Feature - Float and sink construction methodology for tunnel box structure
By Mr Alan BROWN, Mr Bernard TSE and Mr LEE Wai Man
The key purpose of the Wan Chai Development Phase II (WDII) was to provide land for the construction of the Central-Wan Chai Bypass (CWB), which was designed to reduce congestion along the existing Gloucester Road-Harcourt Road-Connaught Road Central corridor. Civil Engineering and Development Department (CEDD) Contract HK/2012/08 formed part of WDII and primarily consisted of the reclamation of land west of Hong Kong Convention and Exhibition Centre, adjoining the reclaimed land at Centre Reclamation Phase III, the construction of the CWB tunnels and Middle Ventilation Building, as well as the construction of a tunnel box structure which spans the existing MTR Tsuen Wan Line (TWL).
The tunnel box structure required an unconventional construction methodology to be developed due to the various constraints imposed on its construction. The box structure is a reinforced concrete tunnel which spans between two sets of bored piles across the existing MTR TWL. The structure forms a trapezoidal shape on plan view with an average width of 9.7 m and an average length of 53 m. The four bored piles of 2.5 m in diameter with a cut-off level of -10.2 mPD were constructed as part of a previous contract. The completed mass of the box structure is approximately 5,000 t. The cross section provides a twin cell structure, which will be connected with the existing adjacent box structure in the future.
Construction of the box structure was subject to a number of challenging constraints, one of which was the presence of the existing MTR TWL directly below the proposed box structure. The chosen construction methodology for the WDII box structure had to ensure that it would not cause any change of vertical loading to the operating MTR tunnel below, which ruled out the traditional construction methods used for tunnel structures, for example the use of in situ casting inside a dry cofferdam by pumping water out, as it would change the effective soil pressure on the railway tunnel. The works therefore had to be carried out in a marine environment.
In addition to the underlying tunnel, the limited available horizontal space for constructing the structure, due to its close proximity to adjacent structures, posed another challenge. An existing seawall and pump house were located directly to the south of the WDII box structure, as well as an existing box structure located directly to the north. In order to place the tunnel box in its designed position, the rubble mound at the base of the seawall had to be removed, which would destabilise the gravity seawall. In consideration of the potential vertical movement induced to the MTR TWL if the existing seawall should be removed, it was decided after careful deliberation to adopt an excavation and lateral support (ELS) system consisting of pipe piles propped against the existing box structure at the north to ensure the seawall was stabilised prior to the removal of the rubble mound. This solution was considered a more structurally sound method to stabilise both the existing seawall and the MTR TWL underneath during construction. However, adopting the ELS system further limited the available space for the WDII box structure to be constructed.
Development of the scheme
Having considered the narrow working space and the necessity to maintain the seawater level, a float and sink construction scheme was adopted. Reinforcement details of the box structure were revised to suit the construction sequence. In order to control the floating state of the box structure, the casting of the structure was divided into six stages. At the first stage, the base slab and a portion of the periphery walls were cast at a dry dock formed inside the site to the northwest of the final WDII box structure position. The dry dock, which had been used previously in the same contract for precast box culverts, was repurposed for the box structure construction. This scheme was positively received and its details progressed through deliberation between the designer and the operation team.
Upon partial completion of the tunnel structure, the dry dock was flooded and the box structure, which floated out, was towed into its permanent position above the foundation piles. The height of the walls concreted within the dry dock was limited and calculated to fully utilise the available draft in the access channel between the dock and the final location of the box structure. To minimise the works while the box structure was floating, reinforcement cages and formwork for the non-concreted sections of the walls were fixed prior to floating. At the same time, the air draft had to be controlled in such a way so that the top of the floating structure would not hit the ELS props stabilising the seawall. This was done by calculating the precise weight of the structure, and timing of the tides during towing, with sensitivity check for a workable range between the upper and lower bounds of parameters. After the floating box was moored at the final location, the remaining elements of the permanent tunnel structure were then cast while in a floating condition. Subsequent construction of the full height of the walls in controlled stages would continue to increase the draft. At the final stage, the box was flooded so as to sink the structure onto the foundation bored piles which were subsequently stitched to the base slab. The development of the float and sink construction scheme was carried out by using structural analysis software SOFiSTiK. Floating states and their resulting stresses were modelled and analysed based on upper and lower bound material volume and densities, as well as additional ballast to balance the floating structure and minimise pitch. A stage-by-stage construction analysis was undertaken, with the extent of in situ pour in each construction stage taken into account, to ensure that the stability and verticality of the structure was maintained while floating and during the concreting works. The draft and pitch of the structure were also checked at each stage, with internal ballast adjusted to control pitching.
