Tailored for a close fit

Outside central London it seems hard to believe that a developer would spend the best part of a million pounds on geotechnical and foundation work for a housing development on a plot measuring only 63m by 20m. But that is exactly the investment Wilcon Homes Western is committed to at 6080 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 ofIK Brunel's gloriously restored SS Great Britain - the world's first iron-clad, propeller-driven oceangoing steamship, that crossed the Atlantic in 1845.

The recent surge in Bristol's property market - especially city centre sites overlooking water - have made the derelict Hotwells site prime real estate and commercially viable for the first time since its brick Victorian 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 ofthe 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 of28 luxury flats.

Geotechnically, the concept involves forming a stitch line - a series ofclosely spaced steeply inclined drill holes (essentially a presplit line without the explosives) - which defines the new slope position.

Support for the new cut rock slope is provided by a series of dowels immediately 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 on the site, says Chris Wade, contracts manager for geotechnical contractor Ritchies, the geotechnical contracting division of Edmund Nuttall.

As work progresses, access requirements are changing constantly and are always demanding - especially getting rigs into high positions for the stitch line, dowels and upper anchors 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 compact slope.

Wade believes Ritchies'origins as a drilling equipment manufacturer have been a big advantage. 'We have fitters who can adapt the rigs to suit the conditions, 'he explains. On this job drills have been mounted on a variety of carriers from long reach excavators to telehandlers as required.

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

Ritchies, whose southern office is only a few miles away at Clevedon, has wide experience drilling in the quartzitic sandstone. At up to 300MPa it is some of the strongest rock in England (and is especially abrasive to drill bits). However the condition of mudstone, which dips out of the face and so has a controlling effect on the stability of the slope, was unknown. It has proven to be worse than usual for the area, and it has had to be pregrouted 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 .

He maintains that the biggest uncertainty was the stability of the old walls at the start of the work. Certainly the company's success on the project appears to vindicate its acceptance of the contract approach.

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

For detailed geotechnical design, Ritchies brought in the expertise of local geotechnical guru Dr Brian Hawkins, who trades as independent consultant HM Geotechnics.

Wade believes the relationship between Hawkins and Ritchies has been a big factor in the project's success.

'As the sandstone is exposed the design is modified to suit what we find, with detailed positions and drilling positions assessed by Dr Hawkins on a daily basis. It has worked incredibly well. '

Drilling is carried out using down-the-hole Conceptual section through slope with stitch line, dowels, anchors and drains.

hammers with air flush. Water mist or a dust collection system is used to suppress dust.

Drilled holes vary from 100mm diameter for the stitch line to 140mm diameter for the dowels and anchors, both ofwhich are spaced nominally at 1. 3m centres.

Ritchies expects to install about 150, double corrosion protected, seven strand Dwyidag anchors during the project, each typically 20m long, including a fixed length of 10m, giving each a target working load of105t.

Dowels comprise grouted single 63. 5mm diameter Gewi bars extending just below the base ofslope level.

Award of the foundation package and some reinforced concrete work to underpin the old walls above the new slope has increased the value ofRitchies work to just under £1 million, to be finished within the 44-week contract.

In total, work will have included 6000m 3ofrock excavation;8km ofdrilling, mostly for the stitch holes, dowels and anchors, but also for slope drainage; 1000m 2of sprayed concrete;

and 200m 3of reinforced concrete work. Throw in localised rock netting and the occasional catch fence to provide safety during the work, and this becomes a most compact and remarkable piece of geotechnical and foundation engineering.