Drivers have been asked to go slow when driving across a major highway bridge in Tongzhou district, after sections of it have subsided, allegedly caused by construction of the second phase of Subway Line 6.

A sign on the Huangmuchang Bridge on the Tongzhou to Yanjiao Highway, reads "the bridge subsidence is caused by the construction of a subway line and the road could be closed at any time so please slow down." 

The depressions, on the southern side of the six-lane highway, measure as deep as 10 centimeters in some places.  

On Thursday, one of the three lanes from west to east was closed to traffic. Several subsided sections could be seen, with obvious depressions from 3 to 5 meters in length. Sections of the stone bridge railing were twisted and broken.

Some workers on the bridge were taking measurements of the subsided sections, but all types of vehicles were still permitted to cross.

An employee, surnamed Liu, from Beijing Capital Highway Development Group Co Ltd said that they have asked a third party to investigate.

"We put warning signs and sent staff there to make an inspection for safety reasons," he said.

A man working on the bridge, surnamed Li, from Beijing No.3 Construction Engineering Quality Test Department Co Ltd, which is investigating the subsidence, said many factors could be responsible for the subsidence, and the subway construction is just one of them.

"The subsided sections are 10 centimeters deep. This could affect driving for sure," he said.

Sun Zhang, a rail expert and a professor with the Railway & Urban Rail Transit Institute of the Shanghai-based Tongji University, said that there are many geological reasons that could have caused sections of the bridge to subside like this.

"It could be related to the leakage of groundwater or pipelines. They should put detectors on the bridge to monitor changes in the subsidence for the safety of drivers," he said.

Taxi driver Wang Hejie, from Beijing Xinyue Taxi Company, said that he can feel the subsidence on the bridge when he drives across.

"There were no warning signs when I drove that way several days ago," he said.

There are not many alternative routes from Tongzhou to Yanjiao, he said.

On Wednesday, villagers in Jijiamiao village voiced fears over their safety, after a 1-meter-deep sinkhole opened up next to a villager's home. Experts are investigating whether the hole could be connected to construction of Subway Line 10.