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Exceptional engineering on this highly complex project delivered an innovative, effective and beautiful solution.
Crossrail is a major new rail system in London. The tunnels for this development were lined with sprayed concrete, which is a rough and uneven surface. We were commissioned to design the system to fit the cladding to the interiors of the station access tunnels (ie. the tunnels for people, not trains).
Crossrail tunnels need cabling for power, lighting, fire systems, data and other services, which have to be covered for aesthetic and security purposes, but easily accessible for maintenance.
On the London Underground, traditional construction allows a tolerance of plus or minus 10-20 millimetres between measurements and production, with the assumption that every fixing (of thousands) will require adjustment. This means a slow, labour-intensive and expensive process, with a big margin for error. Our solution would apply zero tolerance.
The Crossrail tunnel sites were long and narrow, with hardly any room to manoeuvre heavy equipment or store materials. Drilling and fixing into the concrete lining would have created health and safety issues, increased also by people working at height.
We had three key areas of focus for the design: health and safety, productivity and accuracy.
Our aim was to develop an engineering solution to minimise fixings into the sprayed concrete lining for the supporting structure for the 23,000 cladding panels. Our solution followed our zero tolerance approach, provided lighter panels, used fewer materials, sped up installation and improved health and safety.
We used an entirely digital workflow, eliminating the production of fabrication drawings and the potential for human drawing errors. Moulds were 3D-printed or CNC-milled from the digital models. All reviewing and commenting were also carried out digitally.
The cladding panels are mostly curved in one plane, although some are curved in two. We used a range of panel types, some solid and others perforated to include acoustic lining. Cast from glass fibre reinforced concrete (GFRC), the panels are composed of tiny, high-strength glass fibres, surrounded by a concrete matrix. The concrete protects the glass fibres and helps carry the load. The result is a durable material that can be cast to a very fine and decorative finish, but is nearly two-thirds lighter than traditional concrete.
Lighter panels require fewer fixings and a lighter supporting structure, as it has less load to carry. They also require less material, cost less and emit less carbon in fabrication. Lighter panels are also quicker to make because thinner concrete sets more quickly. And, importantly, the combined weight of the panels and supporting structure met the construction criteria for two-person-handling, eliminating the need for heavy lifting equipment.
Our solution is a stainless steel ladder framework that holds the GFRC cladding. Top brackets were fitted to the crown of the tunnel and a levelling brace was attached to the brackets to control rotation. Base brackets were installed to the platform. Ladder frames were then installed between the base brackets and the levelling brace, and then the cladding panels could be fitted to the ladder framework. The entire cladding system needed to sit within 250 millimetres of the wall; challenging, when holding up several tonnes of concrete.
The complex intersections of two tunnels, known as ‘junction transitions’, presented the most complex shapes. Our innovation was special joining pieces, or ‘tusks,’ casting the supporting structure out of the same material as the panels rather than using a faceted steep structure, which took less space and again reduced cost and increased speed of installation.
At the bottom of the cladding, the secondary frame can sit on the platform, but the top must be fixed into the sprayed concrete lining, difficult because of its rough finish and inconsistent thickness. Manually drilling into the concrete, particularly at height, risks injury and is almost impossible to do with millimetre accuracy. With the secondary frame, we overcame the inaccuracy of the concrete spraying and created a highly accurate positioning system for the panels.
We applied this technology and workflow plan to three of the largest and most complex stations on the line – Tottenham Court Road, Liverpool Street and Whitechapel.
It has since been applied to precast facades for buildings on the Shell Centre in London and St James’s Place in Edinburgh by Bryden Wood and Laing O’Rourke.
Our focus on efficiency and innovation delivers standardised solutions that bring a wide range of benefits, with no compromise on design potential and beauty.