Shanghai's plans for a massive metro system will not necessarily result in a huge underground hollow space that will aggravate the city's subsidence, experts said.

A cave-in at the construction site of a local cross-river subway tunnel that led to a wide area of land subsiding earlier this month has aroused public concern over the subway construction in the city.

"Underground engineering is not the major factor causing land subsidence," said Zhu Hehua, a professor with the Underground Construction and Engineering Department of Tongji University.

Zhu explained that every subway tunnel is expected to have strong interior support made of reinforced concrete.

With proper designs drawn up after careful geological studies, metro construction should be able to carry on under any complex geological conditions.

The risk of land sinking is merely a "minor" one compared with other possible engineering hazards in such projects, he added.

Zhu stressed that a convenient and fast citywide rail network is a must to better meet Shanghai's growing traffic needs.

Shanghai plans to invest 100 billion yuan (US$12.1 billion) to build nine rail lines by the end of 2005. This will mean over 188 kilometres being added to the current 65 kilometres of lines in operation.

"It's beyond doubt that Shanghai needs such an urban rail network, although engineering precautions against risks such as land subsidence always take top priority," said Zhu.

The M4 metro line will link the west of the city centre with Pudong District across the Huangpu River. Construction began in 2001 and trials are scheduled to start on the line in 2005.

The cave-in took place on July 1 at the construction site of an underground passageway affiliated to the metro line's main tunnel.

The accident caused drifting sand to pour in, which experts have described as one of the most formidable threats in any underground engineering project. This resulted in large-scale land subsidence.

The city took a wide range of emergency measures to put things right.

Tons of cement were poured onto the ground in that area to prevent further ground subsidence. Buildings that had started tilting were also demolished to lessen the weight on the ground.

The accident-hit part of the tunnel has been sealed with huge walls of reinforced concrete.

Specialists, meanwhile, are trying to find the root cause of the accident, based on the results of an engineering survey of the area.

But the rail line's route will not necessarily be changed, officials said.

"I think no accidents of this type will happen again as the city has gained a lot of experience in handling the mishap," said Zhu of Tongji University.

(China Daily 07/16/2003 page9)