Narrow, densely populated streets made Hong Kong’s West Island Line project extremely challenging. Andrew Bolton reports.
Once or twice a day on the streets of Hong Kong Island’s western district the sound of gongs fills the air.
Workers employed by contractors on the HK$15.4bn (£1.3bn) West Island Line metro project sound them to warn locals that underground blasting is about to take place.
This has been a regular occurrence over the past few months as contractors blast the access shafts and adits which will take them to the alignment of the running tunnels for the new line.
The West Island Line will extend Hong Kong subway operator MTR’s Island Line 3km into the western district, a densely populated high rise area of Hong Kong Island.
The district relies heavily on an overworked road network to give residents and businesses access to Central, the city’s main financial district.
When complete in 2014 200,000 people will be within walking distance of a metro station for the first time.
The completed line will splice into the existing tunnels to the west of Sheung Wan station, the Island Line’s existing western terminus.
From there it will run through soft ground before moving into drill and blast tunnel east of the new station at Sai Ying Pun.
From there, drill and blast tunnel continues through a second station at Hong Kong University until it approaches Kennedy Town, which will be a top down structure.
A lack of open spaces is one the main challenges facing contractors working on the project.
“We have been tied by the number of sites available to us, because the Western District is such a built up area,” says MTR West Island Line project manager Julian Saunders.
Where sites have been available, they have often had to be cleared of buildings before work could start.
“There is a push to get the conveyor system up and running so that spoil can be removed from the work face more efficiently.”
“There were very few sites where we could get access from day one,” adds Saunders.
Because work is taking place in such a densely populated area, construction related noise and traffic must be kept to a minimum and drill and blast tunnelling is also heavily restricted.
Most tunnel spoil will be removed via two underground adits.
One of these will ensure that more than 60% of the anticipated total of 550,000m3 of spoil generated by Contract 704 is removed without trucks.
Instead, 90% of the spoil from the drill and blast operations for the main running tunnels between the new Sai Yung Pun and Kennedy Town stations, including spoil from the station excavations themselves, will be removed via an extensive conveyor system capable of transporting 800t of material an hour.
This will carry material from the running tunnels via an adit to the base of a 45m deep access shaft close to Hong Kong Island’s north foreshore.
The conveyor system will carry the spoil up the 17m diameter Praya shaft and onto an enclosed and elevated conveyor bridge system, which will take the material out to a nearby barging point.
A third shaft, with an associated construction adit, has been excavated to provide access to the running tunnels at Sai Ying Pun station for contractor, Dragages/Maeda/BSG Joint Venture, which is building the section of line between Sai Ying Pun and Sheung Wan.
The access shafts for all three adits have massive noise attenuating sheds constructed above them giving the above ground part of the site the appearance of large warehouses. Each contains a gantry crane which straddles the shaft, ready to move plant and materials up and down as required by the blasting.
Work to drill and blast the adits is now in full swing. Contractor Paul Y has recently completed excavation of two of the three shafts under an advance works contract and these have been handed over to the Gammon/Nishimatsu WIL Joint Venture, which is responsible for construction of the adits as well as the running tunnels from Sai Ying Pun to Kennedy Town and the deep underground cavern stations at Sai Ying Pun and Hong Kong University.
At the end of January, 230m of the 348m, 11m wide and 9m high adit, which will run from the foreshore to Hong Kong University station, was complete and parts of the spoil conveyor have already been delivered and are being assembled at the base of the access shaft.
Tunnelling rates are restricted by the vibration limits imposed and the number of permitted blasts per day.
“At the moment we are blasting around 4m rounds and we are achieving three blasts every two days, largely due to mucking out cycles, which are restrained by construction noise permits associated with spoil removal,” says MTR senior construction engineer Kristian Murfitt who is working on the adit.
“We generally pull around 3.8m per 4m drilled round,” he adds.
Until the conveyor system is in place, trucks removing spoil are only allowed to operate between 7am and 7pm.
Work underground is allowed until 11pm at night, although this is restricted to removing spoil from blast faces to the shaft, ready to be removed from 7am the next day.
“There is a push to get the conveyor system up and running so that spoil can be removed from the work face more efficiently. This will enable the contractor to work 24 hours a day at the work face,” says Murfitt.
Blast operations are also limited by the proximity of the adit to piled building foundations.
At some locations it runs within 12m of the base of piles supporting surrounding high-rise buildings.
The third main construction access shaft is in King George V Memorial Park close to the site of Sai Ying Pun station.
Work from the 62m deep access shaft at King George V Memorial Park towards the running tunnel alignment at Sai Ying Pun station is progressing rapidly, boosted by the use of an electronic detonation system, thought to be the first commercial use in Hong Kong.
More than 100 detonators are inserted into explosive-filled drill holes to trigger each blast, and the timing of each detonation, measured in milliseconds, is vital.
Electronic detonators give contractors more control over the exact moment each detonator is triggered, thus giving them greater control over the sequence of small explosions which makes up each blast.
“We used 1m to 1.5m of water in the shaft. The water kills a lot of the noise,”
MTR construction manager David Salisbury says that the more commonly used NONEL (non-electric) detonators, which have greater timing scatter than electronic detonators, can occasionally result in some overlap of peak vibrations, which increase vibration amplitudes at the ground surface, and can also result in less material being dislodged in a blast.
For the shaft excavation at King George V Memorial Park, electronic detonation also enabled the contractor to flood water into the base of the shaft ahead of a blast to smother explosion-induced vibration and dust.
“We used 1m to 1.5m of water in the shaft. The water kills a lot of the noise,” says Salisbury.
Progress on the King George V shaft and adit has gone so well that MTR has asked the contractor to push west along the running tunnels into the adjoining contract to help speed progress there.
This contract, which includes the adit from University station to the foreshore access shaft, has been delayed by some difficult ground conditions.
Across the project, all tunnelling contracts have now been awarded, although the bored tunnel section at the eastern end of the line is not expected to get going until later this year when the Mixshield TBM arrives on site.
In addition, contractor Dragages/Maeda/BSG Joint Venture has ordered a tunnel dismantling machine (TDM) which will break out 120m of the lining of the existing tunnel west of Sheung Wan station, to allow one of the West Island Line TBM tunnels to splice into it.
“Work on the project is expected to peak later this year, when work is expected to be under way across the length of the route. This means ramping up tunnelling from four to five faces at once to 18,” says Saunders.
By then contractors will be working on running tunnels and passenger access tunnels and lift shafts for the stations which are embedded up to 70m deep in the steep hillside.
The steep slopes uphill from the West Island Line route present another challenge: limiting the risk of serious landslips.
At the western end of the line, work is under way to protect the site uphill from an access shaft at Kennedy Town station from landslips induced by heavy rain.
Contractors are currently completing construction of four material retention dams (pictured), built into the steep undulating hillside above the site of the access shaft.
These will catch debris and water sent down steep gulleys in the hillside during the summer rainy season.
Downhill of the catchment dams is a disused police barracks, which, before the dams are completed, is acting as mud slide protection for the community below and the access shaft.
Dam building is accompanied by the installation of boulder catching fences along with extensive soil nailing and shotcreting to stabilise adjoining slopes.
Detail design consultants Arup/Atkins joint venture and Meinhardt
Main contractor Sai Ying Pun to Sheung Wan Tunnels Dragages/Maeda/BSG JV
Main contractor Sai Ying Pun and Hong Kong University stations and Sai Ying Pun to Kennedy Town Tunnels Gammon/Nishimatsu WIL JV
Main contractor Kennedy Town station and overrun tunnel Gammon Construction