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Tailored for a close fit

A UK developer is ploughing a lot of money into ground works to fit a five storey block of flats onto a tiny brownfield site. Paul Wheeler reports

It seems hard to believe that a developer would spend the ú1M ($1.4M) on geotechnical and foundation work for a housing development on a plot measuring only 63m by 20m.

But that is the investment the UK's Wilcon Homes Western is committed to at 60-80 Hotwells Road in central Bristol.

The site, on one of the city's main thoroughfares, is beside the River Avon and opposite the permanent mooring of IK Brunel's gloriously restored steamship SS Great Britain.

The recent surge in the property market has made the derelict Hotwells site prime real estate and commercially viable for the first time since the original Victorian brick terrace was destroyed by a Second World War bomb.

For more than half a century, giant advertising hoardings concealed the sections of brick wall, perched high on the vegetated steep sandstone slope, that were all that remained of the 10 terraced properties.

Wilcon's plan is to cut back and steepen the rock slope, creating a 20m by 63m long area at its base, giving space to construct a five-storey block of 28 luxury flats.

Geotechnically, the concept involves forming a stitch line - a series of closely spaced steeply inclined drill holes - which defines the new slope position.

Support for the new cut rock slope is provided by a series of dowels behind and parallel to the stitch line, together with three rows of shallow inclined anchors installed as the rockslope is excavated down from the crown. A layer of reinforced sprayed concrete will complete the new slope structure.

The reality is much more complex, with logistics providing the biggest challenge, says Chris Wade, contracts manager for geotechnical contractor Ritchies, a division of Edmund Nuttall, part of the HBG group.

As work progresses, access requirements change constantly and are always demanding, with such restricted access from the road at the front of the site. The solution has been to create steep earthwork ramps, with local earthworks contractor Churngold revisiting the site many times as Ritchies moves on to different parts of the slope.

The site geology was relatively well understood, with exposures revealing sandstone containing mudstone bands dipping steeply out of the slope - broadly typical of the area.

At up to 300MPa the quartzitic sandstone is some of the strongest rock in England. However the condition of the mudstone, which has a controlling effect on the stability of the slope, was unknown. It has proven to be worse than usual for the area, and has had to be pre-grouted in places.

Ritchies was happy to take the project on a design and build lump sum basis. 'It's a question of how you control the risks, ' says Wade. But the company's success on the project appears to vindicate its approach.

Variations include a few extra anchors of lower capacity to cope with geological variations revealed during the work, but despite the need for a constantly developing design methodology, the total number of anchors will be within 5% of the simplified conceptual design.

For detailed geotechnical design, Ritchies brought in the local geotechnical expert Brian Hawkins. Wade believes the relationship between the two has been a big factor in success. 'As the sandstone is exposed the design is modified to suit, with detailed drilling positions assessed by Hawkins. It has worked incredibly well.'

Drilling is carried out using down-the-hole hammers with air flush. Water mist or a dust collection system suppresses dust.

Drilled holes vary from 100mm diameter for the stitch line to 140mm diameter for the dowels and anchors, spaced nominally at 1.3m centres.

Ritchies expects to install 150, double corrosion protected, seven strand Dwyidag anchors, each typically 20m long, including a fixed length of 10m, giving each a target working load of 105t. Dowels comprise grouted single 63.5mm diameter Gewi bars extending just below the base of slope level.

Award of the foundation package and reinforced concrete work to underpin the old walls has increased the value of Ritchies work to just under ú1M, to be finished within the 44-week contract.

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