The Central District Subway System

The Central Pedestrian Subway, which connects MTR Central Station to Hong Kong Station of the Airport Railway, is constructed about 18 m below the existing road surface within the busy downtown area of Central District, with a total length of about 650 m.

Works at the Early Stage

The project commenced in mid-1995 and was subdivided into several major work stages. First, concrete pile wall was constructed along the parameter of the subway by using boring machines. The pile wall acted as the support during excavation and became part of the permanent retaining structure for the future subway. Working in parallel, the contractor commenced the removal of the existing underground facilities within the disrupted areas, such as drain pipes, water pipes, cables and gas pipes etc.

Excavation Under Connaught Road Central

When these preparatory works had basically been completed, a temporary road deck was constructed on Pedder Street and Connaught Road Central for the reinstatement of traffic during the excavation and construction of the subway. The deck was composed of steel/reinforced concrete composite panels supported on steel joists that rested on the concrete piles. After the deck had been completed, the excavation of the subway, which was done mainly in a bottom-up manner under the deck, began. When the downward excavation had been completed to a certain depth, lateral steel struts were installed to stabilise and strengthen the concrete pile retaining structure on both sides. After the first level of struts were installed, the process was repeated until the excavation was completed to the required depth. The main structure of the subway was then constructed by the use of large panel-type formwork to form the walls and the dividing slabs.

Precautionary Works

There were a number of technical problems when works were carried out under Connaught Road Central. Firstly, there were a large number of underground pipes and public facilities   some of these drain pipes with more than 1 m in diameter. The work area did not have adequate space to relocate all these facilities, so many of the facilities had to be protected and hung underneath the temporary road deck during the construction process. Secondly, there were several pedestrian footbridges crossing Connaught Road with their foundation laid within the new pedestrian subway. Special provisions were required to support or underpin the foundations of these bridges. Besides, the dual two-lane vehicular subway that cut through the junction of Pedder Street and Connaught Road further limited the working spaces onsite. And of course, close monitoring of the surrounding buildings to ensure their stability was an inevitable job during the entire process.

Subway Section Under Exchange Square

The subway section that runs under Exchange Square is the most impressive part of the subway. During the early stages of Exchange Square's planning and construction, the Government reserved two strips of underground space for the construction of this subway. A vertical shaft was constructed first at the south of the bus terminal of the Exchange Square. The shaft was used as access and ventilation to facilitate the future excavation of the subway. The shaft was about 25 m long, 6 m wide and 14 m deep in size. Excavation laterally would then proceed at the bottom of the shaft southward in order to form two 3.5 m-high preliminary tunnels. At this stage, temporary excavation support was not required because the existing caisson piles of Exchange Square on both sides of the tunnel were firm enough as support during excavation.

After the preliminary tunnel was completed, a 200 mm diameter mini-pile wall would be installed on each side of the tunnel by the use of boring machines. The wall acted as the support for the second stage of excavation.

Completion

In September 1997, the diaphragm wall separating the subway and Hong Kong Station of the Airport Railway was broken through as scheduled, signifying the final connection of the subway system. The fitting out work and the installation of facilities then commenced.

This large pedestrian subway system is designed to cater for the movement of 40,000 people every hour, and is equipped with facilities such as central air conditioning and a 300-metre travelator system for the swift movement of passengers. This subway was completed and opened to public for use in June 1998. It is the longest and largest pedestrian subway with air conditioning in Hong Kong.

main contractor
Kier-Sun Fook Kong joint venture

 

A Large-Scale Pedestrian Project in Central


Connaught Road Central location of the pedestrian subway as seen in September 1996, when the work was almost at its peak. The vehicular subway that cut through the junction of Pedder Street and Connaught Road Central added to the complexity of this project.

Part of the road surface of Connaught Road was closed to public traffic. Boring machines are busy at work here constructing the in situ concrete pile wall and other supporting structure for the subway.

In this photo, a temporary road deck is constructed prior to the excavation work of the subway. The deck is made of steel/reinforced concrete panels supported on a steel joist frame, resting on the in situ concrete pile wall.

A view of the completed road deck. An access opening for the excavation can be seen in the lower centre of the photo.

As excavation proceeds further downward, the second row of struts require installation. As seen in this photo, the slab of the subway has been placed, while the concrete pile wall is being trimmed, to ready it for the casting of the subway wall.

Under the deck along Connaught Road, the first layer of excavation has been completed to a depth to about 4 m. Lateral support using steel struts had to be installed at this stage before the second layer of excavation could proceed. Temporary drain pipes can be seen hanging underneath the deck.

Inside the subway during the early stage of excavation. The underside of the road deck is depicted at the top. On the left are the in situ concrete piles, which served as the cut-off wall for support of the excavation ,and became part of the final sub-structural wall of the subway. The vertical steel columns are H-piles (known as 'king post') which are used to temporarily support the deck during excavation. Rows of steel pipes just above the soil surface strengthen the pile walls.

The elevated bridge system for pedestrians at the junction of Pedder Street and Connaught Road. In the photo, the bridge is having its supporting piers rested within the boundary of the pedestrian subway.

The entrance to the vertical shaft located at the southern end of the bus terminal under Exchange Square. This shaft was the only access route to the tunnel at the first stage of work under the Exchange Square section.

A concrete truck and a booster pump parked near an access opening. The vehicles carry out concreting work for the subway below.

An anatomical view inside the subway showing how the concrete piles for the pedestrian bridge above are being supported.

Excavation for the double-height entrance lobby at the junction of Connaught Road and Douglas Street. The depth of excavation is almost 20 m lower than the road surface at this point. Because the depth and area of work in this location is massive, a web-like strut system constructed of 0.75 m diameter steel tubes was used to laterally support the soil.

Looking down into the vertical shaft from the ground level. At the bottom of the shaft, excavation was re-oriented horizontally toward the Connaught Road side until it met the other end of the subway.

Heavy steel sections to be used as excavation support are lowered into the subway through a Connaught Road access opening.

A look at the 18 m deep subway interior at the junction of Connaught Road and Douglas Street. Temporary hanging bridges for workers can be seen in upper and lower levels.

The preliminary tunnel form under the bus terminal of Exchange Square. At this stage, the excavated void is basically stable: the top is formed by the reinforced concrete raft cap, the sides by the large diameter caissons (H-piles) supporting the Exchange Square superstructure. From this preliminary tunnel, row of pipe-piles on each side were driven into the soil below to form the support for the second-stage excavation.

View of the same tunnel three months later, when second-stage excavation was nearly completed. The supporting pipe-piles at two sides with the lateral strut-supported system can clearly be seen in the photo. A gantry that moves on a rail (the red beam-like structure at top) is provided for the removal of the excavated equipment and materials.

Looking upward from the bottom of the nearly formed subway. The entire section of the subway space, located some 20 m below Exchange Square, can be seen.

Structural work inside the subway, seen at the peak of activity.

The final subway structure being cast from the tunnel space, using the usual shutter forms and bottom-up method.

The fitting-out work signifying the final
completion of the pedestrian subway. The long
strip-like rack on the lower left hand side
(covered by a plastic sheet) is thepedestrian conveyor.