A two-meter-wide rock crashed on labourers who were working around 650 meters inside an under-construction tunnel (number 10) of Sevoke-Rangpo (Sikkim rail link) at Bhalukhola on Thursday night, killing two and injuring five.

Bhalukhola is about 25km from Kalimpong.

The deceased — Salku Murmu and Naresh Soren — both in their mid-30s were residents of Jharkhand. They were working in a large drilling machine when the rock and mud came crashing down.

Of the five injured, Sufal Hembrem and Thakur Das were referred to the North Bengal Medical College and Hospital in Siliguri. The other three injured — Sukeswar Singh, Ashok Singh and Kundan Singh — are recuperating. Six of the workers were from Jharkhand and one from Bihar.

Almost 86 per cent of the 44.98km link connecting Bengal with Sikkim will run through 14 tunnels. Each tunnel will be eight meters in diameter.

The injured were standing on the floor of the tunnel which was carved out of the mountain. The tunnel is the longest in the project with 5.3km and was dug for just 600 metres.

Ircon International Limited, which is under the Ministry of Railways, is assigned with the entire project.

T.T. Bhutia, general manager of the firm, said the workers were 650 meters inside the tunnel.

“A 1.5 X 2 meters rock fell on them. The tunnel has not collapsed,” said Bhutia.

He said a probe was on to find out the reason behind the cave-in. The Rs 4,000 crore project was inaugurated in 2009 but has over the years been delayed because of various issues, including environmental concerns.

In February 2016, the Supreme Court allowed for changing the status of 8.84 hectares of forest land within the Mahananda Wildlife Sanctuary but with strict conditions that included restricted speed, wireless animal tracking sensors and digging of tunnels only during the daytime.

Bhutia clarified the restrictions, including daylight digging, were only within the sanctuary which had wildlife and not in the area where the Bhalukhola tunnel is situated.

The cave-in had occurred around 10.30pm on Thursday.

The Ircon official said efforts were on to complete the project by December 2023. The initial plan was to complete the work by 2015.

“We could start work on the project only from December 2019 but we were soon hit by the Covid-19 pandemic,” said Bhutia.

In the past, some environmentalists and geographers had raised concerns about a tunnel project in an area where fault lines exist.

B.C. Mandal, geologist who was involved in the design of the project, had earlier told this newspaper: ““We have conducted thorough studies of the region for three years and run the data in software. Depending on our studies, we have changed the alignment of the tracks three times.”

Other experts of the project had said they had come across good rocks in the region like phyllite quartzite and gneiss quartzite while the presence of bad rocks like like phyllite quartzite, silt stone and sandstone were minimal.

Assuaging the fault line concerns, Mandal had said: “In many existing publications, it has been mentioned that two fault line MBT and MCT are located in the area. These have been mapped and exact location identified. We are using the Especial Tunnel Support System (ETSS) and extra precautions are being undertaken by grouting super plasticizer admixture. We are also using crown umbrella protection,” said Mandal.

The designers said the same technology was being used in Jammu and Kashmir where a similar fault line runs.