A PILED geogrid reinforced load transfer platform has provided an elegant solution to complex foundation issues at a new retail development at Downpatrick in Northern Ireland.
The new single storey Safeway Store, car park, filling station and retail warehouse unit has been built on extremely soft ground in a low-lying area subject to flooding.
Developer Safeway (Stores) Ireland required the finished floor and external works to be set 150mm above the 100 year flood level, 2.5m above existing ground.
Site investigations carried out by Belfast-based engineering consultant Taylor & Boyd revealed very deep layers of peat and organic material to depths of up to 30m. This clearly discounted using shallow foundations, which the developer had assumed would be sufficient.
Two deep foundations options were considered, either piling the structure and car park to support a grid layout of concrete beams and precast panels, or using a geogrid reinforced granular load transfer platform supported on a triangular grid layout of precast concrete piles.
The load transfer platform required more piles but proved the most economical and practical. Not only did it provide a more straightforward and quicker solution, but it also offered the client savings in excess of £100,000 over the pile and beam option.
The platform consists of a 2.3m thick layer of well graded granular material reinforced with three layers of Tensar biaxial (SS) geogrids topped with a 1.15m thick layer of bulk fill. Total thickness of the blanket is equal to the clear space between the pile caps.
The biaxial geogrids restrain the granular fill between the pile caps and mobilise the maximum shear strength of the granular layer by positive mechanical interlock, which enhances arching between the pile caps. Once this arching is established, the main load from the embank- ment is transferred on to the piles. The grids also provide long term support to fill below the arch lying on the soft soils between the piles.
'The load transfer platform was designed to ensure that load from the store and parking areas was efficiently transferred on to the piles,' explained Glenn Kerr, partner with Taylor & Boyd who worked closely with geosynthetic specialist Tensar International.
Main contractor Bovis Europe appointed WAM (GB) as contractor for the ground enabling works. WAM site agent David McVeigh said that from a contractor's point of view the load transfer platform was ideal. 'Once the piling was complete we were able to use conventional earthmoving plant. To have had to form concrete ground beams in this type of site would have been extremely difficult. It enabled fast track construction to commence from an early stage in the overall contract.'
The driven precast concrete piles were installed in a triangular pattern at 3.35m spacing over 70,000m2. The granular platform included a layer of Tensar SS30 150mm above the pilecaps, followed by a further two layers of Tensar SS20 at 0.3m spacing.