Ground Improvement
Project Brief
Foundations for a new housing development in Walsall comprising 412 housing plots. The site was a former residential estate which had been demolished to make way for the new homes.
Roger Bullivant Limited (RB) developed a programme utilising two foundation techniques – ground improvement vibro stone columns and displacement piling; a top feed vibro stone column rig, and the quiet hammer rig system were used.
Key Issues
The ground conditions were determined by exploratory boreholes. This revealed the site to be underlain by a variable thickness of made ground over glacial till. The made ground was at variable depths up to 4.1m bgl. While the underlining glacial till, which was present in all exploratory holes, ranged from depths 2.5m – 4.8m bgl. The material typically comprised of medium dense sand or very gravely clayey medium sub rounded sand.
As the majority of the site was overlaid by made ground of unknown consistency this is generally not a competent material for foundation construction. Vibro stone columns were utilised to help densify the ground above the natural ground of the glacial till.
An isolated northern area of the site was deemed unsuitable for ground improvement. This was due to the presence of a black clayey gravel fill with an organic content. A piled solution was deemed appropriate for this area.
Solution
A total of 5122 stone columns were installed equating to 412 housing plots to depths ranging from 1.2m – 2.1m. The scheme was designed to provide a total bearing capacity of 125kN/m² below the foundations. Stone columns were installed to refusal, each with a diameter of approximately 480mm.
Vibro stone columns help densify and stabilise the majority of the site while the pre-cast displacement piling technique created supports within the ashy black clays and gravely fill regions of the site. The pre-cast displacement piles amounted to a total of 466. These were installed to a depth of 10m and achieved a designed maximum safe working load of 350kN. Two pile sizes were used 200mm2 and 250mm².
