Extraordinary though it was, this was far from the only engineering test on the newly-completed project, which comprises two buildings providing 195 apartments, commercial spaces and a new RNLI facility.

We were appointed to the £40 million construction project by clients Brunswick Developments Group and Midgard, working with architects Acanthus LW. The site is on the western quay of the south coast seaside town’s harbour.

The underwater car park with space for 350 vehicles, which has two new buildings atop it, is a feat that has attracted attention from the international engineering community. It sits on skinny 339mm diameter steel tubes driven some 13 metres into the seabed. Surrounded by cofferdam walls, an 800-mm-deep basement slab bears onto these piles. This multifunctional slab;

• resists the 85kN/m2 of water pressure from the surrounding sea
• spreads building loads across the skinny 339mm diameter piles
• transmits loads from the cores and columns of the buildings above sea level
• props up the base of the cofferdam
• allows columns to be located at offsets when needed.

The new carpark structure is built inside a three-sided cofferdam. The fourth side is the existing West Quay, which is itself a double sheet-pile structure with mass concrete infill, constructed in 1976. The design team commissioned investigations into the quay and the seabed, concluding it would be possible to tie the three-sided cofferdam in to the existing quay. A propping system was put in place to ensure that when the seawater was pumped out of the structure, there would be no collapse or weakening of the quay as the water pressure was removed.

A steel liner wall coated with a concrete liner goes up to boardwalk level. It is like an enormous, watertight steel and concrete shoe box. Tide levels in the harbour fluctuate appreciably so unless the engineers could find a way to fix the structure in place, the box was at risk of turning into a boat and floating away. The crumbly chalk seabed was far from ideal for securing tension anchorage, and the solution was to pin the car park down by the sheer weight of the structure on top.

The undersea garage will serve not only the two buildings above it but another nine that will be constructed as part of a new development.

The newly-completed buildings above the parking garage are entirely concrete. Although they are above sea level, they still have to survive a harsh marine environment. All the building materials and components were rated for sea salt and chlorides in the air. The team looked for ways to incorporate as many precast concrete elements as possible for better quality and to limit exposure of reinforcement to the elements.

The superstructure has to work incredibly hard. With a general structural grid of a very long 10.5m x 10.5m grid/span, the team used 275mm thick post-tensioned slabs which helped to reduce the dead load deflection, reducing storey height and allowing the developer to build more storeys within the height restriction on a sensitive planning site.

Visitors to a pub on the promenade are probably unaware that they are sitting on exposed steel tubes and pre-cast beams and slabs. That was the easiest and safest way to build the section overhanging the water, taking into account the health and safety of workers putting those elements into position. It was also quicker. The contractors put the new ground floor next to the pub on top of the piles in just eight weeks.

Higher up, external concrete balconies are connected back to the pre-cast concrete. The buildings have a  height of seven and nine storeys above ground floor level with a series of setback floors rising a further two storeys, presenting more engineering challenges.