The Soil Great Wall in Beijing's Yanqing district is an important part of the Great Wall defense system, with a history of 400-500 years. After given approval by the cultural relics department, the Yanqing section of the Soil Great Wall conducted its first ever rescue repairs this year.

According to the Yanqing Cultural Relics Management Office, the restoration project that began at the end of last April has been completed. The work mainly involved reinforcing the beacon towers No.1 to No.6 and the side walls of the Chadaocheng Pass section of the Soil Great Wall.

The Chadaocheng Pass section of the Soil Great Wall is also known as the Outer Wall. The No. 1 to No. 6 beacon towers stand on the northern and eastern sides of the Chadaocheng Pass. The section has a total length of 205 meters, and forms a semi enclosure around the pass. The existing beacon towers are made of rammed earth or stones, with a residual height of 4 to 6 meters and a side length of about 10 meters. Only the bases of some platforms are left, with the side walls measuring from 0.5 meters to 4 meters in height.

The difficulty with this renovation project lied in the preliminary archaeological exploration, said Shi Lixia, a staff member of the Yanqing Cultural Relics Management Office. "Before the repairing work started, you need to locate the border," she said. "Since the to-be-repaired section of the Great Wall is near the mountain pass and close to the surrounding villages, it has been seriously damaged by natural factors such as wind, sand and flood, and by trampling. The walls have collapsed, and are on the verge of disappearing."

The collapsed stones and clods of the Great Wall as well as the overgrowing weeds and shrubs made the reconnaissance and exploration very difficult. Before the site clearing started, the Yanqing Cultural Committee invited the archaeologists of the Beijing Municipal Institute of Cultural Relics to carry out an archaeological exploration. The experts worked on it for more than three months. By means of partial excavation and trench digging, they cleaned up the six beacon towers and some walls, ascertained the side boundaries and scope of the site, and collected data for subsequent renovation.

Shi said that because this section of the Great Wall in Yanqing is made of soil, which is rarely found elsewhere, the cultural relics department acted with caution with regard to the protection. Based on the principle of "minimum intervention," the rescue mission is aimed to eliminate the hazards and prevent it from deteriorating. "We didn't use supplement materials, nor reshaped it. We did the best to keep its original state," Shi said.

The restoration team used modified yellow mud slurry to plug and reinforce cracks in the walls less than 30 millimeters in width, and mortar grouting for larger cracks larger. The team applied bolting support for unstable chunks and filled in collapsed or eroded voids with adobe. For the walls that had collapsed and turned into soil ridges, the team first cleaned up the weeds and lose soils and then piled up fresh soil.

"The construction of the soil walls was relatively easier compared with that of walls of bricks and stones," She said. "Local materials were used for the renovation. The side walls were mainly made of earth, sand and stone, while the platforms were made of soil cladded in bricks and stones. Gravels and other materials were added to strengthen the walls so they can better withstand weathering."

In addition, after the repair work was done, the workers also sprayed a layer of modified yellow muddy rice syrup on the surface of the Great Wall as a sealing material for waterproofing. "We did experiment with water spray simulation to ensure that 'rainwater' would not infiltrate," Shi said. In the future, low shrubs will be planted on the Great Wall to maintain the water and soil.

To prevent trampling, the cultural relics department set pine logs and used spilling and ropes on both sides of the Great Wall to keep people away. Some warning signs have also been raised, reminding people about protecting the Great Wall. "We will also build a hiking trail next to the Great Wall," Shi said. The soil beacon towers and side walls in the repair area will be used for display purposes.