Authorities are attempting to stabilize a 600-year-old city wall under pressure of subsidence after recent downpours.

As many as seven sections of the ancient wall in the capital of northwest Shaanxi Province have been seriously affected by rain since last month.

The crack that recently appeared on the Nanlou tower at the west gate of the ancient city wall of Xi'an, capital of Shaanxi Province.

"Because of the intense rain, accumulated water collected in some low-lying areas of the wall, and the water infiltrated into the wall and made the inner soil subside," Sun Xinchang, engineer of the Construction Department of the City Wall Administrative Committee, said.

"As a result, the wall suffers from subsidence and cracks on its top and side parts."

The Xi'an City Wall, the most well-protected ancient rampart construction in China, was built in the beginning of the Ming Dynasty (1368-1644) with rammed earth inside.

The local government repaired the ancient city wall in the 1980s, using new bricks, leaving the existing soil inside.

Fan Dake, deputy director of the administrative committee, said the soil inside was of poor quality and it subsided quickly as rain leaked through the wall.

"In some parts of the wall, there are cracks with the width of 3 to 5 cm and the length of 5 to 6 m," he said.

"If the urgent repair is not carried out, some parts of the ancient wall will collapse."

Urgent reinforcement work in six parts suffering from serious subsidence have been carried out, and work on the final part at risk of collapse will commence shortly.

In order to better protect the ancient city wall, construction machinery will not be used, and work will largely be carried out using manual labor.

Extensive waterproofing and new, higher quality soil mixed with lime will replace the existing packed soil in the wall.

It is expected the project will be completed by next month at a cost of 2 million yuan (US$256,410).

On July 25, 2006, some 30 m of the wall in its southern parts collapsed during repair work after rain. That collapse was also caused by water leakage, Fan Dake said.

The Xi'an City Wall, about 13.75 km in length is 12 m high and up to 14 m wide on the top and 18 m wide at bottom.

It is the world's largest existing ancient rampart.

(China Daily August 15, 2007)