Drilling through the rubble to prepare an escape route for the 41 workers trapped inside the Silkyara tunnel was set to resume on Friday morning, a day after another snag hit the rescue efforts, an official said.

Former advisor to the Prime Minister's Office Bhaskar Khulbe said boring through the rubble with an auger is likely to resume by 11.30 am and the operation might successfully be completed by the evening if everything goes well.

Citing the data scanned by a ground penetrating radar, he said that there is no metallic obstruction up to five metres beyond the 46-metre point through the rubble.

The drilling machine's platform has been revamped and strengthened by shotcreting through the night, Khulbe told reporters at Silkyara.

"We have 12-14 metres more to go. And I hope if all goes well we might conclude the operation by Friday evening," he said.

The workers have been trapped for the past 12 days after a portion of the under-construction tunnel on the Uttarakhand Char Dham route collapsed on November 12, cutting off its exit.

Khuble said around 2-metre part of an inserted pipe had to be cut off as it got compressed due to friction while drilling after the auger encountered an obstruction and exerted more pressure on Thursday.

The crushed part of the pipe has been cut off which has reduced the length of penetration up to 48 metres achieved through the rubble to 46 metres now, Khulbe said.

Chief Minister Pushkar Singh Dhami and Union Minister of State for Road Transport and Highways Gen VK Singh who visited Silkyara on Thursday are still in Uttarkashi.

Dhami spent the night at a guest house in Matli while Gen Singh stayed at the Nehru Institute of Mountaineering, Uttarkashi.

On Thursday, boring through the rubble was put on hold again, apparently after cracks appeared in the platform on which the drilling machine rests.

The dampener came hours after the operation resumed earlier in the day, following a six-hour delay to cut through an iron girder that came in the way of the auger machine late Wednesday night.

This is the third time that the drilling exercise had to be halted since the multi-agency rescue mission began.