Sacrificial reinforced concrete bulkhead walls (120 to 180 mm thick) were provided to serve as permanent formwork for subsequently cast concrete walls, and to bring the top of the structure above water. Internal props were used to support the thin concrete bulkhead walls against the hydrostatic pressures.
The effect of the resulting residual stresses in the permanent walls and slabs was assessed during the design. Hydrostatic pressures acting on the partially completed structure while floating resulted in a different stress distribution than a wish-in-place structure. The temporary stresses induced in the incomplete structure during construction were compared to those experienced in the permanent case and, where required, additional reinforcement was added to cater for any increase in forces over the conforming design.
Construction of WDII box structure
The calculated upper bound mass of the structure (including props and ballast) at the first stage of the construction (floating out of dry dock) was circa 2,400 t while the calculated lower bound mass deviated by almost 5%. The calculated draft of the structure was 5,542 mm and 5,321 mm for the upper bound and lower bound solutions respectively, both within the available draft of 5,820 mm during high tide when the structure was to be removed from the dry dock.
After completion of the base slab and bulkhead walls, the dry dock was flooded to ensure the structure floated in accordance with the design. The measured draft of the box structure was within the calculated ranges from the upper and lower bound analysis. During high tide, the structure was carefully towed out of the dry dock and guided into position above the bored piles and between the ELS for stabilising the existing seawall, by means of a series of winches.
When moored in position above the bored piles, a series of in situ concrete pours were undertaken over the following months to complete the external and internal walls and part of the roof slab. The internal ballast was adjusted in every stage to ensure minimal tilting. When the external walls and the majority of the roof slab were completed, the structure was flooded to sink it onto the bored piles to which it was stitched. The sections of the roof slab above the pile locations were not concreted to allow for access for stitching. Divers were used to undertake the stitching works within recesses formed in the base slab to allow for the connection to the piles.
After completion of the stitching works, the structure was partially dewatered to allow the remaining roof slab to be concreted. During this operation, the bored piles were temporarily subject to tension due to buoyancy, as the permanent backfill above the roof slab was not in place yet. The strength of the stitch and the piles at this stage was carefully checked to ensure there was adequate resistance against the uplift forces. The casting of the remaining length of the roof slab completed the construction of the box structure, after which internal props were removed and the structure was backfilled to the design level.
The adoption of the float and sink methodology allowed the box structure to be constructed within the limited available space between the seawall and the existing box structures, minimising any risk of damage to the underlying MTR TWL during construction. The innovative solution also minimised the amount of temporary works required, and allowed the tunnel box to be constructed in an effective and time efficient manner. The WDII box structure was completed economically, safely and on schedule using the float and sink methodology, which proves to be an advantageous solution for structures subject to constraints such as inability to implement cut-and-cover construction within a cofferdam.
Under the WDII, China State-Build King Joint Venture was awarded contract no HK/2012/08, with AECOM as the engineer supervising the contract and Tony Gee and Partners (Asia) as the designer of cost saving design.
About the authors: Mr Alan Brown is a Chartered Engineer, a Member of the Institution of Civil Engineers and a Senior Engineer at Tony Gee and Partners (Asia) Ltd. Mr Bernard Tse is a Chartered Engineer, a Member of The Institution of Structural Engineers and a Technical Director at Tony Gee and Partners (Asia) Ltd. Mr Lee Wai-man is a Construction Manager at China State-Build King Joint Venture.
Figure 1: Final position of the WDII box structure (L) and cross section of the box structure when floated (R)
Figure 2: Aerial view of the WDII box structure within the dry dock prior to floating and towing into position
Figure 3: The WDII box structure floating after being towed from the dry dock