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Richard Woo

A group of architect firms has called for the re-adoption of manual caissons for foundation works to speed up construction and eliminate defective piling.

The Association of Architectural Practices (AAP) said foundation works by hand-dug caissons, which is banned, should be resumed on condition that new mitigation measures were imposed.

In a proposal recently submitted to the Construction Industry Review Committee chaired by Executive Councillor Henry Tang, AAP said the manual caisson method could speed up construction by more than one year in some cases.

The Government decided to abolish manual caisson operations in 1993 due to the high accident rate involving caisson workers and the significant number of lung diseases as a result of the poor work conditions.

A caisson is a watertight chamber used for foundation construction work, one of the piling systems widely used in Hong Kong before 1993.

AAP is an organisation of professional architecture firms.

Chairman Dennis Lau Wing-kwong said they the association had told Mr Tang that to eliminate defective piling, the most reliable course was to resume the caisson system.

The problems of inadequate length and improper founding conditions have been discovered in other common piling systems such as pre-cast concrete piles, bored piles and steel H-piles.

Mr Lau said the caisson system, unlike the alternatives, did not use machines for piling. Hand-dug caissons completely exposed bedding conditions so that they could be examined and sampled before concrete was placed. This enabled the quality of the foundation work to be assured.

Foundation works also could be completed more quickly, he said.

Mr Lau estimated foundation works using the caisson system could finish the job in 30 per cent to 50 per cent less time than other piling systems.

One of the constraints affecting most other piling systems is the necessity of huge mechanical plants and equipment. Sites in Hong Kong, due to their small sizes, usually could allow only two or three bulky plants to work simutaneously, he said.

In contrast, a large number of caissons could proceed in parallel, making foundation works speedier.

Problems with mechanical plant became more obvious when building construction took place on sloping sites, which was common in Hong Kong.

To accommodate heavy piling machines, a platform had to be built before foundation works could begin. These platforms cost between $40 million to $50 million.

Even with a platform, the process was relatively dangerous, Mr Lau said.

The Buildings Department would be cautious in approving a building using such a platform, meaning it may take builders six months to obtain the permit, he said.

Adding the four or five months delay due to the slower work by mechanical piling, and the two to three months it took to demolish the platform, a non-caisson system could add one year to foundation works, said Mr Lau.

He added that besides sloping sites, construction over railway depots and over tracks, which also was a common situation in Hong Kong, would make other piling systems technically impractical due to the restrictive working spaces.

He said resuming the use of the caisson system also would help the 4,000 to 5,000 unemployed caisson workers.

However, in order to minimise potential threats to caisson workers' safety, the AAP proposed several measures be imposed, such as the provision of a larger caisson if the system was resumed.

Artificial lighting and mechanical ventilation was proposed to maintain decent and dust-free conditions in the shaft, while sensor devices should be installed to detect poisonous gas.

Ladders should be available for workers to escape in case of accident.

Mr Lau believed a well-equipped caisson was no more dangerous than working in a mine.

He said despite the caisson system's inherent risk, the focus should be directed towards ensuring a proper working environment instead of banning the operation altogether.

'This is analogous to the case of athlete's foot. We tackle the disease by maintaining a hygienic environment instead of having our feet cut-off,' he said.



Architects in appeal for return to faster caisson foundations