ED NEWMAN-SANDERS, technical director at Atkins, discusses the use of prefabrication in the construction of Crossrail’s new Custom House station, and suggests this modular approach might be the future for major rail infrastructure projects

Prefabrication and modularisation is revolutionising construction in the commercial and education sectors, increasingly demanded by clients and made ever more feasible by BIM. However, its application to major rail infrastructure projects remains relatively limited. With its ability to cut down costs, construction times and disruption to the surrounding communities, prefabrication techniques are an attractive and ever more possible approach to construction and design.

Custom House, the new Crossrail station in London’s Docklands, presented a unique opportunity to deploy these novel methods of design and construction. Due to open in December 2018, and the only above ground station being built in Crossrail’s central section, Custom House will be an important transport interchange with the adjacent Docklands Light Railway (DLR), local bus services and the ExCeL Centre, in addition to providing the focus for regeneration in the London Borough of Newham. Serving as inspiration for the whole project and local community, the new station has to embody Crossrail’s vision and identity, thus requiring iconic architecture.

The unique challenges at Custom House
I am part of the team behind the building of Custom House, consisting of Atkins, Laing O’Rourke, Arup and Allies & Morrison. The design has had to take into account a number of constraints at the development, including: a very narrow site; existing utilities; the existing DLR running along the southern boundary and remaining fully operational throughout any construction sequence; a busy footpath and congested Victoria Dock Road running along the northern boundary; and a public right of way running across the top of the site to link pedestrians between Victoria Dock Road, the DLR station and ExCeL Centre. This had to be maintained at all times, and also accommodate people with reduced mobility.

Further constraints included the main site being bounded by the Victoria Dock tunnel portal to the west and the Connaught portal to the east. This meant the contractors on each contract had to temporarily work within the other site’s boundaries to complete their work.

A line of high voltage cables also overhangs the DLR to the south of the site, stretching from pylons to the east and west of the station. While not overhanging the main station site directly, they were close enough to be a major risk to any lifting operations on the site.

Solving the problems strategically
Our strategy for the construction of Custom House included pre-fabricated and standardised components, with a ‘kit of parts’ forming the platforms, columns, concourse slab and roof. This unusual and innovative approach had a number of advantages. It minimised work on site that, in turn, drove down the programme time, preliminary costs and the impact on the local community.

Off-site manufacture also required fewer deliveries and vehicle movements around the site, reducing traffic, noise and effects on air quality.

Shifting construction activity from site to factory also improves working conditions and reducing health and safety risks, while the more controlled conditions of the factory ensure more consistent and higher-quality production.

The development of a pre-cast concrete solution brought other benefits to the construction phase, allowing swifter installation by gantry crane of repetitious units, a benefit made more acute by the proximity of live overhead power cables and the restriction this imposed on the construction sequence.

The concrete components for the station were fabricated within Laing O’Rourke’s Explore facility in Nottinghamshire, as the project’s main contractor. These were then delivered to the site for installation on a ‘just in time’ basis.

Laing O’Rourke’s operations director Steve Jones said: “The seamless integration of the ‘virtual’ design model and the off-site manufacturing plant allowed us to create highly precise major structural elements, delivered exactly when we needed them. It’s an innovative strategy that has great potential for the many railway infrastructure projects in the years ahead.”

Designing an ambassador for Crossrail
As Crossrail’s only new above-ground station in the central section, designers Allies & Morrison had the opportunity to design the station as a free-standing building rather than an interior fit-out.

The concrete superstructure consists of a series of pre-cast reinforced concrete frames, columns and floor units, held together with hidden connections. The platform structure comprises pre-cast concrete panels spanning a system of primary beams, supported typically by three or four lines of beams resting on piles, constructed through four metres of soft alluvium ground below.

There are three new overhead bridges linking the DLR, ExCeL and the London Borough of Newham to the new station. All three structures comprise steel fabricated primary box beams with a composite slab cast on permanent pre-cast concrete planks.

Modelling all of the components at Custom House in 3D provided a valuable tool in improving communication between teams around building, site inductions, logistics, sequencing and health and safety. The 3D model was also digitally linked and synchronised with the project programme to visualise and plan the complex sequence of installation. This was in turn linked to Laing O’Rourke’s factory database.

By using unique QR codes on each component, the status of each of the 880 pre-cast components could be tracked, planned and recorded from the design stage through to casting, delivery and installation on site.

Once on site, mobile devices were used to scan the QR codes, which brought them up the appropriate quality for completion. This was an efficient way of carrying out all quality and health and safety checks, maximising traceability and simplify the handover process.

Conclusion
Using a ‘kit of parts’ approach, the close collaboration of Atkins, Laing O’Rourke, Arup and Allies & Morrison developed a solution that was elegant, durable, costefficient and safe to erect on the constrained site, with the majority of the fabrication process taking place in a controlled factory environment.

The team not only overcame the unique challenges for design and construction presented by the site, but also delivered the iconic ‘ambassador’ for the new Crossrail network they were seeking.

Given the many benefits a prefabricated construction method can bring to the design and construction of rail infrastructure projects, I expect the use of this method to significantly increase over the next few years. Custom House has been an exemplar project for demonstrating the value of this type of construction and design approach and the possible applications of prefabrication will only increase as technology develops.