Sometimes DIY can be fun.But, as client B&Q learned when one of its stores foundered on Irish peat, there are times when it is best to leave it to the experts. Joanna Booth went to Londonderry to find out how Keller got home improvement back on firm foundations
IN AMONG the picture-postcard beauty of rural Northern Ireland's softly rolling hills, Buncrana Road in Londonderry is not a particularly prepossessing place. But, seen through the commercial development-tinted spectacles of one of the country's largest home improvement chains, the 10,000 sq m site, flanked by residential property and on a busy road, was too good an opportunity to miss.
B&Q signed a design and build contract with local firm Heron Bros to construct a superstore, car park and garden centre on the brownfield site, formerly home to a clothing factory.
It seemed a prime location above ground level but when the team dug a little deeper a very different story emerged.
Although one end of the site had stable rock close to the surface, the level of this dropped off very quickly, giving way to made ground, and then peat and soft clays.The ground level dipped in the centre of the site and the crater needed to be filled with 3 m of upfill to level out the area.
The original intention had been to drive 3,000 precast piles from fill level, carrying the load on a concrete slab. In June the team realised this design was too expensive to be viable, which threw the fate of the store into doubt.
Further difficulties arose in the shape of an old river basin 8 m below the surface, winding its way across the site in a culvert.
The final nail in the coffin was the overhead power line that also crossed the site, limiting headroom to the extent that it would not have been possible to accommodate a large rig.
Ground engineering specialist Keller was initially only called in to look at ways of supporting the car park, but quickly realised it could provide a more cost-effective solution for the entire construction.
'Peat is a soft, organic material and compresses very easily, ' says James McNeill, Keller's business development manager for Ireland.'The weight of the soil in the upfill alone could create up to 400 mm of settlement.'
The load needed to be transferred through the peat and made ground. Keller's design team recommended using a mixture of vibro stone columns and vibro concrete columns to support the loadings.This would reduce the cost enough to keep the project on budget.
After dynamic probes ascertained the exact location of the peat, Keller's design team mapped out which ground engineering method was most suitable for different areas of the site.
Where there is less peat, vibro stone columns, the cheaper option, are being used.This covers the perimeter areas, underneath the garden centre and half the car park.
Keller is using its Minicat rig so it can work beneath the power cables, driving 2,800 columns to depths of up to 5 m.
Conditions are so soft that pre-augering has not been necessary for any of the columns; the vibration alone being sufficient to drive the poker through the made ground.
The rig-mounted poker is driven to the required depth and stone is vibrated into the soil, strengthening it and increasing its innate load-bearing capacity.To combat subsidence, a bottomfeed method is being used, whereby the 25-40 mm stones are fed down the inside of the poker and into the ground at the base of the pile as the poker is extracted.
Vibro concrete columns are being used in the most peatheavy areas, situated in the centre of the site beneath the car park and the store itself.
'You can tell when the poker hits the peat, ' explains Mr McNeill.'It just drops away, the ground is so soft.'
Instead of introducing stone into the soil, concrete is pumped down the poker and into the soil. Pressure is maintained as the poker is withdrawn.The columns are designed to found in the gravels beneath the peat, so the depths range from 2.5 m to 7 m.
To further anchor the columns into the ground, Keller is creating bulbs on the bases.After pumping out concrete at the base level, the rig driver re-enters the concrete with the poker, displacing the ground. Keeping the poker moving up and down, concrete is pumped continuously until a bulb is formed.
With the exception of the concrete columns supporting the store frame loads, most of the 997 of these piles are also constructed with 900 mm enlarged heads, which work as in-situ pile caps.After excavating the top of the pile, the same formula of concrete is poured into the former for the pile cap.
'We use a very low slump mix - the target is 60 mm, ' says Mr McNeill.'It is only just pumpable.The CFA mix is so much higher that on those piles you have to wait and trim them when they are half-set. It makes the tops less sound than with our method.'
To distribute the load across the VCCs, a 900 mm load-transfer platform is being constructed over the columns. Coarse, angular fill is reinforced with three layers of geogrit, a high-density, hightensile plastic mesh. It provides ground-bearing conditions by arching the load down into the columns beneath.
'It means we avoid using a concrete slab, 'Mr McNeill explains.
'It is often used in road construction over soft ground. It sounds odd, but the performance of the car park is the really critical factor with these stores. People are loading up their cars with bags of cement and concrete blocks. If you get settlement, you end up with trolleys running away.'
The Keller techniques will provide an institutional loading of 35 kN/sq m, which should be enough to support the trolley of even the most prolific DIY enthusiast.