Project Features

Project Challenges

I. Challenge – Passed through traffic arteries by Mined Tunnel

Cross Harbour Tunnel (“CHT”) is the busiest road tunnel in Hong Kong, with daily traffic flow up to about 115 000 number on average. The CWB project needed to construct a mined tunnel of approximately 160m long, 50m wide within the rock stratum 20m underneath the CHT. To safeguard the continual operation of the CHT at the same time proceeding with the tunnelling works satisfactorily was regarded as one of the most challenging parts of the CWB project.

Having analysed the implications of different excavation methods to the surrounding, the “Drill-and-Break” technique which would generate the least vibration was adopted for the construction of the mined tunnel. A series of precautionary and contingency measures including the installation of automatic deformation monitoring system (ADMS), additional steel weights, standby dewatering system, etc. were also implemented to ensure the integrity of CHT during construction.

Around 820 000 m3 of rock were excavated for the construction of the mined tunnel. The average uniaxial compressive strength of the rock reached 119MPa, which is about 4 times the strength of concrete. These hard rock caused a higher-than-expected wear and tear on the construction plants. To tackle this problem, the project team adjusted and deployed a variety of machineries and employed experienced frontline staff to optimise the excavation procedure. During the peak, there were 6 workfronts with over 30 machineries in different varieties and 90 technicians deployed for the construction of mined tunnel, achieving an excavation rate of 500 m3 of rock per day. With the efforts made by all sides, the Mined Tunnel was successfully breakthrough in October of 2014. Throughout the construction period of the mined tunnel, the ADMS readings were stable and within safe zone, which indicated that no adverse impact on the CHT portal structure was made by the works.

The works of mined tunnel underneath CHT was widely recognized by both local and worldwide engineering discipline. The project team was invited to attend different conference and publish articles in academic journals to share their experience. The works was also highly regarded and won numerous awards including the “Tunnelling Project of the Year 2016 (Over US$500M)” of the New Civil Engineer (Journal of Institution of Civil Engineer, United Kingdom) and “Major Project of the Year 2016 – Certificate of Appreciation” by International Tunnelling and Underground Space Association.

Mined Tunnel
Excavation works
New Civil Engineer – Tunnelling Project of the Year 2016 (Over US$500M)
II. Challenge – Construct tunnel under the sea without permanent reclamation

Victoria Harbour is one of the most precious resources of Hong Kong. To preserve the appearance of Victoria Harbour, CWB adopted the “Cut-and-Cover” method coupled with temporary reclamation for tunnel construction in Ex-Wan Chai Public Cargo Working Area and Causeway Bay Typhoon Shelter. By doing so, the temporary reclaimed land could be removed upon completion of the tunnel and the seabed and original appearance of the harbour could be restored. The procedures of temporary reclamation are outlined below:

  1. Remove marine deposits at seabed such that the seawall blocks could be secured in place
  2. Install seawall blocks along the perimeter of the proposed temporary reclamation area
  3. Fill soil to form a temporary working platform
  4. Install diaphragm walls to provide support during deep excavation and tunnel construction (majority of the diaphragm walls were used as side walls of the permanent structure)
  5. Excavate soil inside the diaphragm wall progressively to designated depth with lateral supports installed by layers
  6. Upon completion of excavation, construct tunnel structures in a bottom-up order (from base slabs, side walls, middle walls to roof slabs)
  7. Backfill the space above the tunnel to the original seabed level and remove the temporary features above the seabed upon completion of the tunnel section

Temporary reclamation was implemented in phases with a view to reducing the occupied area of sea and providing sufficient berthing spaces for 500 vessels inside typhoon shelter, thus maintaining their daily operation.

The CWB project removed approximately 640 000 cubic metres of marine sediments within CBTS and hence improved the water quality.

Step 1
Step 2 and 3
Step 4 and 5
Step 6
CBTS during temporary reclamation
CBTS upon reinstatement
III. Challenge – Construction works at Island Eastern Corridor Link next to busy traffic

Some sections of the Island Eastern Corridor need to be modified to connect and provide space for constructing the eastern tunnel portal of CWB. Since the construction works was carried out in parallel with the busy traffic, the need to proceed with the construction works while minimising the disruption to road users become a challenge to the project team.

In view of this, modification works of relevant section of IEC was carried out in stages with appropriate temporary traffic arrangements implemented at the same time. The sequence of modification works is outlined below:

  1. Construct the new eastbound bridge
  2. Divert the eastbound traffic to the new bridge and divert the westbound traffic to the original eastbound bridge
  3. Demolish and reconstruct the westbound bridge in situ
  4. Divert the westbound traffic to the new westbound bridge upon completion of new westbound bridge
  5. Demolish the original eastbound bridge and construct the east tunnel portal of CWB

Moreover, in view of the limited space for bridge construction, balanced cantilever method was adopted to construct the new bridge. Launching girder was maneuvered to attach the precast bridge segments to both sides of the pier segment one by one until a continuous bridge deck was formed. The project team used 650 precast concrete segments, with each weighed 120 tonnes in average, to construct the new eastbound bridge. The whole modifications of IEC took two years to complete.

Bridge segment erection
Bridge segment
Launching girder
Original IEC
Existing IEC
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Last review date:12 Jan 2022