Rescue operations to evacuate 41 workers trapped in Uttarakhand's Silkyara tunnel resumed on Thursday morning after an overnight hurdle delayed the drilling by several hours.

Former advisor to the prime minister's office Bhaskar Khulbe said an iron mesh thad had come in the path of the drilling machine creating an escape passage for the workers was removed in the morning but has delayed the rescue operation by 12 to 14 hours.

Removing the mesh in a claustrophobic environment inside the pipe was difficult and the problem was compounded by the lack of oxygen, Khulbe told reporters.

"It took us six hours to remove it. But the good news is that we have cleared the hurdle which came yesterday after drilling up to 45 metres had been done," he said.

Now the process of assembling to go beyond 45 metres which requires welding the pipes has been restarted. The drilling will also resume soon, he said.

It will take around 12 to 14 hours more to complete the whole operation of reaching the workers. After that, it will take three hours to take out the workers one by one. That will be done with the help of NDRF, Khulbe said.

Union Minister of State for Road Transport and Highways Gen V K Singh and National Disaster Response Force (NDRF) Director General Atul Karwal have arrived in Silkyara for an on-the-spot review of the rescue efforts.

Chief Minister Pushkar Singh Dhami is also likely to arrive soon.

The drilling of 800 mm diameter steel pipes was halted on Wednesday night after the mesh came in the way of the auger machine, officials said.

Till 6 pm on Wednesday, up to 44 metres of the escape pipe had been inserted into the debris, an official update said in Delhi.

Earlier, officials said the American-made auger machine had to drill through a 57-metre stretch of debris to reach the 41 workers, who were trapped when a portion of the under-construction tunnel collapsed 11 days ago.

The drilling from the Silkyara end was put on hold on Friday afternoon when the auger machine encountered a hard obstacle around the 22-metre mark, creating vibrations in the tunnel that caused safety concerns.

The drilling resumed around midnight Tuesday.

As the machine drills through, six-metre sections of steel pipes, just under a metre wide, are pushed into the escape passage. Once the pipeway reaches the other end, the trapped workers are expected to crawl out.

Meanwhile, all arrangements have been made to provide immediate medical care to the 41 trapped workers after they are evacuated, officials said.

A 41-bed separate ward has been readied at the Community Health Centre in Chinyalisaur for Silkyara tunnel evacuees and 41 ambulances wait outside the tunnel to rush them there as soon as they crawl out.

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