Cheng Yongmao (left) reviews timely the repairs of the renovation project of the east section of Jiankou Great Wall in Huairou district of Beijing.[Photo by Ma Wenxiao/For China Daily]

The total length of the renovation project of the east section of Jiankou Great Wall in Huairou district of Beijing is 1,094 meters. Cheng, an engineer of Beijing Huaijian Group, is responsible for the technical guidance. For a man who has been repairing the Great Wall for nearly 20,000 meters in 16 years, this section of the Great Wall is the most dangerous. It is known by hikers as "the wild Great Wall".

"Almost the entire section is on a cliff with a slope of 70 or 80 degrees, It's a mountaineering job, and a oneway trip usually takes at least one hour, so you've got to have good physical strength to repair the Great Wall," he says. To climb the Jiankou Great Wall, one has to climb with both your hands and feet. Despite his years, Cheng insisted on climbing for inspection at least once a week. He could remember concretely every damage on this section of the Great Wall.

On the night before leaving for the Great Wall, Cheng was sitting in front of the computer, drawing a detailed sketch of the construction nodes using CAD software. Nowadays, the technical aids and equipment for repairing ancient buildings have made great progress, but the guidelines for reparation have not changed.

Cheng sticks to the reparation principle－minimal intervention－to protect the integrity, authenticity and safety of the Great Wall. Following the original repairing process, he summed up the "five-following" method－following the layer, the slope, the bend, the old and the damage, so as to maintain its ancient style, and pass on the reparation wisdom of the Great Wall to future generations.