A SECTION of roadway at the intersection of busy Lujiabang Lu and Xizang Nanlu suddenly subsided last Sunday afternoon disrupting traffic for six hours.

No pedestrians or vehicles fell into the 2-metre-deep hole and the cause of the cave-in is under investigation.

Workers summoned to the roped-off section of the intersection poured 15 tons of sand and sediment into the hole and then covered the filled-in section of road with a steel slab.

As the subsidence occurred just 10 metres from a Metro station being built for the new M8 line, doubts were raised about the overall safety of the work being carried out on the city's vast underground rail network.

However, experts said there was no need to panic.

According to witnesses, at first only a basin-sized section of the road surface sank covering about one square metre.

Police immediately rushed to the scene and blocked-off the road. The repair workers found the subsidence seemed to have stopped when the hole was two metres deep.

"Everything is now back to normal," said Huang Jianping, a traffic control assistant working at the intersection.

Experts said they believed the road's foundations had been eroded by water but that nothing would have happened if traffic conditions along that section of road had been normal.

However, work on the nearby underground M8 station meant that three of the four lanes of Xizang Nanlu had been reserved for construction work. This forced all other traffic into the one lane remaining and this was where the subsidence occurred.

Gao Ruihua from the Shanghai Tunnel Engineering Co Ltd (STEC) said that although the hole was just 10 metres north of the M8 metro station under construction, the subway work had nothing to do with the roadway's collapse.

"The accident didn't happen in the construction area of the M8 and it didn't have any impact on the subway project either," he said.

He added that STEC had carried out a thorough survey, especially on land subsidence along the subway route, to ensure construction safety.

A senior engineer, surnamed Yu, with the Shanghai Municipal Civil Engineering and Management Bureau said that because Shanghai was a city built on clay and sand, some road cave-ins were inevitable.

The cave-ins had various causes. In the Zhongyuan area in north Shanghai, more than a dozen cave-ins had occurred in the past few years, mainly due to sewerage construction work.

Some roads built many years ago had lower standards of construction than today. With new buildings and facilities being built, some of the city's roads might not be able to handle the increased weight on them, Yu said.

"But we have been fully aware of the situation and have been closely watching out for possible cave-in spots and taking measures to reinforce the roads," he told the Shanghai Star. Yu added that most cave-ins showed warning signs long before any major subsidence occurred.

In the past few years he and his colleagues were finding an increasing number of similar hazards due to the pace of construction and greater traffic flows.

Yu said such cave-ins were normal in a city experiencing rapid urban construction.

"They don't result in injuries and can be repaired quickly," he said.

"So there is no need to panic."