Gammon takes the bother out of building Hong Kong’s deepest tunnel
Constructing a tunnel isn’t one of the easiest things for a contractor to do, even at the best of times.
So when that tunnel is going to be Hong Kong’s deepest tunnel, you’ve got to really put your thinking cap on.
That’s what Gammon Construction did on the North Point to Wanchai section that it is responsible for on the HK$9.2 billion Harbour Area Treatment Scheme (HATS) project for the Drainage Services Department.
For company director Kennedy Cheung Tat-fai, a qualified engineer, the problems arising from working at a depth of 160 metres below sea level, equivalent to a 50-odd storey commercial building, were tricky indeed.
First and foremost was the water pressure.
At the depths concerned, water pressure was a big problem, reaching 16 bar compared with 4 bar for a roadside watermain burst on the surface.
There was also the Cross Harbour Tunnel and the MTR Corporation’s subway tunnels, structures best left undisturbed.
With that in mind, Gammon introduced several innovative measures to make sure construction proceeded as smoothly as possible.
First was making sure it knew where its workers were in the tunnel, vitally important when the drill-and-blast method is being used.
To solve this problem, Gammon turned to RFID or radio frequency identification, commonly used in the logistics industry, for its Personal Tracking System.
Each worker is issued a RFID chip which is read by card readers sited every 100 metres along the tunnel.
The card readers have a range of 50 metres.
“With this system, at any time, we know how many people are working in the tunnel and at what location,” Cheung said.
It is the first time such a system has been used in a tunnel in Hong Kong, he added.
With labour in short supply and the twin constraints of safety and speed to consider, Gammon took a leaf out of the book of the quarrying industry in Australia.
To remove muck arising after the blast safely and faster, it decided to use the Automatic Shaft Mucking System.
Doing away with manual operation, the system carries up muck at a speed similar to a passenger lift, four times faster than a conventional shaft lift.
“The operator sits up in an office on the surface,” Cheung said.
Activities after blasting were speeded up also.
Gammon deployed 3D laser scanning to accurately survey the situation of the tunnel after blasting, typically taking only 15 minutes to complete.
“If it was manually done, it would take four to five hours. This is the biggest difference,” Cheung said.
Inspections of the tunnel face by geologists and surveyors were aided by use of computer tablets that sent information such as the number of soil nails required back to the surface immediately for colleagues to follow up.
“They are vibration-proof, water-proof and dust-proof,” Cheung said.
The working environment and care of workers were also looked at.
“These two issues are what will attract workers to come to our site and not go to the site next door,” Cheung said.
Tunnel construction is characterised by extreme heat and relative humidity, something that partly prompted workers on a MTR Corporation (0066) Express Rail site to hold a quick strike last August.
To make sure workers didn’t start dropping like flies, Gammon put in chiller plants to ventilate the tunnels with chilled air such that the temperature was kept at 26 degrees centigrade or below with relative humidity capped at 90 percent.
To prevent lighting from adding to the heat, LED lights, which emit less heat for the same light output, were installed despite the cost being four to five times that of a normal fluorescent light.
Apart from throwing only one-fifth of the heat, there were no toxic gases released if the lights were damaged.
Getting along with neighbours such as the Society for the Prevention of Cruelty to Animals building next door to the drop shaft in Wanchai was another consideration.
To cut down on the noise arising from drill-and-blast, Gammon erected a fully enclosed noise enclosure such that the only signs of construction activity were dump trucks going in and out to remove muck.
So far there have been no complaints from the SPCA.
“Even the dogs didn’t bark,” Cheung said.