Subsidence of the track surface is the main reason for the improvement works taking place at China’s grand prix venue in Shanghai, it has emerged.

The April event has only a provisional date on the 2011 calendar, with re-approval of the 7-year-old circuit to only be possible after an FIA inspection in March.

It was previously believed that the necessary improvements at the $240 million facility were in the area of the track surface and rain drainage.

The local race organisers have now confirmed that the main problem is subsidence – damage to the track surface due to downwards movement of the ground – at three corners.

“There should be no problem,” Yang Yibin, manager of race promoter Juss Event, is quoted by the Shanghai Daily.

“We’ve communicated well with the FIA, our plan has been approved and the grand prix will be on time.”

The report said the serious subsidence is present at turns 1, 8 and 14.

The Shanghai International Circuit was built in 2003 on former swampland.

“Subsidence is normal because of the soft soil in Shanghai, which the circuit was built above,” Yang said.

And circuit engineering manager Sun Liang told the Oriental Morning Post that artificial elevation changes were built into the layout by using “complex materials”.

“Because some parts of the circuit are on the ground while some parts are not, as time goes by it’s normal for some subsidence,” he said.