Rudi Wolf on site during the mission to rescue four miners trapped in a collapsed mine for 36 days in Pingyi, Shandong province. JU CHUANJIANG / CHINA DAILY

For Rudi Wolf, the past month has been the most challenging and tiring he has ever had to endure. It has also been one of the most rewarding.

The 55-year-old was a technical consultant for the rescue team that worked for 36 days to return four trapped miners to the surface on Friday night.

"The job here has been the most difficult of my life," said the German drilling expert, who was called in to help shortly after the gypsum mine collapse in Pingyi county, Shandong province, on Dec 25.

Wolf, who works for German drilling equipment company Prakla and has 35 years of experience, had worked on similar operations in Germany and Chile.

"The reason the rescue operation took so long is because we have very, very bad ground here," he said, explaining that the geological conditions at the Pingyi mine are complex and unstable. The ground is made up of layers of claystone, gravel and limestone, so it is "hard, then soft, then hard again", which meant the drilling crews had to keep changing their methods and delayed the operation.

Wolf estimated that the team had to switch more than 20 times between air hammer drilling, reverse circulation air drilling and reverse circulation mud drilling until the tunnel reached the miners, who were about 200 meters underground.

There was also a layer of underground water, so rescuers had to think of ways to seal the tunnel to prevent putting the stranded men in further danger, he said.

Hundreds of people had been working around the clock to help rescue the miners since the collapse.