01.08.06
Talking concrete
James Best, senior bridge engineer, Halcrow, tells us about his company’s recent work on the DLR City Airport Extension Viaduct
The Docklands Light Railway (DLR) City Airport Extension is an exciting project that provides an essential link between the airport and Canary Wharf. The areas adjacent to the project route are undergoing a period of revitalisation, and the 4.4km DLR scheme includes intermediate stations that will promote development along the route, helping communities such as Silvertown and North Woolwich.
In February 2003, the appointed concessionaire, City Airport Rail Enterprises, let a design and build contract for the project to AMEC Group Ltd for a fixed price of £140 million. Halcrow was appointed as engineer, responsible for undertaking the detailed design, including all civil and structural works. Weston Williamson was appointed as architects for the stations, while Rizzani de Eccher was appointed sub-contractors for the casting and erection of the viaduct superstructure.
The design of the viaduct and stations took just over two years to complete from the appointment date. The construction of the viaduct substructures started in December 2003, and the viaduct structure was completed 20 months later in February 2005, and opened for service ten months later.
Viaduct Structural Form
The route follows a linear alignment threading between warehouses and buildings and over numerous access roads. This favoured a pre-cast concrete segmental construction using an overhead gantry, so minimising disruption to local businesses and communities. A concrete structure offered a better whole-life cost solution, with a reduced maintenance regime. The use of concrete sections also lessens radiated structure noise, reducing the impact of the structure in the urban environment.
Segments
The design aimed to optimise and add value to the concrete viaduct sections as well as cater for the construction methodology. The segment sections and arrangements were detailed to improve striking of the concrete formers and aid their lifting and placing, as well as meeting the design requirements for the service condition.
The viaduct was designed for permanent loads, in combination with erection load cases, live loading, wind and snow loading, braking and traction loads, and derailed train loading. The live loading was equivalent to 0.5RL loading as outlined in the British Standards. A three-dimensional frame model was used to investigate global load effects, and a finite element model was used to investigate local load effects.
Simply Supported Spans
Just less than 2km of the project is made up of simply supported spans using pre-cast concrete dry-jointed segments. The spans are typically 37m long, but vary in lengths to suit access roads and other alignment constraints. Variation in span lengths was achieved by reducing the number of segments in the span, which meant all the segments could be a constant length of 3.1m, so reducing the number of variations in the segment moulds.
The simply supported spans were post-tensioned using six tendons with strand numbers varying to suit span lengths. The tendons run in HDPE ducting external to the section, with anchorages cast into the end segments. Durability of the viaduct was a key consideration, and a multi-layered protection system was adopted following the recommendations of Concrete Society Technical Report 47.
Station Spans
Due to the increased load the station structures have a smaller span (typically 27.7m) and use the same segment types but with modified cantilever extensions. Careful consideration was given to the form of the viaduct segments at the stations to minimise the variation to the typical segment formwork and provide an aesthetically interesting solution. The adopted solution involved elongating the segment cantilevers and using inclined props with cast-in sockets to receive the inclined prop. This meant that no modification was required to the lower portion of the segment concrete section, so minimising the modification to the existing segment formers.
Balanced Cantilever Structures
Three glued-segmental balanced cantilever structures are used to span obstructions beyond the range of the simply supported spans. These were typically four-span continuous structures with spans of 40 to 58m. As with the simply supported spans, the balanced cantilever structure used common pre-cast segment types repeated in various arrangements to suit the pre-stressing requirements. The balanced cantilever segments also increased in depth to form a haunch adjacent to the piers and reached 2.85m in length. The site team worked closely to ensure that the balance cantilever structures were erected to the design profile using predetermined stage construction profiles.
Substructures
The viaduct is supported on groups of 600 and 900mm diameter continuous flight auger piles, reinforced over their top 12m. A significant number of buried obstructions were encountered, from an air-raid shelter to abandoned piled foundations. The design and site teams worked together to survey, analyse and redesign many of the pile caps without delaying the construction programme.
The octagonal stem piers were profiled with a feature flare opening to form the pier head. As the featured flare has a complicated geometry a single pier type was developed; this provided sufficient head space to accommodate all configurations of the erection gantry support legs and bearings, and enabled a common formwork mould to be used so reducing construction cost, and improving production programme.
Concrete
Timely completion of the project was dependent on maintaining a segment production programme with a mould cycle time of 36 hours. As the concrete segments were match cast on site in an open casting-shed it was important that the programme could be maintained in low winter temperatures. Considerable attention was given to the concrete mix design to assure the early strengths required for the striking of the segment formers, particularly in low temperatures. Two mixes were developed using Ordinary Portland Cement (OPC) and Rapid Hardening Portland Cement (RHPC). The switch to the RHPC mix was made in low temperatures following results of concrete strength tests.
Conclusion
The design and construction of this innovative project to a tight schedule proved demanding, and involved close integration between design requirements and construction methodology. The successful completion of a valued service for London City Airport and the local communities, two weeks ahead of schedule, is a tribute and a triumph to the effort and teamwork of all involved.
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