Rudi Wolf talks to reporters at the site of the rescue. [Photo by Ju Chuanjiang/chinadaily.com.cn]

The rescue team has changed the drilling methods more than 20 times, adopting various methods such as air hammer drilling, reverse circulation air drilling and reverse circulation mud drilling, until the pipes reached a depth of more than 200 meters to connect to the area where the four miners were living, Wolf said.

And there was also a layer with water underground, so rescuers had to think of ways to seal the pipe sent down to save the survivors so water will not seep into the pipe, he said.

Fighting against time to save the survivors, Wolf, like hundreds of others, has been extremely busy since the start of the operation.

He has been working for about 15 hours every day, seven days a week in the past five weeks, and he said he worked for 38 hours in a row when he first arrived at the site.

"I spent New Year Day on a night shift," he said.

When the four survivors were lifted out after being trapped for 36 days on Friday night, he was also very happy and excited and joined in the applause with other rescuers on site, he said.

Now the operation has been successful, Wolf said he is eager to go back to his home in Germany and "take my family into my arms", although he still does not know when he can leave.

Wolf said the Chinese government provided good support for the rescue, and he worked well with Chinese colleagues, including an official surnamed Wang, who is the chief commander of the rescue team.

"He is a very good, smart man and took the right decisions. I must give him my compliments," he said.

The working experience has also been quite a rewarding one for Wolf, who is also a trainer of drilling technologies.

"I will show everything to my German students. I learned a lot here," he said.