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When you first set eyes on Las Arenas, your natural reaction is to blink just to check you aren鈥檛 seeing things. An entire bullring hovers 5 m in the air. And it is only after blinking that you notice that the whole thing is being held aloft by a series of spindly struts. Walk cautiously under these walls and another shock is waiting: the ancient walls of the bullring teeter on the edge of a 26 m deep hole where bulls and matadors once fought for their lives.

What is the point of these extraordinary structural gymnastics? Well, the modern trend of turning unusual old buildings into retail and leisure facilities has finally struck at the heart of what was one of Spain鈥檚 most venerated traditions, albeit one that is in decline in Barcelona. A project to transform the bullring into a leisure and entertainment venue was only commercially viable if 1200 cars could be parked underneath the building, as the tight site is in the middle of Barcelona, and right next to a Metro line. Furthermore, the bullring is on a raised site, meaning that its foundations are actually 4 m above street level.

These factors presented structural Expedition with a unique challenge. 鈥淚t鈥檚 a mega retained facade made complicated by the five storey basement and the fact the ground has been lowered underneath the bullring by a storey,鈥� says director Chris Wise. 鈥淵ou might do one of these things on a project, but not all three and survive.鈥� But survive he has, with the help of the arm of Bovis Lend Lease who have been responsible for overseeing the work of Spanish contractor Dragados.

Credit: I帽igo Bujedo

A disused and neglected treasure

The whole crazy idea came about by chance. Laurie Abbott, a senior director at architect Richard Rogers Partnership, happened to drive past the abandoned Las Arenas one day with Spanish architect Luis Alonso. 鈥淚 said that鈥檚 interesting and asked Luis what was happening to it, and he said 鈥榥othing鈥�,鈥� says Abbott. Alonso offered to approach the site owner who agreed to a meeting with Abbott. 鈥淭hey said if you want to make a proposal that鈥檚 fine. It was a gamble on our part.鈥� It was a gamble that paid off as the site owner agreed to go along with the idea.

Abbot worked closely with Wise on the proposal. Despite the technical difficulties both men wanted to retain the bullring as part of the scheme. 鈥淚t was a nice, strong simple building and I鈥檓 sure the people of Barcelona would want to keep it,鈥� says Abbott. 鈥淚t was also an interesting technical problem.鈥�

The people of Barcelona might be fond of Las Arenas, but they had not done anything to look after it. It was abandoned in the 1970s after a spell of being used as a venue for bands 鈥� The Beatles played there once. It was in such poor condition that the plan was to rip out the old stands leaving just the perimeter wall, which according to Wise was the only element worth hanging on to. 鈥淲hen we surveyed the wall we found it was in a terrible state. It was full of rusty old iron, pigeons were nesting in it and it was leaning over by up to 350 mm,鈥� says Wise. 鈥淚n some places if we had taken the parapet off the wall would have fallen over.鈥� Indeed the wall was in such a bad state that bits were falling off it 鈥� a woman was seriously injured by falling debris before work started.

The first job was to stitch together large cracks in the walls with metal bars. Then work could commence on supporting the facade so the mound of earth on which it sits could be removed, leaving what was left suspended in mid air. This is part of the final architectural solution; the idea is people can wander freely into the new building under the old facade, which acts purely as a visual element. The new building and the roof will be entirely structurally independent of the original wall.

Credit: I帽igo Bujedo

The perfectly circular facade of the original bullring has been retained by supporting it from underneath and at the sides with temporary works

Supporting the facade

Ultimately the facade will be supported on a series of V-shaped steel supports. But it had to be temporarily propped so the earth mound could be excavated and the permanent supports slid into position. Four mini piles were sunk under each arch in the facade. Then 15-20 m long sections of earth were excavated under the foundation so formwork could be positioned to cast curved concrete beams on either side of the old wall 鈥� these ring the facade and will distribute the point loads from the supports evenly around the fragile walls. Holes were drilled through the wall to correspond with holes cast in the beams. Threaded steel bars were inserted through these, enabling the two concrete beams and the old wall sandwiched between them to be clamped together as one unit. The old wall turned out to be hollow, so concrete was used to fill the cavity.

The mini piles were attached to the concrete beams and temporary supports erected to laterally support the facade. Work could now begin on demolishing the bullring interior and lowering the earth bank on which it was sitting down to street level, ready for the next delicate stage of the job, which was to construct the five-storey basement.

Digging deep

The adjacent Metro meant digging a vast hole then building the new structure was out of the question. 鈥淏ecause of the Metro we have had to construct the basement using top down construction,鈥� says Peter Gibney, the for Bovis. 鈥淭he fear was that with the underground water and the earth pressure, the whole thing could have caved in.鈥� Top down construction means building the structure first, then excavating the earth afterwards. In this case, the structure consists of a retaining wall to hold the earth back around the perimeter of the basement, and a series of piles to support its concrete floor slabs. Only the structure adjacent to the old facade is being built using top down construction, as this will keep the surrounding ground in place and support the facade. The basement and slabs in the centre of the bullring will be built conventionally once the perimeter structure is complete.

The retaining wall is outside the facade as the basement extends underneath it. A 60 mm wide trench the depth of the basement had to be excavated for the retaining wall. 鈥淭here was a lot of rock so the teeth of the scoop were constantly getting broken,鈥� says Gibney. A mechanical hammer could be used to break up the rock but only in areas well away from the Metro. Once the trench was excavated it was filled with reinforcement and concrete.

Credit: I帽igo Bujedo

Piles were also sunk ready to support the floor slabs. These were inserted in a circle inside the old facade, and also outside in the places where the retaining wall is further away from the original facade and the slabs need additional support. Plunge piles were used to ensure the piles were vertical. A large hole was bored then a guide plate was fitted at the top and the steel column inserted into the hole. A weak concrete mix was poured in around the column to ensure it stays in the correct place. It can be removed later.

Once the structures were in place underground, the ground-floor slab was cast. This contained cast-in beams to transfer the weight of the retained facade 鈥� which sits in the middle of the slab 鈥� to the structure on either side. Once the slab was cast, the ground underneath it could be excavated down two floors so the next one could be worked on. The intermediate floors will be put in later, in tandem with the structure in the centre of the bullring.

Currently Dragados has reached level 鈥�4 and only has to excavate down another 3 m to reach the lowest point. But being below the water table is causing problems. 鈥淲e are now pumping out water and are delayed as we didn鈥檛 start pumping early enough,鈥� says Gibney. Once the water level has been reduced, more piles will be sunk inside the bullring to support the roof structure, which will sit on four big piers. Then the lowest level slab, a 1 m thick raft foundation will be cast and the comparatively straightforward work of completing the basement in the centre of the bullring, and the rest of the building, can begin.