Feature

Castle in the Sand

A sliver of beach is all that lies between the Atlantic Ocean and Jade Signature, a 62-storey luxury oceanfront tower in the Sunny Isles Beach neighbourhood of Miami, Florida. The recently completed scheme features 192 luxury apartments, all of which span the full width of the building, affording their occupants views out of both the ocean to the east and the city skyline to the west.

Designed by Swiss architect Herzog & de Meuron the tower is supported on a concrete structure clearly expressed on the facades by sculpted vertical columns and projecting floor slabs, which provide the apartment’s large outdoor terraces and shade the glazed facades from the Miami sun.

It is not the sun, however, but the hurricane-force winds that posed a major challenge to the project’s structural engineer McNamara Silvia: “The structure is designed to deal with some of the highest winds anywhere in the world,” says principal Andrew Sullivan.

The tower is unusual in that its footprint is a parallelogram, minimising the shadow it casts on the beach to the east and capturing the greatest amount of direct sunlight for the ocean-facing apartments. As a consequence, the building’s beachfront elevation is angled approximately 45 degrees to the beach. The apartments, however, are arranged perpendicular to the beach (see floor plan, right). “The design is all about maximising views, which meant we had to be careful about where we placed the shear walls,” explains Sullivan.

To preserve these views, the building’s two reinforced-concrete lift cores and the integral shear walls, which are key to resisting lateral wind forces, are also at a 45-degree angle to the main elevations. “The building’s configuration meant that it behaved in the wind as if it was a square building instead of a rectangle,” says Sullivan.

In addition to its distinctive footprint, the tower is unusual in that it tapers gradually from bottom to top on its east and west facades, which means its floor plates become progressively narrower. “The perimeter columns stack, but they are offset by 3in [75mm] all the way up the building to account for the smaller floor plates,” says Sullivan. The perimeter columns also change shape from floor to floor. “A sweeping column might turn into a column with less of a sweep and then into a rectangular column and even into an hour-glass column,” he adds. In the words of Herzog & de Meuron: “The project uses structure in the most basic way, a structure that you have anyway, and turns it into something sculptural”.
Where possible, the engineers took advantage of the form of the columns to help support the tower’s expansive balconies. “The sweeping columns helped because some of the trapezoidal-shaped balconies are fairly deep – up to 20ft [6m] – so where we had the opportunity we used the sweeping columns to help prop the cantilever,” says Sullivan.

Perhaps the most unusual aspect of the scheme, especially to those familiar with the ground conditions in this area of Miami, is that the residents’ parking is located underground. Burying the car park, rather than housing it in a podium at the tower’s base, as is the case with most neighbouring developments, allows Jade Signature’s residents access to the lobby area from the street – the lobby opens out onto the swimming pool, the pool onto the beach, all of which are at the same level.

“The designers wanted a project that when you pulled in off the main road you didn’t have to drive up a big podium to gain entry. They wanted people to be able to pull up to the front door and be able to see the beach and water,” Sullivan says. “We were involved early in the project, mainly in regards to coming up with a good, cost-effective scheme to create a 40ft-deep [12m] excavation right on the beach for the underground parking.”

Coming up with a way to build a 425-car, three-storey garage in porous limestone and sand at the edge of the ocean was not without its difficulties. “Before this building was here you could have taken a shovel and by the time you’d dug down 2ft you’d have hit water,” says Sullivan. “So keeping 38ft [11.5m] of ground water out of this space was a large part of the structural design” (see “How to build an underground garage on a beach”, above).

Although the basement was probably the most challenging aspect of the scheme to construct, Sullivan says that creating the subterranean car park did help with the design of the foundations below. “The positive from the removal of all of the soil was that there was a load swap with the weight of the building that does not penalise your foundations.”

With the tower’s construction complete, many of the apartments are now occupied, giving their residents the opportunity to experience first hand the architectural use of concrete to create a unique living space on the ocean front.

How to build an underground garage on a beach

Ground conditions on the ocean-front plot where Jade Signature was to be constructed were far from ideal. The site was covered with about 4.5m of beach sand, overlaying a 3m layer of peat on top of layers of limestone and sand. In addition, the ground was saturated, with groundwater just 600mm below the surface.

Before excavation of the parking garage could commence, contractor Suffolk Construction worked with foundation contractor Malcolm Drilling Company to stabilise the ground around the perimeter of the excavation site and also beneath the site. For this, they used a low-strength, low-permeability form of concrete created in-situ with a technique called deep soil mixing. The plan was to create what the contractor terms “a giant, waterproof bathtub” to protect the excavation from groundwater infiltration during construction.

Malcolm Drilling designed a 2.7m-diameter hybrid auger specifically for the project. As the auger bores into the ground it crushes the limestone and mixes it with the sand while simultaneously injecting cement slurry into the mix as a binder. The pulverised limestone and sand serve as a natural aggregate to create a column of concrete-like material that eventually hardens. Malcolm Drilling repeated this process hundreds of times to form the bathtub’s four, 3.6m-thick walls.

To create the floor of the bathtub within the rectangle of walls, water was used as the drilling fluid until the auger had bored down to the level at which the basement floor was to be constructed. At basement floor level, cement slurry was again injected as a binder and the auger continued its journey downwards for another 6m. Again, the operation was repeated over the entire 69m x 46m footprint of the building to form the floor of the bathtub. In total, 2,000 deep-soil-mixed columns were used to create the walls and floor of the bathtub.

Before excavation of the dry soil inside the bathtub could start, a series of 45m auger-cast-in-place piles were installed into the ground through the base of the bathtub. These piles now carry the weight of the tower, however they were initially used to keep the concrete bathtub in the ground and to prevent it being pushed upwards by the pressure of the ever present groundwater.

Once the piles were in place, the dry soil inside the concrete tub was dug out to reveal the bathtub’s deep soil mix walls and floor.
The creation of the deep-soil-mixed bathtub enabled Suffolk Construction to cast the 2.4m-thick concrete mat foundation on which the tower now stands in the dry, 11.5m below the water table. The mat was constructed with 2,900 tonnes of steel rebar and 7,500m3 of concrete from 1,200 deliveries and poured over a 24-hour period. With the mat complete, construction of the tower on the beach could commence.

Photos: Field Condition; Model: McNamara Silvia; Floor plan: Fortune International Group