Hong Kong West Drainage Tunnel
International Spotlight

Use of the three Gia shuttlecars connected together allows the contractor to transport up to 34.5 cubic meters of spoil in a single trip without car switching.

Hong Kong experiences an annual average rainfall of some 86 in., one of the highest among cities in the Pacific Rim. It is located in a subtropical region and has an oceanic climate, making it vulnerable to heavy downpours and tropical cyclones.

With continuous urban development making the northern district of the island one of the most densely built areas of Hong Kong, the surface water runoff has considerably increased in recent years, reducing the flood carrying capacity of the existing drainage systems that were built decades ago and aggravating the flooding problems.

Affected low lying areas include Sheung Wan, Central, Admiralty, Wan Chai and Causeway – the financial and business districts of Hong Kong. This therefore disrupts the economic activities and business operations and leads to huge economic losses.
While the Drainage Services Department has made improvements and extensions to the aged systems, the improved drainage systems still fall short of current flood protection standards. They remain inadequate to deal with the flooding caused by heavy rainfall or typhoons.

To alleviate the flooding in Northern Hong Kong Island and thus minimize public inconveniences as well as economic loss during heavy rains, Hong Kong’s Drainage Services Department has implemented the Hong Kong West Drainage Tunnel (HKWDT) Project, which began in November 2007 and is scheduled for completion in 2012.

HKWDT Project

The scope for the new HKWDT project involves construction of a new 11-km long drainage tunnel deep in the ground in the mid-levels of the island from Tai Hong to Pokfulam. The scheme will intercept and convey the storm water from the upper catchment areas directly into the sea near Cyberport. The project also includes 34 intakes and 8 km of connecting adit tunnels and ancillary works.

A Dragages-Nishimatsu joint venture is responsible for constructing the tunnel in two sections. The first section, 4.5 km long and 6.25 m in diameter, will run from Tai Hong and under the Aberdeen tunnel. The second, 6 km long and 7.25 m diameter, is from the Aberdeen tunnel to Cyberport. The contractor is using two TBMs for this project.

“The construction sites are situated within the hub of the city and close to residential and commercial districts. This presents great challenges with the construction works,” Director of Drainage Services Lau Ka-keung said. “To shorten the construction period with a view to reducing construction nuisance to the public, the contractor is required under the contract to deploy two tunnel boring machines to carry out the tunnel construction simultaneously.”

Crews are using two 8HR2-B Häggloaders and three sets of Gia locomotives and nine shuttlecars for the excavation of the adits and spoil removal at the western portal for off-site clearance.

With 27 of the adits measuring 2.59 m width x 2.25m height and a single adit measuring 3.5 m width and 3.435 m height, the Gia equipment was purpose-chosen for its ability to operate in such compact tunnel construction.

A key factor in the joint venture ordering the Gia equipment followed representatives visiting the Gemerska Polona project in Slokavia where an identical Gia configuration was being used to excavate a 4.2 km long tunnel with a cross sectional area of just 11.5 sq m, highlighting the advantages of the system in small dimensional tunnels.

The 8HR2-B Häggloaders, generally fitted with two digging arms to speed the mucking out process at the face, has been modified to include just one arm due to space constraints. The arm is also used for scaling the blasted profile.

When used in conjunction with its built-in conveyor, the Häggloader is able to continuously load the blasted spoil from the face into the Gia shuttlecars in an uninterrupted, spill-free loading operation; offering loading capacities of up to 3 cubic meters per hour.

For the first 5 to 6 m of the adits from the main tunnel junction, the contractor will use rock splitting techniques to open the adit, preventing possible damage to the high voltage supply and conveyor belt systems in the main tunnel.

Thereafter drill-and-blast methods will be introduced. Blasting is carried out once a day working on, according to JV plant manager, Masanori Ishii, up to 12 faces simultaneously. This includes initially, for example, two faces of the first adit, three faces of the second adit and three of the third.

Spoil is removed from the face after blasting by the Häggloader and loaded directly into the Gia shuttlecars.

Mucking Operation

Using three Gia 115 CE shuttlecars allows loading from car to car using internal conveyors to ensure the fastest loading system possible. With each 1,700 mm car featuring an 11.5 cubic meter (22,000 kg) capacity along its 11.2 m length, use of the three cars connected together allows Dragages-Nishimatsu to transport up to 34.5 cubic meters of spoil in a single trip without car switching.

Up to eight cars can be coupled to suit different size construction sites and, by matching the number of cars to the volume of blasted rock, it is possible to transport up to 100 cubic meters of spoil in a single trip. The shuttlecar floor is lined with Hardox 500 wear parts to ensure long life and low service requirements.

At the discharge point outside the tunnel at the western portal, all three shuttle cars discharge the spoil from the front car using the internal conveyor system in an operation that takes less than 5 minutes.

The cars are hitched to a Gia DHD25 hydrodynamic diesel locomotive that has a maximum speed of up to 30km/h. The 25,000 kg class locomotive is powered by a 144-kW engine.
For the shorter adits, the contractors is using a skid loader to remove the spoil, directly loading the Gia shuttlecars in the main tunnel, in place of installing rails for the Häggloader. Currently the contractor is achieving TBM advance rates of 450 m per month in the main tunnel, although this recently slowed to around 200 m when water was encountered.

The Hong Kong West Drainage Tunnel spans 11 km under the city.

Rock conditions are however generally good, although a number of faults have been identified. Two types of rock are predominant – granite at both portals with a toughness factor of 200-300 MP and up to 400 MP in the central section.

Drop Shafts

A total of 34 intake shafts are to be excavated to intercept waterflow from existing drains or streams along the 11 km route. Most will be constructed using the raise boring method initially using a small diameter pilot drill down to the adit. At the bottom a larger reamer is installed to the drill rod. The reamer then bores upwards to the ground excavating the shaft along the way. All excavated material falls to the bottom for retrieval.

The finished drop shafts feature a mix of three diameter sizes – 1.5 m, 1.8 m and 2.3 m and a variety of depths varying from 17 to 170 m.

In a 49 month contract, completion of construction is scheduled for December 2011. The consultant engineer for the project responsible for planning and design is Ove Arup & Partners Hong Kong Ltd.

When the Hong Kong West Drainage Tunnel goes online in 2012, the new drainage tunnel alleviate flooding in Northern Hong Kong Island, in particular the low-lying areas along Sheung Wan, Central, Admiralty, Wan Chai, Causeway Bay and Happy Valley, and will substantially improve the overall flood protection level of the Hong Kong Island. Most importantly, the potential risk of nuisance to the public, loss in economy and disruption to traffic arising from flooding will be much reduced. In the long run, these will be a great benefit to Hong Kong as a whole.

This article was based on a press release from Gia Industri AB, based in Sweden. Web: www.gia.se.