Clarke Quay station and tunnels
SINGAPORE, 1997 - 1999
Geotechnical works associated with the construction of a major underground station for the Singapore MRT North East Line.
As a further development to the infrastructure of Singapore, the new North East Line adds a further 16 stations to the existing Mass Rapid Transport network. Clarke Quay Station is in the heart of the City and serves an area of the city centre along the Singapore River in full redevelopment. Linked to a commercial development above and doubling as a Civil Defence shelter, the station is a complex civil engineering structure in its own right. Additionally, the contract also included the construction of the tunnels both north and south of the station towards the Dhoby Ghaut and Peoples Park stations.
The North East Line construction packages were let as Design & Build contracts. For Contract 708 Clarke Quay Station and Tunnels, the station box was to built adjacent to the Singapore River and the geotechnical conditions were difficult with a significant layer of marine clay to overcome and heavy foundations to be constructed for the commercial development above the station. As a member of the Main Contractor Joint Venture, Bachy Soletanche were heavily involved in the evolution of an alternative foundation scheme to simplify the station construction and propose economic methods for the Station's geotechnical works.
In the event the piled foundations of the station were deleted and the whole station box lowered to found directly on good soil. However, due to the presence of the particularly thick layer of marine clay, it was necessary to use Jet Grouting in some areas to improve the soil and therefore increase the bearing capacity. Jet Grouting was also used at the station tunnel break in and break out and at two of the station pedestrian access tunnels. Additionally, bored pile foundations were required for the station entrances and secant piles were used at Dhoby Ghaut for a future access.
The Clarke Quay station box was formed with a 1 200 mm thick diaphragm wall, excavated using a combination of conventional cable grabs and the KS3000 hydraulic grab. Complex traffic management was required to allow the busy city centre traffic to flow smoothly at all times the works programme was tight with many areas critical to the success of the overall project. During the excavation of the wall there were unforeseen difficulties to overcome, particularly old driven concrete and timber piles and debris from an old shipyard located by the Singapore River. However, the experienced site team and the modern equipment were able to cope with the surprises in the ground.
A particular feature of the station temporary works design was the seven levels of strutting required for the bottom up construction technique. The sketch on the left shows the strutting layout.
Some heavily loaded foundations were required outside the station at the northern edge for the future commercial development above. Barrette foundations were chosen given the high loads involved.
At the Dhoby Ghaut Station end of the work site, an additional problem was identified by the Client. Here the new tunnels crossed only a few metres below the existing MRT running tunnels. The particularly poor ground conditions in this area necessitated special measures. At this locations the ground was highly fractured mudstone and a fault zone had been identified during previous MRT works. In order to protect the existing running tunnels a 36 m long horizontal pipe pile roof (umbrella arch) was constructed.
With the close proximity of the existing tunnels very tight tolerances were required to ensure the 300 mm diameter pipes could be installed between the existing operational tunnels and the future NEL tunnels. A Down-The-Hole hammer was used to drill in the pipes through the very fractured layers. Despite the difficult nature of the ground the pipes were successfully installed and sealed to provide further protection to the existing structure.
A diaphragm wall is a reinforced concrete wall that is made in situ. The trench is prevented from collapsing during excavation, reinforcing and casting by the use of supporting bentonite slurry. The slurry forms a thick deposit (the cake) on the walls of the trench which balances the inward hydraulic forces and prevents water flow into the trench. A slurry made of polymers can also be used.
Jet grouting is a construction process that uses a high-pressure jet of fluid (generally 20 – 40 MPa) to break up and loosen the soil at depth in a borehole and to mix it with a self-hardening grout to form columns, panels and other structures in the ground. The parameters for the jet-grouting process and the desired final strength of the treated soil depend on a number of characteristics, such as the soil type, the technique used and the objective to be reached. In granular soils, the high-pressure jet breaks up the grains through erosion, while in a cohesive soil, such as clay, the jet breaks the mass up into small particles. High pressure is needed to produce the kinetic energy required for the jet through a small-diameter nozzle. Waste material from the process (a mix of soil, water and binder) is recovered at the surface before being taken away for disposal.
A pile is a structural element driven into the soil for transferring loads and prevent deformation. Its slenderness ratio is not limited.
Pile shafts can be uniform and rectilinear, telescopic and belled out.
Piles can be installed either separately or in groups. They can also form a retaining wall, a mixed curtain wall, contiguous piles, secant piles and composite curtain walls, such as Berlin walls and similar. Piles are also used as precast beams to be placed in the structure of the building they support.
Soletanche Bachy has helped build most of the world's major metro systems, performing soil consolidation grouting and constructing cut-and-cover tunnels with cast-in-situ diaphragm sidewalls, bored tunnels and major underground openings.
Soletanche Bachy has developed a number of techniques for use in railway works; cut and cover tunnels, highways, underground stations, viaduct etc.
In the tunnelling field, Soletanche Bachy’s subsidiaries Soletanche Bachy Tunnels and CSM Bessac specialise in management of tunnel sites and also, for the latter, the design and construction of tunnel boring machines.
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