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MILITARY PRECISION

PILING & FOUNDATIONS

A ancient graveyard, a listed building and the shadow of July's London bombings face the piling team on a City site. Damon Sch³nmann reports from the home of the oldest regiment in the British Army.

A stark reminder of London's suicide bombings meets anyone entering the site of the Honourable Artillery Company (HAC) on City Road.

The oldest regiment in the British Army, established by Henry VIII in 1537, in happier times hosts cricket matches on its secluded pitch in the heart of the capital. But the pitch as well as the rest of the site is screened off from public view, because it is still being used as a temporary tented mortuary, set up in the wake of the 7 July attacks.

Once the visitor gets past the police checkpoint, it is clear that something else is happening here, as cement lorries rumble past Harris fencing.

This is down to the HAC charity, known as The Company, which owns some of the surrounding land and Armoury House, in which the Albert Room hosts banquets as well as acting as a drill hall.

The 12m wide Albert Room is the target of all the construction activity. It is being lengthened by 14m at both ends to incoporate what are now backroom areas, resulting in a total length of 53m. A basement is also being added.

Access for main contractor Killby & Gayford on the £5M scheme has been hindered by the fact that the Albert Room is listed. Also, an ancient graveyard butts up against the north edge of the building.

Known as 'the cemetery of Puritan England', it contains the remains of John Bunyan, members of the Cromwell family, Daniel Defoe and William Blake.

Site manager Dennis Mulvihill says: 'The graveyard has 300,000 bodies and because lots of gravestones are near the Albert Room boundary wall, we've encased the gravestones and put up fencing to protect them, as well as debris netting to shield the work from the public.' Killby & Gayford will excavate down to 4m for the basement in a scheme that was amended after planning objections. Mulvihill says: 'The original idea was to underpin along the north side but the Corporation of London didn't want that as there might have been the risk of bones dropping out [into the basement during construction].' The stratum is made ground over about 10m of gravel overlying London Clay.

May Gurney won a £250,000 contract to secure this edge of the planned basement with a different method. 'We're using a CFA contiguous wall to replace the underpinning along this edge, ' says the subcontractor's southern area manager Nick Sharp.

The company will install CFA piles 8-12.5m deep; 135 of 450mm diameter at 550mm centres and nine 600mm units at 750mm centres. These will resist the surcharge load from the graveyard, which is about 1.5m above the Albert Room's ground level, and support the existing foundations of the building. Workers are building them in a sequence of every fourth pile to allow the concrete to go off undisturbed.

On the south edge, where another wall within Armoury House butts up against the Albert Room, underpinning concrete built in three 1.5m deep layers will support the existing foundations. Although the underpinning has saved a bit of space here compared to a piling solution, a few 450mm piles will still be used along the east and west ends of the Albert Room's southern boundary.

'But what's interesting on this job is we're using two rigs on one job, a Drillfrance CB9 CFA rig and a H³tte HBR 202 TF-A minipiling rig, ' says Sharp.

This is because the roof of the listed central area of the Albert Room cannot be removed to allow a standard rig to operate.

This restriction does not apply to the two 14m long ends of the building, into which the 25m long Albert Room is being extended, so the CB9 can work here.

But there is one slight cause for concern. To operate in the furthest extension from where the rig will enter the gutted structure, it will need to travel under the sensitive listed central area.

Sharp says: 'The CB9 is 14.8m high when derigged for manoeuvring, and the clearance is only 15m, so it's going to be pretty tight.' May Gurney was also apprehensive about the ability of the mini-rig to get through the ground. 'We were a bit worried about the dense gravels and the rig not being powerful enough, but it's eating it and going well, ' Sharp says.

Each fiight for the H³tte is 1.2m and after depth is reached the sections come off one at a time as a metre of concrete is pumped through the hollow stem.

'We intend to do the mini-rig piles in the central area first but there will be a bit of an overlap once the bigger rig comes in at the end of September, ' he says.

May Gurney is scheduled to finish on site next month with Killby & Gayford completing in autumn 2006.

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