Hundreds of unique precast concrete petal forms were needed to create Heatherwick Studio and Arup’s Little Island off west Manhattan. Nick Jones finds out how it was done
In Manhattan, nature has often relied on sleight of hand. In the 19th century, Central Park’s hills and meadows were created by a small army of labourers, while more recently the High Line has conjured a green route through the West Side on a disused freight rail line. Now, just off shore on the Hudson River, Heatherwick Studio has added to this tradition in typically theatrical style.
Little Island is essentially 2.4 acres of rolling landscape elevated over the water. Viewed from the park itself, it is an expanse of open grassland and sloping lawns, swathed in native trees and shrubs. But from below, the pier’s structure is revealed in all its complexity. A thicket of slender piles resolves into a canopy of supersized plant pots, which hold up the greenery above. It looks random, the pots an assortment of shapes and sizes, reflecting the uneven structural dynamics of the “natural” topography they support. But, of course, that’s all part of the trick...
This sense of randomness was central to the conception of Little Island. The project replaces Pier 54, one of a long parade of maritime piers that once formed New York’s busy harbour. Its main backer, the media tycoon Barry Diller, wanted to do something different, rethinking the pier less as a flat, linear jetty, and more as a public space where people could gather or watch open-air events. Heatherwick looked at the existing wooden piles and wondered how a structure could form organically from these rather than simply being a surface on top.
After a few early iterations, the idea developed into a roughly square area of contoured parkland, rising up to 65ft above the water. In section, the precast concrete piles, with their vast tulip heads, rose and dipped in an asymmetrical wave, while in plan it was even more irregular, the pots looking like the cells of a soap bubble. “The original idea was very amoeba-like,” says David Farnsworth, principal at structural engineer Arup, whose job it was to work out how to build this. “And they really liked the idea that the piles would be random, not seen as a marching row.”
Arup set out to find an underlying pattern in the randomness. This would be key to the pier’s buildability. The structure needed to be made from concrete, for its strength and durability in a marine environment, and precast, due to the logistical challenges presented by building in the middle of a river in one of the world’s busiest cities. But to make bespoke formwork for 132 differently shaped pots would have been unfeasible. The structural load of the pots also needed to be made more consistent: regardless of size, they were all sitting on the same 3ft-diameter piles.
Farnsworth and his team began to rationalise the amoeba-like plan into a pattern of Cairo pentagons – an irregular five-sided shaped that can be arranged into repeating and overlapping hexagons. These pentagons were then further distilled into five triangular “petals”. “This unlocked the ability to get repetition of form and meet the architectural intent,” says Farnsworth. “We took the Cairo pentagon plan shape and extruded the petals upwards to generate a number of variations, creating the unique topography. That was a nice little trick to figure out – using the same formwork for multiple unique petals.”
Arup and Heatherwick drew up a parametric definition for the outer surface geometry of each pentagonal pot. “We knew there would be a complicated coordination dance between what was needed in terms of depth of soil, access requirements for the paths, the weight of each pot and so on,” says Farnsworth. “Once we got the definition for the outer surface, we could use parametric modelling to generate the interior surface geometry for the structure, as well as eventually the steel reinforcement.”
Then all they had to do was build it. “Pretty quickly we saw that we needed to digitally fabricate the formwork in order to get all of these curving pieces to come together and look like they were made as a single monolithic tulip,” says Farnsworth. At the time, no such facilities existed on the East Coast, but they found a willing partner in Fort Miller Company, a precast supplier near Saratoga Springs in upstate New York that was prepared to invest in the five-axis robotic machinery needed to read the surface geometry from a parametric model and mill it into foam formwork.
Arup’s rationalisation of the form meant that the 132 pots could be cast from just 39 sets of petal forms. These were used to create 12 different basic Cairo pentagon pot types, with two different depths and tilting on five different axes, from which a further eight different pots could be cast. “We took the curved 3D petal shapes, and then poured the concrete up to different closure forms,” says Farnsworth. “That created the elevational variation on the same shape.” The pots were completed with a star beam configuration of triangular precast planks, which sit on top of the petals and are held in place with steel connection plates. Needless to say, the planks are all unique, angled differently depending on the elevation of the pot.
The concrete itself was a 5000psi mix of white cement and lightweight medium and fine aggregates, with an as-struck finish. To improve durability, it has a low water-cement ratio and added chloride inhibitors. The process included casting about 20 test petals and two full-scale pot mock-ups. “They needed to try out a lot of specifications to find the right balance of meeting all the durability requirements, and getting the concrete to flow through these densely reinforced, complex shapes,” says Farnsworth.
One of the trickiest aspects of the digital fabrication was the rebar layout, which had to follow the complex geometry of the forms and needed at least 3 inches of cover to withstand the marine environment. “We thought that, since we have a parametric model, maybe we could continue to run it to generate the 3D rebar models. There were literally hundreds of thousands of rebar elements, all unique because of the different lengths and shapes of the petals.” In the model for each petal, the rebar would be tagged with a unique identifier, corresponding with an RFID tag on each bar in the factory.
Formliners posed another challenge. “We experimented a lot. We were using polyurethane, which is great for rolling and single curves. But as soon as you introduce double curvature it wants to bunch and wrinkle. So we found a firm that supplies sprayed polyurea, which hardens over the foam and enables multiple uses for each form.”
Once all of the elements had been cast, they were taken on a flatbed truck to the Port of Coeymans, 135km up the Hudson from New York, where the pots were assembled. Farnsworth says he was delighted with the seamless joints between the various curving petals: “The way the digital fabrication enabled all of these complex shapes to come together as one holistic element is pretty amazing – something that wouldn’t have been feasible with manual formwork.” Now, they were ready to be loaded onto barges for the final leg of the journey.
Even though so much of the construction had been done remotely, the site work was still an exercise in extreme precision. For the pots to line up correctly, the 3ft-diameter piles had to be guided by GPS so that they hit the bedrock – which was up to 200ft deep – within half an inch of their target. Once the steel shoe was secure in the bedrock, the top of the precast pile could then be cut off at the required height (with the offcuts used to make an artificial reef). The pots, which weigh up to 70 tonnes, were lowered by marine crane onto a steel guide column protruding from the centre of the pile.
A 9-inch gap, to be filled with in-situ concrete on HDPE formwork, was factored in between the pots to allow some leeway for the positioning of the piles, and all but two of the pots could be accommodated within these tolerances. “For one of those we were able to pull the pile over, and we caught the other one early enough to tweak the design before we fabricated the pot.”
The structure was made rigid with a 5-inch-deep in-situ topping slab, for which the precast planks on top of the pots acted as permanent formwork. “Some piles are 75ft above water, and a further 60ft above their point of fixity – they’re pretty tall and slender,” says Farnsworth. “The topping slab restrains those piles against shorter piles, which are a lot stiffer.” This slab left the surface of the pier looking like “an awesome skate park”, but was soon buried with 3-5ft of soil and rigid insulation, taking the load on each pile to between 250 and 350 tonnes.
None of this will be obvious to people picnicking on the open lawns above, admiring the 400 species of trees and shrubs, or enjoying a concert in the 687-seat amphitheatre at the top end of the park. Nor will they think much about the kilometres of undulating pipework, power lines, storm drains and other infrastructure that weave their way beneath the island’s surface. But there are likely to be just as many people on the water or on the shore admiring the huge sculpted pots from which this similarly sculpted nature springs. “Some of the best views of the pier are from underneath,” says Farnsworth, “just looking up and seeing this cathedral of precast concrete.”