The explosion echoed across the silent valley. It had taken nearly five hours and 50 sticks of dynamite, but the People’s Liberation Army had managed to blow up a building where the bodies of eight men still lay after last week’s earthquake.

An air force officer directed his men by walkie-talkie as they laid the explosives to bring down the building, at the site of the Tianchi phosphate mine. Thirty accountants were meeting when the quake hit; emergency teams could not get at the last eight of the 18 people who died. The structure was deemed too dangerous to enter.

“It is our duty to rescue the living and to recover the dead,” the officer said. “We will do whatever it takes.” Helicopters rattled over the Tianchi Mine No 1 pit to airdrop supplies to a town still inaccessible except by a day’s walk.

More than 130,000 troops have been mobilised as the vanguard of the huge relief operation to help five million people left homeless by the 7.9 magnitude tremor — and to find the 50,000 dead and nearly 30,000 still missing 12 days on.

The soldiers have been greeted with relief and rejoicing by survivors. The cry has been the same in dozens of devastated villages. One man, who lost his son when the Hongbai Middle School crumbled, saw the troops trudge into the hilltop town just 13 hours after the earthquake struck. “When the People’s Liberation Army arrived I thought there was hope that we would find him. We knew they had come to help us.”

At the foot of the Dragon Gate Mountains, thousands more soldiers distribute food and water to survivors. They chat and pause for a cigarette as they help the homeless to build wooden-framed huts. They clamber over ruins with shovels, digging for the dead and then clearing the rubble away.

The PLA is expected on the scene when it is necessary to shore up river banks in the summer flood season. Less often are they ordered to the front line of rescue and relief operations, and still less often do they live alongside ordinary people, sharing their rations and carrying the elderly on their backs.

But the scale and complexity of the job has presented a valuable chance for the PLA, which has not seen action since a brief war with Vietnam in 1979, to brush up its command and control skills. It is prepared for many weeks under canvas. One young soldier said: “The saddest thing is to see the faces of people hoping their loved ones will come out of the mountains. It makes you want to do everything you can to help.”

The operation has not been without hiccups. Officers have described inaccurate airdrops that forced men on the ground to walk for hours to retrieve the food and get it to those in need. Communications have also been hampered by a lack of military telephone lines and incompatibility among units. One squad resorted to their mobile phones when field units failed to work.

But, given the size of the catastrophe, the Government knew at the start it could cope only by sending in the military at once.

Bad weather caused delays in the first crucial hours. But Premier Wen Jiabao ordered all haste to reduce the death toll and called in helicopters to reach the epicentre, Wenchuan, cut off by huge landslides and fallen bridges, within 24 hours.

When the PLA balked at the risk, Mr Wen bellowed at commanders to remember their duty to serve the people whatever the risk. A squad of 15 paratroopers wrote their wills and leapt from a plane into thick cloud above Wenchuan. They succeeded without losing a man.

With scant hope of finding anyone else alive, the PLA’s focus is on the grim task of digging out bodies and helping the living to recover their belongings buried.

At the Tianchi phosphate mine, the PLA officer waited for the boom. “Then it will be safe to go in to retrieve the bodies and the strongbox that may hold a lot of money.”

He said the job was worth the effort and pointed up the valley where 48 soldiers had been needed to extract the body of a man trapped in his car. “First we put the body in a bag, then we have a moment of mourning and then we put the victim aside and move on.”

This team would head next to a Buddhist temple on a neighbouring hillside that collapsed, burying its pilgrims. “We will find them,” a soldier said. “It’s not easy but it’s our duty.”

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