By Catherine A. Cardno, Ph.D.
A three-story parking garage that is being built on the seabed within Brighton Marina in the United Kingdom will remain permanently submerged.
An 11-building complex is under construction in the United Kingdom’s Brighton Marina. The complex will be a mixed-use development of buildings ranging in height from 2 to 40 stories. Meinhardt
October 21, 2014—Parking garages are often built underground, but a new 350-space garage currently under construction in Brighton, United Kingdom, is taking that a step further. The garage is being built atop the seabed within the inner harbor of Brighton Marina and will remain permanently submerged. Two 6- to 10-story residential and retail buildings and a wide promenade will be built atop the underwater garage—on the marina. The garage itself is sized to serve a total of 11 buildings that will be constructed as part of a new development.
Brighton is a well known seaside town on the southern coast of England, primarily developed during the early 19th century, when George IV, then the Prince Regent, was a frequent visitor. Tom Kimbell, an executive architect and director of London-based Acanthus Architects LW, the firm's project director for the development, explained that the new development will encompass a mix of architectural styles. Kimbell wrote in response to questions posed by Civil Engineeringonline. "The formalistic approach for the proposed development draws from the heritage of the bold architecture arranged along Brighton's seafront, particularly the syncopated rhythm of the barreling Regency facades and predominant use of smooth white render," Kimbell noted. The development will use the "'Brighton white' residential architecture as a reference point," he said.
London-based Wilkinson Eyre Architects served as the concept architects for the development. Its buildings, ranging in height from 2 to 40 stories, will be arranged around a large public promenade. A 40-story tower will mark the eastern portal to Brighton with a "slender, sinuous, and distinct composition," Kimball added.
The entire development "is going to be built on an arrangement of foundations that are going to be constructed in the marine environment," says Nick Gillespie, CEng, a director at the London office of global engineering firm Meinhardt and the project's structural engineering director.
The first phase includes two mixed-use buildings and a promenade built atop a submerged parking garage. Slender steel tubular piles reaching depths of 13 to 14 m will support the parking garage and buildings. Meinhardt
The first phase of this development is currently under way. Dubbed the "West Quay development," this £41.5-million (U.S.$7.2 billion) phase includes the submerged parking garage and the two towers and promenade that will be located atop it. Acanthus Architects LW redesigned the car park, which had ramped slabs, to include a level slab arrangement and optimized vehicular circulation routes, according to Kimbell. "The car park box also contains a double-height energy center which will ultimately provide power and heating for the entire development," Kimbell noted.
To build the parking garage, a three-sided sheet pile cofferdam was constructed and nestled against the development's namesake—the existing West Quay within the marina's outer harbor. Initially the parking garage design called for a four-sided cofferdam, but value engineering study determined that the existing quay—itself formed from a sheet pile cofferdam that was then filled in with concrete—could be used as the fourth side.
"It's a risky thing to do a three-sided cofferdam because you need to interface with an existing structure that's quite old, that's already corroded, and you're not quite sure how watertight [it] is," Gillespie explains. "The existing quay had no design requirements to be watertight."
So the design team commissioned investigations into the quay and the seabed and determined that it would be possible to tie the three-sided cofferdam in to the existing quay with grout socks. Once that was done, a propping system was erected within the cofferdam to support the quay to ensure that when the site was dewatered there would be no collapse or weakening of the quay as the water pressure was removed, Gillespie explains.
The seabed consists of moderately strong, high-density white chalk known as Newhaven Chalk that is permeated by numerous, relatively narrow, bands of flint, according to Gillespie. This layer is overlain by deposited silt measuring up to 2.5 m in depth.
The three-story submerged parking garage will be constructed within a three-sided cofferdam that has been tied into an existing quay to create its fourth side. Meinhardt
Once the cofferdam was dewatered—which took place last month—the layer of seabed silt had to be examined to determine if it had to be treated before construction could begin. "The silt had compressed quite well after it had been dewatered," Gillespie notes. "It settled by about 75 mm." Below the silt, "the chalk is a very good founding material," he says.
Steel tubular piles measuring 338 mm in diameter were vibrated to depths of 13 to 14 m to support the parking garage and buildings. This is "quite thin for piles," Gillespie notes. "Traditionally with most buildings you have piled foundations with pile caps, ground beams, and in-situ slabs," Gillespie says. "What we ended up doing for Brighton Marina—because they had smaller-diameter piles at closer centers—we ended up making the basement foundation for the entire development a uniform thickness of a solid flat plate."
This 800 mm thick bottom plate performs many functions, Gillespie notes. The plate resists the 85kN/m2 of water pressure from the surrounding sea, which will be equivalent in height to a three-story building. It also transmits loads from the buildings' cores and columns and props up the base of the cofferdam, and it is thick enough to allow the columns to be located at offsets when needed.
This arrangement is a "fairly neat form of construction," Gillespie notes. "It might use a bit more concrete than pile caps, ground beams, and a basement slab, but it's easier and cheaper to construct."
The parking garage structure was completed with concrete liner walls—located on the interior side of the cofferdam walls—that were made waterproof by the use of a concrete admixture, according to Gillespie.
Arranging the construction sequence so that the site remained water-free and the initial structures remained firmly in place was a balancing act. The height—and more specifically, the weight—of the buildings located atop the waterline is dictated by the upward forces of the submerged parking garage. If those forces are not managed properly, the three-story concrete box could "become a boat and float away," Gillespie says.
"The actual steel sheet-pile cofferdam is used for temporary waterproofing, and to dewater the site, but there are pumps that are in place to keep the excavation dry during construction," Gillespie explains. "Those pumps need to remain on and operational until such time as the entire building above is constructed," he says. If they do not, "water would come into the excavation, apply pressure to the concrete box, and then that would come up out of the water," Gillespie says.
The two buildings that will be built atop the parking garage will be entirely concrete. "Given the marine environment, which is obviously quite an aggressive environment, we're using concrete for all the primary structural elements," Gillespie says.
The two buildings have been designed as white 'pebble-shaped' forms that appear eroded to the seaward side, according to Kimbell. The buildings' landward sides exhibit a "more conventional orthogonal masonry treatment," he noted.
The buildings will include slip-formed, cast-in-place concrete cores, precast double-story concrete columns, and 260 mm thick posttensioned floor slabs that typically span 10.25 m, according to Gillespie.
"There is going to be a significant use of precast concrete in the construction of the development," Gillespie says. "There is a big move in the U.K. at the moment for off-site manufacture, and precast construction is coming back into fashion in a big way." Prefabricated elements are often cheaper, according to Gillespie. Prefabrication is also a more sustainable method of construction because work can be completed in a localized space, waste is limited, and material is more tightly controlled, he notes.
Work is progressing steadily on-site. The first placement of concrete for the basement slab took place this month, according to Gillespie. The car park is expected to be complete by March 2015.