BEIJING • More than a century after the fall of imperial rule, the walls that once protected China's mighty emperors are beginning to show their age.

The walls of the Forbidden City, the ancient imperial centre of power in Beijing that is now formally known as the Palace Museum, have weaknesses like loose bricks and bulging, cracking surfaces.

Last Saturday, the authorities launched a restoration project to keep the walls from collapsing.

"People usually think the walls are much more solid than our wooden palace architecture," said museum director Shan Jixiang. "However, the result of our detailed investigation has told us they are not."

A survey of the 3,437m-long wall has turned up the main problems: some sections have been hollowed out and are sagging; grass and tree roots have infiltrated gaps, posing a threat to the wall's stability.

The western walls have sustained the most damage, Mr Shan said. A 233m section on that side has been chosen as the first target for renovation. Plans for the rest are pending.

The Forbidden City dates back to 1420. Its walls are about 9.3m high and 8.6m thick. They have an earthen core, protected by outer bricks.

Historical records show there were several restorations in the 17th and 18th centuries, following heavy rainstorms or earthquakes.

In 1988, a section of the north wall collapsed. In 1999 and 2000, the museum undertook a preventive maintenance project, but it mainly focused on the surfaces.

Mr Shan said restorers are looking in-depth this time, using high- tech tools like ground-penetrating radar. "We want to thoroughly cure the 'illnesses'," he said.

Mr Zhao Peng, a museum engineer who is leading the restoration project, said his team will reuse as many original bricks as possible. Any new materials used must conform to the look of the existing brick, he added.

Mr Shan said he expected the whole project covering all endangered sections to be completed by October 2020, when the Forbidden City celebrates its 600th birthday.

CHINA DAILY/ASIA NEWS NETWORK