Authorities have established communication with more than 40 people believed to be trapped and said they were all alive and were being provided oxygen through pipes.
Part of the tunnel collapsed on Sunday morning when a group of workers were moving out and replacement workers were going inside the Silkyara tunnel in the town of Uttarkashi.
A joint rescue operation is under way and rock breakers and mountain-digging machines are being used to try to reach those stranded.
About 20 metres of earth has been removed using excavators. Authorities believe about 35 metres of soil is left to be removed.
“Rescue operation is ongoing. We have managed to communicate with them and they’re all alive. But we are facing challenges as loose debris is falling due to the digging,” Devendra Patwal, chief of the state disaster response force, told The National.
Arpan Yaduvanshi, Uttarkashi police chief, said that two pipes have been inserted to where those stranded are based. One pipe is being used for food and communication, while another provides oxygen.
“We were able to communicate with them at 12:30am. They’re all safe. We have sent them water, and raisins. We are sure that we will be able to evacuate them all. We are trying to stabilise the surface so it is hardened, and we can progress with our operation,” Mr Yaduvanshi said.
Photographs from the site showed piles of concrete blocking the tunnel, with twisted metal bars poking in front of the rubble.
State chief minister Pushkar Singh Dhami visited the site on Monday.
“We want to assure the families of those stranded that the state government and administration are making every effort to rescue them,” he said.
Those trapped were working on the construction of a tunnel between Silkyara and Dandalgaon to connect two of the holiest Hindu shrines of Uttarkashi and Yamunotri.
Mr Patwal said that they’re making all efforts to rescue the men safely but added it was “difficult” to say how long it will take.
“Rescue will take time, we can't be sure how long it will take,” he said.
The tunnel is a part of Prime Minister Narendra Modi’s Char Dham project, to improve connectivity to four Hindu pilgrimage sites in the state.
Nestled in the lap of the Himalayas, Uttarakhand is known for its natural beauty and houses dozens of major Hindu temples. It is a popular tourist destination.
However, the ecologically sensitive region has been facing the brunt of climate change and rampant construction.
The state was hit by floods in 2013 that devastated Kedarnath, one of the holiest Hindu shrines, killing more than 5,700 people. A glacial lake burst in 2021, triggering a flash flood at a hydropower project construction site that killed more than 200.
About 50 people were killed this summer after unprecedented heavy rain caused flash flooding and landslides.