On 23 July 2016, a technical visit to the Shek O Casting Basin for Immersed Tube Pre-cast Units Construction Works organised by the IOM3 Hong Kong Branch was held.
Before the site visit commenced, an introduction and a presentation of the project background were given to the participants by Construction Manager, Mr Fumihiro AIKAWA, and Senior Construction Engineer, Mr Cano NGAI, of the Mass Transit Railway (MTR) Corporation.
Mr Cano NGAI (L) and Mr Fumihiro AIKAWA (2nd L) of MTR giving introduction and presentation to the participants
In the presentation, Mr Aikawa and Mr Ngai briefed all the major scope of work for Shatin to Central Link (SCL) Contract 1121 being constructed the cross harbor tunnel to extend the East Rail Line (EAL) from the North Ventilation Building at Hung Hom Landfall to the Causeway Bay Typhoon Shelter (CBTS). As Immersed Tunnel (IMT) was selected as the construction method, a suitable casting basin is a pre-requisite for commencement of the immersed tube prefabrication work.
Shek O – The Unique Casting Basin in Hong Kong
The ex-Shek O Quarry is the unique location in Hong Kong that is suitable for such large scale precasting work. It is located on the south-eastern part of Hong Kong Island, on the western side of D’Aguilar Peninsula and next to Shek O Country Park. Its favorable conditions for prefabrication of immersed tube tunnel sections includes its large and deep basin, which is capable for pre-casting of the whole immersed tunnel (11 elements) in one go.
In fact, the ex-Shek O Quarry had been used for prefabrication of immersed tube tunnels for the Western Harbour Crossing (WHC) and the Lantau and Airport Railway (LAR) Contract 502 Western Immersed Tube (WIT) in the mid-1990s. After the completion of the pre-casting works for both tunnels, the casting basin was handed over to the Shek O Quarry operator and then back to the Government for rehabilitation of the Quarry in 2010.
Shek O Quarry Developments from mid-1990s to 2010
Current Lookout View of Shek O Casting Basin
On-site Setting Up for Pre-cast Unit Construction
Three major items are needed to be set up in Shek O site to facilitate the pre-casting of the units. The first one is the temporary dock gates that were used to dam the two existing openings at the basin to allow the pumping away of contained seawater. A dock gate
was formed as a gravity-type retaining wall made of seawall blocks with a sheet pile wall in front to seal in the water.
The second item is the barging point to facilitate the delivery of major construction materials by sea and relieve the traffic loading on Shek O Road.
The last item to be set up is the on-site concrete batching plant. It is the most critical facility for the reinforced concrete tunnel box pre-casting since the supply of concrete will directly affect the progress of work.
Panoramic View of Shek O Casting Basin
IMT Tunnel Construction Arrangement & Method
Once the casting basin was dewatered, formation work was carried out and concrete paving placed for precast unit construction.
The 1.66km long pre-cast immersed tunnel is divided into 11 elements with typical length of 156m. Each element will be further divided into nine bays for on-site construction. The typical construction sequence of an immersed tunnel is described as below.
1. Install steel plates at bottom and both sides (approx. 1.7m high) of tunnel external walls as waterproofing membrane.
2. Construct the tunnel base slab with wall kicker.
3. Construct the tunnel wall and roof slab.
4. Pre-stressing (post-tensioning) work and apply waterproofing to external wall and roof.
5. Install ballast tanks inside tunnel and steel bulkheads to seal up the tunnel openings at both ends.
6. Finally, the basin will be flooded and dock gate removed for towing of the completed pre-cast units to their location for the future fourth rail harbor crossing for Hong Kong as part of the SCL works.
Participants with Immersed Tube Tunnel Precast Unit at Shek O Casting Basin
Participants at Shek O Casting Basin
During this visit, the participants had a better understanding of the construction of immersed tube pre-cast units.
Mr Tim Leung (L), Hon. Treasurer, and Mr Kelvin Choi (R), Hon. Secretary, of IOM3(HK) presenting certificate of appreciation and souvenir to Mr Cano Ngai (2nd L), Senior Construction Engineer, and Mr Fumihiro Aikawa (3rd L), Construction Manager of MTR
On behalf of the IOM3 HK Branch, we would like to express our sincere thanks again to MTR for the kind arrangements of this site visit to Shek O Casting Basin for Immersed Tube Pre-cast Units Construction Works, in particular we would like to express our gratitude and deep appreciation to the project team of MTR SCL Contract 1121 for receiving us. Our members enjoyed very much the visit and were particularly impressed to have Mr Aikawa to give the detailed background of all the immersed tunnels constructed in Hong Kong. Everyone from the visit shared their great appreciation.
Aikawa F. 2015. Shek O Casting Basin for Immersed Tube Pre-cast Units Construction, Project Link Issue No. 40, Nov/Dec 2015, p.12 – p.14.