The initial narrow shaft it will dig will have to be doubled in diameter to permit a man to pass through.

Once it is fully assembled and operational, engineers said it will take up to four months of drilling to reach the trapped miners down the gold and silver mine that collapsedon August 5.

Most of the miners appeared to be in good spirits in a 45-minute videothey sent to their families after their three-week ordeal.

Signs of depression

But officials said that a handful of them appeared to be struggling psychologically.

"Five of the miners are isolated, are not eating well and do not want to appear on camera - this is what we call depression," Jaime Manalich, the country's health minister, said.

The video footage shot by miners showed most of the group coping with confinement [AFP]

The minister said a psychiatrist would attempt to treat the men over an intercom system dropped down to hot and cramped shelter, that the miners have described as "hell".

The video footage shot by the miners and broadcast in Chile late on Thursday, showed most of the group coping with their confinement.

"We've organised everything really well down here," one of the miners said, pointing to a corner reserved for medical supplies.

"This is where we entertain ourselves, where we have a meeting every day, where we make plans. This is where we pray."

Chilean authorities have already taken steps to boost the men's mental resilience for the ordeal that still lies ahead, notably by reaching out to organisations and individuals with experience in prolonged isolation.

Four officials from Nasa, the US space agency, were due to arrive in Chile to provide expertise, while submarine commanders in Chile's navy have already given advice.

Legal battles

As rescue operations and psychological assistance ramped up, so did legal actions against the owners of the mine where the men are trapped.

San Esteban Mining, the company responsible for the gold and copper mine in northern Chile, was ordered on Thursday by a local judge to freeze $1.8m in revenue so that it can pay future compensation to 26 of the families of those trapped.

One family has also filed a lawsuit over the accident, accusing the mine company and government inspectors of criminal negligence by allowing the facility to reopen in 2008 after a worker accident led to its closure in 2007.

Laurence Golborne, Chile's mining minister, announced late on Friday that he will revise mine safety regulations and increase monitoring of the sector.

"We can't guarantee there will be no more accidents, but we can make company leaders more aware of the importance of worker safety,"  Golborne said.