Fears over blast plan for HK Island tunnels

PUBLISHED : Sunday, 04 January, 2009, 12:00am
UPDATED : Sunday, 04 January, 2009, 12:00am

A decision by the Drainage Services Department to use explosives to blast almost 20km of tunnels on Hong Kong Island has raised concerns.

The planned use of traditional drill-and-blast methods, instead of tunnel-boring machines, has caused some discomfort within the industry, sources say.

The 19.5km of tunnels, budgeted to cost about HK$6.2 billion, form the backbone of a HK$13.25 billion project to send sewage from Hong Kong Island's main centres under the harbour to the treatment works at Stonecutters Island.

That plant is also to be upgraded as part of the enterprise.

Drill and blast, as the name suggests, uses controlled explosions to advance a tunnel. However, many recent projects have opted for tunnel-boring machines, which act basically like massive electric worms, grinding away at the rock and spitting the waste out behind them.

But problems with a faulty machine and a dispute with a contractor led to the first phase of the Harbour Area Treatment Scheme being completed 41/2 years late and HK$2 billion over budget.

The second phase is expected to get under way next month.

Perhaps the biggest single contributor to delays on that project was that groundwater seeped into the tunnels at up to 100 times the rate forecast.

That played a large part in the decision not to use boring machines, according to the department.

'One of the key considerations is that the drill-and-blast method facilitates grouting at the excavation face to better control water flows into the tunnel,' a spokesman said. He added that there had been no complaints from bidders for the project.

Grouting is a process whereby a high-pressure sealant is pumped into the rock for a distance ahead of the actual works, the aim being to block any fissures through which groundwater might flow.

The department has also sought to calm concerns about the noise generated by blasting associated with the project.

'Blasting will be carried out in tunnels located deep underground - more than 70 metres below sea level in Aberdeen and Cyberport - so noise is not expected to be a problem at the ground surface,' the spokesman said.

No explosives would be stored at the work sites. Instead they would be brought in daily by specially designed vehicles.

However, another tunnelling engineer pointed out that boring machines could work 24 hours a day and were not dependent on the just-in-time delivery of explosives or highly skilled staff.

'A boring machine, if you bring in the right type and set it up well for the ground conditions and the rock type, will go like a dream,' he said. 'But if you get it wrong, it's your biggest nightmare.'




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