Sky Pool floats against London skies as an unmatched swim experience in clear acrylic

In a striking feat of structural design by HAL Architects and Eckersley O'Callaghan Engineers, the transparent pool is suspended between two rooftops in London’s Embassy Gardens.

by Jerry ElengicalPublished on : Jul 07, 2021

In Embassy Gardens, a residential neighbourhood by joint venture developer EcoWorld Ballymore near the banks of the River Thames in London, a transparent 15-metre-long cuboidal vessel of water suspended 10 storeys above the ground between two apartment buildings, creates a striking visual spectacle. With a panoramic vista of the English capital and the United States Embassy in its proximity, Sky Pool provides an incredibly unique experience for swimmers and spectators, unavailable anywhere else in the world.

The structure provides an incredibly unique experience for swimmers and spectators | Sky Pool by HAL Architects and Eckersley O’Callaghan Engineers | STIRworld
The structure provides an incredibly unique experience for swimmers and spectators Image: Simon Kennedy

The concept for the project - that of a seamlessly transparent swimming pool between two building rooftops - was developed and realised through a collaboration between HAL Architects, Eckersley O'Callaghan Engineers, and Reynolds Polymer Technology. After Sky Pool's first announcement back in 2015, the project attracted significant media attention both in Britain and abroad for the audacious structural and architectural maneuvers it proposed.

To make the structure completely transparent, acrylic was used as its primary construction material | Sky Pool by HAL Architects and Eckersley O’Callaghan Engineers | STIRworld
To make the structure completely transparent, acrylic was used as its primary construction material Image: Simon Kennedy

Originally envisioned as a swimmable aqueduct in an urban housing development, the concept evolved to take full advantage of unmatched views offered by the site. To have an entirely transparent pool design, glass was the first material explored, as per the vision of Sean Mulryan, Chairman and Chief Executive of Ballymore. However, the idea didn’t go forward once the designers ascertained that the joints and wall thicknesses required to support nearly 150 tonnes of water in the structure would hinder its transparency.

The designers wanted to make full use of the spectacular views on offer in its vicinity | Sky Pool by HAL Architects and Eckersley O’Callaghan Engineers | STIRworld
The designers wanted to make full use of the spectacular views on offer in its vicinity Image: Simon Kennedy

Subsequently, HAL Architects and Eckersley O'Callaghan Engineers were inspired by applications of transparent acrylic in large aquariums and marine centres and eventually resolved to adopt it as their primary material. Hal Currey, Director of London-based HAL Architects, explained the thought behind this decision in an official statement: “The intention was to keep the design simple for it to look as transparent and effortless as possible. Acrylic offered aesthetic benefits since it appears colourless and its similar refractive index to water would allow it to seemingly blend and ensure that joints in the structure would not be visible.”

Analysis of natural frequencies of the pool structure and water sloshing frequencies | Sky Pool by HAL Architects and Eckersley O’Callaghan Engineers | STIRworld
Analysis of natural frequencies of the pool structure and water sloshing frequencies Image: Courtesy of Eckersley O’Callaghan

The finished pool floor consists of seven sections of transparent 360mm thick acrylic fitted together, with side walls that are 180mm thick and nearly three metre deep. Angular bond joints were used between the panels to conceal them from swimmers and observers. Two stainless steel rods run along the 15 metre span beneath the U-shaped acrylic section to provide additional reinforcement. Graham Coult, Technical Director at UK-based Eckersley O'Callaghan Engineers, in a conversation with STIR, describes the most challenging facet of the project as "the accommodation of the various movements of the structures arising from differing loading conditions." He elaborates, "Sky Pool is fixed to two buildings with their own individual movement characteristics, and the movements can occur over durations varying from minutes to years. To further complicate matters, the structure is built from stainless steel and acrylic which have different amounts of thermal expansion."

