At 5:40 AM on June 27, Shanghai was shocked out of its slumber by the sound of a 13-story tower block crashing to the ground. But the fatal accident may prove a bigger wake-up call for the construction industry, say experts, who have accused building firms of sacrificing quality for quick profits.

An investigation report last Friday revealed the collapse had been caused by the foundations being undermined by a combination of dug-out soil being piled 10 m high against one side and the digging of a 4.6-m-deep underground car park on the other.

A migrant worker surnamed Xiao, who entered the doomed building to collect his tools, was killed in the collapse, which, lying on its side and with its doors and window almost intact, is a shocking site.

It sparked outrage among the 500 homebuyers, many of whom almost invested their life savings in the Lotus Riverside project and are demanding a full refund and compensation from Shanghai Meidu Real Estate Development.

More frightening, however, is the fact Shanghai Zhongxin Construction, the firm working on the 11-block Lotus Riverside residential complex in Minhang district, was allegedly warned of the danger by the supervising company hired to oversee the project.

"We warned the construction company many times," said Wang Jinquan, head of Shanghai Guangqi Construction Supervision. "We have been telling them about the mud pile since the end of last year but we have constantly been ignored."

He explained that supervisors were in a "weak" position, adding: "We have to eat". He refused to elaborate further.

Experts like Shi Jiangang, director of the real estate studies institute at Tongji University in Shanghai say the incident highlights the lack of an effective construction supervision system in China, while others have called for a new independent body to be set up to oversee safety and quality standards.

"The fact the building collapsed tells us everything," said Shi. "They do not need to check to prove it was of shoddy quality; it was something the supervisors failed to find out or act upon."

Construction supervision was introduced in the 1980s, when ambitious city expansions were just beginning, said Ouyang Guanghui, deputy general manager of Shanghai Project Management, a company that has supervised a variety of projects, including the record-breaking Shanghai World Financial Center.

He said that after signing a contract with the developers, teams from the supervision firm visit a site to draw up a "plan of supervision" and then meet developers and the construction company to discuss it, as well as necessary materials, tools and qualifications for on-site staff. Once preparations are agreed, supervisors sign an order to begin work.

"The government gave supervisors some power to control quality, but not much," said Ouyang. "The supervisors are hired by the developers and report directly to them about any problems. If supervisors raise any potential dangers but are ignored by the developers, they are expected to inform the local authority's quality control department.

"If they do so, and it causes the project to be delayed, the developer might try to postpone or reduce the supervisory firm's fee. Some developers just want construction completed as soon as possible, so they can sell the building and make money."

He said building weaknesses would only be highlighted in extreme conditions, such as an earthquake, while for others, such as the case in Shanghai, all it took was a pile of mud and the fact it was rainy season.

Sub-standard construction is a major concern in China, where vast infrastructure projects are the norm as the country tries to keep pace with its fast economic growth.

In 2006, former deputy minister of State construction, Liu Zhifeng, said the average longevity of a residential building in China was only 30 years.

The collapse at the Lotus Riverside complex is not the only one to hit Shanghai in recent weeks, with the pit of an expansion project at the Southern Hotel just 1.5 km away falling last Thursday. It happened during heavy rain and is believed to have caused sinking to nearby residential blocks.

Four people were also killed when a bridge in Tieli, Heilongjiang province, crumbled to the ground on June 29. Initial investigations blamed the tragedy on an overloaded truck, while the State Council has ordered local authorities to step up safety maintenance on local bridges.

"It is all about money," said Shi at Tongji University. "As the country's property market is booming, developers cut costs to build faster. In choosing between quality and cost, some will go for lower cost and faster progress, sacrificing quality."

Building supervisor Wang said not moving the mud pile would have saved the Lotus Riverside developers "about 5 to 6 million yuan (US$730,000-880,000)" and explained the construction company had planned to use the mud, which came from the car park dig, for lawns in the complex.

In Shanghai, which is developing rapidly in preparation for the 2010 World Expo, 8,000 projects are being built, including 70 million sq m of residential space, he said, adding: "Since May, more than 2 million sq m has been sold every month."