Drilling had penetrated through about 21m of debris by Friday morning, Devendra Singh Patwal, a disaster management officer, told Reuters.

A total distance of nearly 60m must be drilled to reach the workers.

The men have been trapped since Sunday when a section of the 4.5km Silkyara Bend-Barkot Tunnel, being developed for National Highways and Infrastructure Development Corporation (NHIDCL), suffered a part collapse. A jacked 120mm* diameter steel pipe has enabled workers to be supplied with food, water and oxygen.

Initial efforts to create an escape route by jacking a 900mm-diameter pipe through the collapse debris – and also to excavate with an 800mm auger – failed because boulders blocked the auger.

A more powerful auger – of the same type that was used in the 2010 Chilean mining emergency – was deployed and in a LinkedIn post yesterday ITA president Arnold Dix confirmed that “several new segments have been completed in the newest 800mm diameter tunnel attempt”.

“The ground is a mixture of soft ground and hard boulders and collapsed structural steel ribs and lattice girders. There is a very high risk of encountering impenetrable obstacles,” Dix said.

He added that ITA’s international emergency response team, comprising SME and industry experts, equipment and ground improvement experts in emergency tunnelling were ready to assist.

“Collectively they (we) have developed a range of evacuation intervention options,” Dix said.

But, he added, “The experts on site, are known personally to our international team. Without any doubt the local experts are best qualified and competent to deal with this emergency. We – myself personally too – have complete faith in the on-site emergency team and their careful approach to safely rescuing the trapped tunnel builders.”

Silkyara Bend-Barkot Tunnel is located in India’s Himalyan region. The 4.5km-long tunnel project is being developed by NHIDCL to be an all-weather route and reduce travel distance by about 20km between Dharasu and Yamunotri.

In 2018 NHIDCL awarded the contract to Navayuga Engineering Co Ltd to build, operate and maintain the road link. The contract includes the tunnel plus a short approach road.

* Updated: previously given by source as 200mm