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Wates springs into action

PILING - Building luxury flats over two railway tunnels was never going to be easy, but Wates has opted for a quiet life by putting everything on springs.Adrian Greeman reports

CONTRACTOR Wates has been putting more springs into its £23 million design and build housing project than you could find in the most luxurious king-size mattress.They are everywhere across its central Birmingham site.

'All the buildings we are doing have to be isolated acoustically from the ground, ' says project manager Shah Shahnavaz.'Everything to do with the foundations is a vibration issue.'

The reason for these extensive measures is not any complex need for recording studios or musical performances, like those in the nearby Birmingham Symphony Hall but simply for comfort.

'People will be paying a lot of money to live in the apartments here and will expect to have peace and quiet, 'he says.And, without the springs and their associated vibration resisting elastomeric bearings, they would not get it.

Just below the site run two railways tunnels, one disused and one live.

Both could be a problem, the unused one because it might be brought back into use for a future light railway scheme and the other because it carries the main line from London into Birmingham New Street station.

To control the constant rumbling of heavy trains, each of the blocks on the site will have isolation systems built in.This has required the construction of a variety of complex foundation structures.

'There are three main sections to the project, ' explains Mr Shahnavaz.'A main block of apartments up to six storeys high with six service cores, an office block in the centre of the site and an apartment block of two storeys.'

The latter block is a reconstruction of a late 19th-century wharf storage building for the Worcester and Birmingham canal, part of the narrow canal network that runs through much of this central Birmingham area.The site is surrounded on two sides by water where the canal bends sharply towards the redeveloped Gas Street basin.

'We have to retain the facade for that building and are rebuilding the structure behind it, ' says co-project manager Tony Bevan.About 18 apartments and a bit of retail space will fit into the wharf, while the main block will have 136 flats.The offices will be used by Birmingham City Council, as part of the arrangement for acquiring the land.Wates has used Arup for the structural design together with Aedas AHR Architects.

Mr Shahnavaz says foundation work across the site has involved a variety of methods and complications.The first problem was the tunnels.As one is an important live link, an unusual degree of care was needed to work anywhere near it, particularly with the piling, which supports much of the main structures.

The tunnel was inspected nightly for signs of any damage, when engineers would walk through it, taking record photographs.The Wates team has worked together with Network Rail on this.

'We have used CFA piling mainly up to 20 m deep and where this was done within the influence zone of the tunnel - between a 2 m and 5 m distance - we had to use a steel sleeve for each pile, ' says Mr Shahnavaz.

Much of the piling was close to the tunnel because the structure must 'bridge' it.No additional loads are above those already existing are permitted.

Once the tunnel problems were sorted, the next issue was acoustic insulation. In general, to achieve this, buildings cannot sit directly on the pile caps, but instead go on elastomeric bearings that sit in-between the pile caps and the ground beams and bases of the walls.

'It is not just the main structures; all the services must be isolated too, ' says Mr Shahnavaz.'It goes to the nth degree - even the lightning conductor has its own acoustic connector, a so-called 'lattice mat', which is a connection of copper tapes.'

It gets more complicated for the six different lift and stair cores in the main apartment building.These must each be 'floated' on elastomeric bearings ('each one of the bearings must be precisely measured for load, ' says Mr Shahnavaz), but then they also have to be restrained laterally.'The whole core could slide about otherwise, ' explains Mr Bevan.

As a result, another set of horizontal elastomeric bearings must be used at the sides.These sit in a gap between the core concrete wall and thrust blocks outside, which are built up as part of the pile caps. Each bearing is around 130 mm thick and must be compressed by 5-10 per cent once in position in a gap 175 mm wide.

'The only way to do this is by using a jack, so we had to find something that would fit into the remaining gap of 45 mm, ' says Mr Shahnavaz.'We found some flat jacks that can develop the required load of 450 N, but they are expensive.These go in permanently.They are sacrificial and, once they reach load, they are grouted in.'

For each core, up to 16 horizontal bearings had to be loaded at the same time; unequal load could have caused the cores to move sideways. Specialist subcontractor Abbey Pinford used a multiple-port compressor unit to load the jacks and monitor them simultaneously.

Sliding was also a problem over the basement car park, which underlies about half of the main building.Acoustic bearings could not be put in below ground, so the top of the one-storey basement retaining wall took the bearings instead.To prevent lateral slide here there are steel pegs projecting from the wall at intervals. Each peg has a rubber sleeve where it projects above the elastomeric bearings, so that when the superstructure walls and columns are cast there is vibration damping around the pegs.

A particular challenge on the main building has been a large 15 x 15 m floor slab that sits over the tunnel, supported at either end on a ground beam and a pilecap.The 900 mm thick slab of concrete, poured in one go, has to have a void underneath of at least 100 mm to prevent noise transmission.

Achieving this posed a conundrum, since creating a void would require some kind of filler in the formwork when the slab was poured. But it could not stay in situ because even a filler transmits noise. So to make the cavity Wates used a clayboard filler underneath.As, given enough moisture, this dissolves, the idea was that it could be flushed out when the slab was ready.

'We wrapped it in plastic and then put pipes through the slab when it was cast, so that we could pop the plastic later and flush the board, ' explains Mr Shahnavaz.

The slab had to be designed with an opposite camber, so it could settle to a flat position when the clayboard was removed. But to limit the distortion of the pipes, which Mr Shahnavaz points out 'can only move 15 mm before breaking'careful design work and even more careful site work was required to achieve the result.

Meanwhile, there was an even bigger problem to solve.The canal wharf n See page 52 n From page 51 has some historic significance - it had been part of a city waste-transfer station and its old listed facade needed to be kept in place.This, too, required isolation damping, relatively easy to achieve in the new part of the project, but more complex in the old wall.

Basic support for the facade during construction is provided by a steel support frame set up during early work on site. Five main support columns are held up by clusters of 4 m-deep mini-piles along the canal-side towpath, with a few more piles behind. But to get the acoustic isolators into the heavy masonry wall, a support beam had to be built along the bottom.

'We began by knocking out slots in the brickwork and putting in steel support posts about 1,200 mm high, ' explains Phil Evans, who supervised the facade work for Wates.These posts comprised three sections joined by bolted steel end plates, and were inserted into cavities knocked out of the wall at roughly 1 m intervals.At the top and bottom was a padstone, and each post was dry-packed in for firmness.

With support along a 7 m length of wall, the brickwork could be knocked away to the depth of the posts. Concrete beams were cast around the top third of the posts and the bottom third sections, both of which were sacrificial and left in place as part of the reinforcement.

'We then moved along, leaving a 3 m section of wall intact for stability, and did another 7 m length beyond it, and so forth, coming back to finish the infill sections afterwards, ' says Mr Evans.

In the 450 mm-deep space left between the new beams compressed springs could be inserted ('just like Zebedee's in the Magic Roundabout, ' says Mr Bevan), along with lifting jacks.The springs were pre-tensioned, held at 80 per cent of load by bolted steel frames, and just slid in.

The final stage of the process was then to lift the new top beam by a few millimetres on the jacks, which allowed the centre sections of the temporary support posts to be unbolted and removed.The beams were then lowered on to the springs to a measured load, which as it passed the springs' pretensioning level allowed their tensioning frames to be unbolted and removed as well.'It all had to be done in a very controlled manner, ' says Mr Shahnavaz.

Work on the canalside building has been proceeding apace, and the first apartments will be available by spring next year when the former municipal land is handed over to developer Charles Church. Phase two of the scheme will continue on the main block. But it is all very conventional from here on in, says Mr Shahnavaz, perhaps somewhat relieved.