The Sky Pool is the most distinctive feature of the Embassy Gardens residential development, promoted by EcoWorld Ballymore in the Nine Elms district of London, adjacent to the new United States Embassy. Located on the tenth floor, approximately 35 m above street level, the pool connects two ten-storey residential towers, serving simultaneously as a swimming pool, a pedestrian bridge, and a structural element. With an overall length of 25 m, including a clear span of approximately 14–15 m between the buildings, it represents one of the most innovative applications of structural acrylic in contemporary architecture.
The project addressed the challenge of accommodating a full-length swimming pool at roof level without occupying the space required for the buildings' mechanical services. The adopted solution was to transform the pool itself into the primary load-bearing structure spanning between the two towers. The assembly consists of a monolithic cast-acrylic (PMMA) central section and two stainless-steel end tanks, supported directly on the buildings, which incorporate the access stairs, lighting systems, and part of the filtration equipment.
The acrylic side walls, approximately 3 m high and about 180 mm thick, act as deep beams capable of resisting simultaneously the weight of the water, hydrostatic pressure, and wind loads. The base slab, approximately 360 mm thick, together with transparent bonded joints between the acrylic panels, creates a continuous structure with virtually uninterrupted transparency. The pool contains approximately 150,000 litres of water, and the complete assembly has a total weight of around 200 tonnes.
The structural engineering was undertaken by Eckersley O'Callaghan, who initially investigated a glass-and-steel solution before ultimately selecting cast acrylic. Although acrylic has a significantly lower design strength and substantially greater deformability than glass, it offered several decisive advantages: almost invisible bonded joints, a refractive index very close to that of water—which considerably reduces optical distortion—and the elimination of visible metal frames and mechanical fixings. While structural acrylic of comparable dimensions had previously been used in large public aquariums, this was the first time it had been employed as the primary load-bearing element of a swimming pool spanning between two buildings.
One of the most remarkable engineering aspects of the project is the way it accommodates the relative movements of the two towers. Rather than being rigidly fixed to the buildings, the pool rests on bridge-type bearings that allow controlled sliding and absorb movements caused by wind-induced sway, differential settlements, and thermal expansion. The assembly is held together by prestressed stainless-steel tie rods and spring systems that compensate for the thermal expansion of the acrylic while maintaining constant tension and ensuring watertight joints. As a result, the entire pool behaves as an independent "floating box", capable of moving slightly relative to each tower while preserving its structural integrity and waterproof performance.
The structural design was developed using finite element analysis (FEA), considering numerous loading scenarios, including self-weight, hydrostatic pressure, live loads from bathers, wind actions, differential settlements, thermal expansion, and dynamic behaviour. The acrylic thickness was determined not only by the material's short-term strength but also by its long-term creep behaviour, ensuring structural safety and durability throughout the service life of the structure.
The pool was manufactured by Reynolds Polymer Technology, a U.S.-based company specialising in large-scale acrylic structures. Fabrication required the construction of a purpose-built oven to perform the annealing cycles necessary to relieve residual stresses generated during the bonding process. Once completed, the structure was transported from the United States to the United Kingdom and lifted into position on the tenth floor, where it was installed on its permanent bearings with millimetre-level tolerances, completing one of the most remarkable assembly operations in contemporary structural engineering.