150 tonnes of water, the equivalent of 15 double decker buses | Sky Pool by HAL Architects and Eckersley O’Callaghan Engineers | STIRworld
150 tonnes of water, the equivalent of 15 double decker buses Image: Courtesy of Eckersley O’Callaghan

As a means of resolving this, the pool is anchored by two five-metre-long stainless steel tubs on the roofs of both buildings. "We chose a more traditional material for the sections where the pool interfaces with the buildings as the filtration, lighting design, and other equipment could be more easily incorporated and would also remain hidden," explains Coult. "This also allowed the weight of the pool and the water to be transferred to the building and for building movements to be accommodated using bearings more commonly used in bridge structures,” he adds.

Effects of differential building movement on structure and bearings | Sky Pool by HAL Architects and Eckersley O’Callaghan Engineers | STIRworld
Effects of differential building movement on structure and bearings Image: Courtesy of Eckersley O’Callaghan

The bridge bearings permit the structure to slide, accounting for the effects of strong winds as well as thermal expansion or contraction. Furthermore, the inclusion of two tensioned high strength cables, 38mm in diameter, compacts the assembly into a sturdy 25 metre long unit.

The structure neatly slotted into place after a lengthy design and fabrication process, with granite finishes incorporated around it | Sky Pool by HAL Architects and Eckersley O’Callaghan Engineers | STIRworld
The structure neatly slotted into place after a lengthy design and fabrication process, with granite finishes incorporated around it Image: Simon Kennedy

Reynolds Polymer Technology, based in Colorado, United States, was commissioned under a design and build contract to fabricate the acrylic structure and assemble the final installation in London. Due to the considerable size and complexity of the project - as potentially one of the largest pieces of load-bearing acrylic ever fabricated - the company had to build a new outdoor manufacturing facility and create special new equipment. After 18 months of fabrication, following a lengthy four-year design process, the pool was transported from Colorado to Galveston, Texas, and then to London by trans-Atlantic ferry.

Sky Pool has made the apartments in Embassy Gardens look more attractive | Sky Pool by HAL Architects and Eckersley O’Callaghan Engineers | STIRworld
Sky Pool has made the apartments in Embassy Gardens look more attractive Image: Simon Kennedy

On the day of installation, in September 2020, a 500-tonne mobile crane delicately lifted the structure into place over a period of two hours. As per the designers, there was less than 25mm available for adjustment during the process. Fortunately, the pool neatly slotted into the desired spot. Granite finishes were incorporated around it, and final on-site tests were conducted to ensure its structural performance. In addition, a pedestrian bridge was built adjacent to the structure to facilitate easy movement between both rooftops.

Plan | Sky Pool by HAL Architects and Eckersley O’Callaghan Engineers | STIRworld
Plan Image: Courtesy of HAL Architects

The results are dramatic - to say the least. As stated by EcoWorld Ballymore in a press release, the remarkable view of this shimmering crystal clear vessel of water suspended against the skies has been a major driving force in making the Embassy Gardens neighbourhood a more attractive destination for prospective homeowners. In a sense, it has enhanced the residential design of the complex beyond traditional limits, adding a view of an unparalleled feat of structural design ingenuity.

Section | Sky Pool by HAL Architects and Eckersley O’Callaghan Engineers | STIRworld
Section Image: Courtesy of HAL Architects

Founding Director of Eckersley O'Callaghan, Brian Eckersley, reflects on the endeavour in a statement about the project's execution saying, “We are really proud of what we have achieved with Sky Pool. From an initial concept which people thought could not be done, to seeing it designed, fabricated, installed, and now opened is a fantastic engineering feat that we hope will act as an inspiration to all.” Co-Founding Director James O’Callaghan further adds - "Sky Pool is a true testament of our commitment to innovation and we look forward to seeing people’s reactions as they swim across.”

Project Details

Name: Sky Pool
Location: Embassy Gardens, London, United Kingdom
Client: Ecoworld Ballymore
Total Length: 25m, 15.8m(Acrylic Section)
Width: 5m
Height: 2.9m, 1.2m(depth of pool)
Architect: HAL Architects
Structural Engineer: Eckersley O’Callaghan
Fabrication/Engineering: Reynolds Polymer Technology

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