoing projects include excavation, and the disaster risk is high.”
He added, “When the topography is the same, then having a ban on new construction while letting ongoing ones continue seems like discrimination.”
Dutta explained that tunneling does not necessarily need to stop its work during monsoons, but that constant monitoring was needed of moisture and water levels in the rocks and soil, and that measures needed to be put in place to drain out water quickly if needed. He added, “If their monitoring shows that cracks are forming, then they must halt the tunneling.”
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