Listed cells at the heart of an abandoned London magistrates' court had to be retained as part of a new development. Alan Sparks went to see how it was done.
Guests at London's new Courthouse Hotel, next door to the world famous Palladium theatre and opposite the equally famous Carnaby Street, will be following in some illustrious footsteps. In its previous incarnation as Great Marlborough Street Magistrates Court the site saw the first scenes of the Oscar Wilde legal drama that eventually saw him jailed for gross indecency, and The Who pop group charged with drugs offences.
Echoes of the site's previous history will resound in the new hotel, as English Heritage and planners have demanded the main court buildings remain and the female prisoner cells be retained. Incorporated into the hotel as a feature in the cocktail bar, the three cells posed a major challenge to structural engineer FaberMaunsell, situated as they were on the ground floor of a single storey basement building that was to be demolished while a further storey was to be excavated beneath them.
'We came up with half a dozen cartoon strip style alternatives for dealing with the three cells. It was such a unique challenge, completely captivating the design team to the point where we could not switch off from trying to find a solution. In fact, some of our best ideas came while sat at home watching Eastenders, ' says FaberMaunsell associate director Gem Ahmet Main contractor Vascroft eventually threaded needle beams beneath the cells' slab and supported it on a larger steel beam and steel column frame sitting on a mini piled base. This meant that the whole of the cells' weight was directed through these piles rather than the walls that needed to be demolished.
Excavation of the extra basement storey followed, transforming the piles into one storey columns restrained by what used to be their pile cap above ground. Once this was complete the hotel's foundations could be laid - a 1.5m deep raft that increased to 2m deep below the building's core. Now the new structure could be built around and up to the cells before the concrete columns and former pile cap were removed.
'A raft foundation was chosen ahead of a piled structure mainly because column positions were still being discussed, less disturbance was created, it was quicker and also helped control any settlement with an even distribution of the building's weight, ' says Ahmet.
Behind the old courts, where the cells stood, a hotchpotch of tack-on buildings backed on to traffic wardens' offices that fronted the street behind. Here there was a very special challenge. The 10m high party wall that separated the site and the sensitive Palladium theatre next door was laterally stabilised by these offices - which were due to demolished.
Although this sounds like a simple wall propping exercise, there was a multitude of potential spanners that this 300 year old wall could throw into the works. Ahmet explains: 'Extensive analysis and historical research of this wall was essential.
'Lower levels were built in the early 1700s before a bazaar and circus took over in Victorian times. Since then an ice rink and now the theatre have inhabited the site. Each use has seen the wall evolve. And for us to understand fully how the wall was going to react during construction, we had to get to the bottom of its history.'
A full height steel framed prop, supported off sacrificial plunge columns, was fixed to the wall and the concrete framed hotel structure built around it.
Once the hotel had emerged above the wall, then the frame was removed and the gaps in the slab filled in.
But this was only part of the risk to neighbouring structures.
With the extra basement storey, underpinning was needed to the existing courts, the Palladium and the street to the rear. The new basement delves 5m deeper into the site's made ground, gravel and London clay. This depth effectively ruled out hit and miss traditional underpinning, which would have taken a couple of months to complete.
Another option was to use secant piles, but here noise and disruption to neighbours was a big negative. In the end, Keller's jet grouting system was chosen.
Work began with a 100mm bore being drilled through the existing basement and below the party boundary. Pressurised water was then pushed into the ground to create sufficient room for the grout mix that followed down the same tube.
Slurry was then extracted via the outer skin of the tube, leaving behind strengthened soil of around 10N/mm 2compressive strength.
'This system was very quick, taking half the time traditional methods would have needed.
And it left us with a stiff face that could be excavated with confidence, ' says FaberMaunsell project engineer, Alex Wong.
Trial pits beneath the Palladium indicated that the ground would be ideally suited to the jetgrouting system, but just a couple of metres to the side of this pit, clay was struck. 'This meant that we had to revert to traditional underpinning techniques for this shallow section, which cost us a little time, ' explains Wong.
From first floor up to the seventh floor plant room, the concrete frame is relatively simple, repeating itself on each level. This repetition lends itself to potential savings. Here prefabrication has been adopted, where possible, to speed up erection. Complete bathrooms are delivered to site from Italy in secure individual 4.5t pods.
Floor depths are kept down to 200mm by using Omniadeck planks, which span between 200mm thick reinforced concrete walls. These 50mm concrete planks work as permanent formwork and provide a smooth finish for follow-on trades.
Steelwork is tied in to the walls, which enables the them to be designed as fully restrained 'I' beams spanning between columns. This means that the client can easily change room arrangements in the future.
At first floor level, however, the story becomes more complicated. A hotel needs large lobbies and large clear areas as well as bedrooms. 'This requires nine different transfer structures at the first floor, with more again at the storey below, ' says Ahmet.
'None are more complicated than the transfer slab over the preview cinema. Here we had to carry seven storeys of cavity blockwork landing at midway on a 7.6m spanning slab. Originally we used a beam and slab design at a total depth of 1.5m, but the architect baulked and we tried to trim it down.'
Eventually a slab of just 750mm was justified, designed as a beam. 'More crucially our design had to satisfy a deflection check using plate design, as deflection will have a major bearing on the long term stability and amount of maintenance required on the seven storeys above, ' adds Wong.
To cope with this deflection, structural propping will remain in place for the transfer slab until late in the construction programme to maximise the flexural strength of the concrete.