Client: Westfield Group
Engineer: William J Marshall & Partners
Type of works: supply and installation of a bespoke ACO channel drainage system for one of Europe's largest shopping centres.

London’s newest retail destination, Westfield London, is one of Europe’s largest and most prestigious. Its pioneering design blends shopping with leisure and live events in a unique environment defined by striking internal and external public spaces. Clearing rainwater from the open walkways and terraces is a bespoke drainage system that reflects the styling and architectural quality of the surroundings.

Westfield Group is the world’s largest retail property owner with a global network of 119 high-quality shopping centres across Australia, New Zealand, the United States and the United Kingdom. In all, the group operates over 10 million square metres of premium retail space. The latest addition to its global portfolio, Westfield London, represents a £1.7 billion investment in the redevelopment of a 42-acre site in London’s White City. Hailed as an innovative and dynamic meeting hub for a new generation of consumer, the centre is home to 265 premium retailers (80% of which specialising in fashion) and to over 50 restaurants and eating concepts. A large central atrium where special events and shows are staged is surrounded by a series of individual covered retail environments and by two external terraces – the Southern Terrace on the south side of the centre and Wood Lane to the west - where the main entrances and restaurants/cafes are sited.

To ensure the overall project was completed within the shortest possible timeframe, Westfield appointed a series of architects, consultants and contractors to work on different areas of the centre simultaneously. Consulting engineers on the Wood Lane terrace were William J Marshall & Partners. As Associate, Tristram Hambly, explains, the design of the surface drainage system presented a particular challenge; “As a feature space embracing two main entrances, the Wood Lane terrace plays a key role in setting the quality expectations for the centre. The architect’s concept introduced a series of curved steps and planters that would take arriving visitors from street level up to the elevated entrances. Following the curves of both the planters and the top step level, the original proposal was to use a narrow 10mm, faceted stainless-steel slot channel drain to remove all the surface water. With different radii on each planter and step, we thought it would be extremely difficult either to make a faceted system that was acceptable to the architect or to manufacture a network of bespoke curved drains to accommodate the tight tolerances required.

“Having worked with drainage specialist, ACO, on the specification of channel drains elsewhere on the project, we consulted its built environment division, ACO Building Drainage, on the design of a curved system. ACO’s team suggested that the specification be changed to a wider, 50mm channel inlet. This would retain the minimal visual impact of the surface gratings, but would be less prone to blockage and easier and quicker to clean – a critical factor that would, during routine maintenance, reduce inconvenience to visitors.”

Westfield agreed to promote the change and, to harmonise the external environment across the centre, extended the new drainage specification to the Southern Terrace where only straight channel runs were to be installed. To maximise manufacturing efficiency, ACO designed the curved and straight systems with the same channel body, the only difference being that on the curved system the inlet is positioned to one side so that it can fit flush against the planters and steps. Manufactured from 3mm thick, grade 304 stainless steel, each channel body is 120mm wide and 190mm deep and fitted with a series of adjustable leveling feet that allow up to 25mm vertical movement. The straight channels were supplied in 1m long sections while the curved units were fabricated in lengths to suit the radius being installed. As manufacturing commenced well ahead of the planters and steps being installed, dimensions for the curves had to be taken directly from the architect’s drawings. Each section was then assigned a unique serial number to ensure the contractors knew exactly where it had to be installed.

The gratings, manufactured from 4mm thick Grade 304 stainless steel and each 50cm in length, have a laser cut overlapping slot design intended to prevent ‘wash-over’: a situation where surface water runs over the inlet without being intercepted. For the curved runs, the gratings were again cut to precise dimensions from the original drawings and supplied to site ready assembled with the channel body to avoid any confusion during installation.

In total, 250m of curved- and 250m of straight-channel have been installed across the two terraces. Set out in 33 separate runs (18 on Southern Terrace and 15 on Wood Lane), each discharges to the site’s mains drainage network via a vertical spigot located over point gullies set in the slab below the channel.

“The installed drainage system adds a striking visual dimension to the finish of both terraces with the strong curves in the planters and the entrance steps accentuated by the precise line of the gratings,” says Tristram Hambly. “Without absolute control of the manufacturing process this would not have been possible.”

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