Western Harbour Crossing

Out of the ten Airport Core Projects, the Western Harbour Crossing is the only one which is operated under a Build-Operate-Transfer (BOT) type contract for a franchised period of 30 years. It is also the longest and largest cross-harbour tunnel with three lanes of traffic in each direction.

The $5.7 billion contract started in August 1993 and continued for a period of 47 months. Besides the construction of the immersed tubes which form the cross-harbour tunnel, the contract also included the carrying out of the traffic interchanges, the ventilation shaft/building with the required mechanical facilities, and the tunnel approach sections on both sides of the harbour. In addition, an administration building complex and a toll plaza with 20 tollbooths were also included in the contract.

To facilitate the casting of the immersed tubes that formed the tunnel sections, the government allocated part of the quarry areas in Shek O for the contractor to form into a working yard for the casting works. The yard was in the form of a dry dock in which 12 immersed tubes were cast in three batches, four tubes at a time. Each batch required about 7-8 months to complete. After each batch had been completed and sealed on the ends, sea water was allow to flood the yard until the tubes floated. The tubes were thendragged to a preparation yard at Tseung Kwan O for further placing in the harbour at a scheduled date.

Over 950,000 cu m of marine deposit was dredged for the 1,360 m trench in the seabed to receive the immersed tubes. The base of the trench was about 40 m wide and averaged 12 m deep with a 1:2 slope cut on the sides.

Two temporary survey towers were erected on each immersed tube before the tube was dragged to its harbour location. Together with other control points on each side of the harbour, the tubes could be placed in the seabed with precision up to 20 mm. The sinking process was controlled by two winch pontoons. Water was pumped into the immersed tube to give the unit negative buoyancy with the loads transferred onto the pontoons. Controlled sinking was achieved by ballasting the unit with around 1,200 tons of sea water. When sunk to the base on the seabed which had been prepared for the immersed tube, the joining action to the adjoining tube was achieved by lateral water pressure. With the level and alignment receiving final adjustment from the winch pontoons atop, sand was then poured into the base to stabilise the position of the tube.

To provide ventilation to the 1.36 km tunnel, ventilation facilities with gigantic fan and duct systems housed in a ventilation building are provided on each side of the harbour. The building was built on a 38 m shaft constructed of contiguous diaphragm wall panels. The ventilation shaft also formed the coupling structure between the immersed tube and the approach ramp.

On the Sai Ying Pun side of Hong Kong, complicated traffic interchanges are required to handle the vehicular flow coming from or entering into the tunnel from western and eastern directions. A series of interchanging facilities including at-grade approach roads, elevated bridges and footbridges were built in the reclaimed land at the entrance of the tunnel so reserved for the purpose. On the Kowloon side, the interchange involved is simply a direct connection onto the Kowloon Expressway through the approach ramp and the tolling square.

There is a 200 m approach ramp leading traffic from the immersed tube of the tunnel to the road level on both sides of harbour. The approach ramp was formed by the use of diaphragm wall or sheet pile walling system, and constructed using cut-and-fill. In certain sections, a top-down method was used.

main contractor
Nishimatsu Kumagai joint venture

 

Western Cross-Harbour Tunnel

The largest cross-harbour tunnel in Hong Kong

The casting yard for the tunnel tubes at Shek O Quarry. The yard was subdivided into two main casting basins, one for the casting of tunnel tubes for the Western Harbour Crossing (on right side of photo), the other for the airport railway tubes. This photo was taken in February 1995, when the first batch of the tunnel tubes had been completed and were removed from the casting basin, while preparation was carried out immediately to prepare for the casting of the second batch of tunnel tubes.

The casting basin for the airport railway tubes with four tubes at their final completion stage. All that remained was to finish the necessary tensioning works (the workers are doing the stressing work on the right hand tube) and install the sealing gate at both ends of the tunnel tubes.

Excavation in progress for the water gate and sea channel.

Close-up of a typical tunnel tube under construction.

The casting yard with the second batch of tunnel tubes near completion. The photo shows that the water gate and the sea channel for the casting basin for the airport railway tubes was still under construction.

Another view demonstrating the formation of the casting basin. The basin was in fact a 3.2 ha dry dock, 14 metres deep and 11 metres below sea level.

The casting arrangement of the tunnel tubes. Each section was cast in six identical sub-sections using in situ method. In order to ensure the tunnel tubes would float when flooded, the base of the tubes were seated on metal plates. The dimensional assurance of the overall tube length was controlled by the junctioning section at both ends, which was cast separately using a standard steel junctioning/coupling frame.

The setting up of the formwork and seating arrangement preparatory to the casting of a tunnel tube. In the photo, the slide-action steel shutter used on the external side wall, the top and bottom section of the retractable form for the inner sections, and some self-standing and attached-on scaffolds, can be seen.

A cross-sectional view of the formwork arrangement inside the tunnel tube. Note also the coupling detail at the end of the tube.

The steel shutter system which formed the external side wall of the tunnel tube. This shutter system could be released and slid sideways on track rail to the next sub-section to repeat the casting process.

A virtually completed tunnel tube awaiting sealing of the section ends. The steel sealing plates are stacked and ready for the final installation.

Close-up of a sub-section of the tunnel tube during the casting process.

A standard retractable formwork unit used to form the inner sections of the tunnel tube. This particular formwork unit was used for the casting of the airport railway tube; Those for the Western Harbour Crossing are much larger.

A tunnel tube being towed to the approximate location in Victoria Harbour. The two survey towers mounted on the tunnel tube and as several survey points stationed on both sides of the harbour were used to control precise positioning of the tube in the prescribed location. The two barges were used to control the sinking of the tunnel tube. Though the weight of each tube was close to 15,000 tonnes, careful pumping of sea water allowed the sinking to be easily controlled by these two 350-tonne barges.

The box-sectioned water gate for the casting basin. The 45 m water gate was formed by four identical water tank-like steel boxes, which could be filled with water as required. They were dragged aside to allow passage for the tunnel tubes later. The basin's pumping facilities for the basin can also be seen in this photo.

One water gate section temporarily stored in the casting basin. This gate section was earmarked for the casting basin of the airport railway tubes.

In December 1994, the first batch of tunnel tubes were completed and towed to a temporary preparation yard at Junk Bay prior to their sinking.

The junction between the seaborne tunnel tube and the landborne approach tunnel at the Kowloon side. The ventilation building will be built above this junction with the ventilation shaft immediately above it.

The construction of the approach tunnel section used a relatively simple cut-and-fill method. The sides of the approach tunnel in Sai Ying Pun were constructed of diaphragm wall, which was strut-supported during the excavation process. The structure under construction in the background was the ventilation building for the tunnel.

The early stage of excavation for the approach tunnel as seen on the Kowloon side, with the retaining reinforced concrete diaphragm wall on both sides of the cut exposed.

The interior of the approach tunnel on the Sai Ying Pun side as seen during the excavation and construction stage.

The final finishing work inside the tunnel tube. Note the hang-on precast concrete panel that formed the service void under the ceiling of the tunnel tube. The three square openings on the top front form the air shaft leading to the ventilation building.