Housing Development

Busk Lane, Church Fenton

Ashbury Homes (part of the Bellway Group) bought the site of the former RAF barracks, where the buildings were still in situ and during the demolition the ground was noted as being very poor. The site was purchased on the recommendation of piling the entire site due to wet/damp silt and sand deposit which were initially noted as running deposits, which on first evaluation from specialist sub-contractors was deemed unsuitable for a Ground Improvement Solution.

Due to the long working relationship of the then Technical Director of Bellway Yorkshire and our own Technical Director, the question was asked can we do anything to gain a betterment to the site over and above piling. Town and Country Vibro came to site and looked at a number of trial pits, assessing that the ground was not in fact a running silt sand deposit but was actually just wet/damp, which based on a bottom feed Vibro Stone Column solution could actually improve the ground to in excess of the required 125kN/m² bearing capacity and reduce total settlements to less than 25mm. Initial scepticism was brought to the fore as the site showed significant movement when tracked over due to the mobilisation of the pore-water pressures within the ground, exhibiting in noticeable cow bellying.

Town and Country Vibro Limited had to produce initial settlement design calculations, with treatment depths going down to a competent soil down at between 4-5m below the existing ground level. The treatment showed that within a couple of days from installing the columns, and proposals to install the deep drainage through the road box sections, the ground stiffened up significantly, with water noted coming up from the columns installed. The columns actually acted as large band drains within the upper poor soils, and when tested showed settlements well below those calculated. The issue here was not poor ground, but a high water table that made the upper fine granular soils react such that they became partially liquified when exposed to load, i.e. a machine tracking over the ground.

The use of Vibro Stone Columns has significantly changed the makeup of the ground conditions such that the entire site is now free from physical cow bellying even without treatment, but with treatment a required bearing capacity of 125kN/m² can be achieved. Town and Country Vibro engineers have been taught how to question all soil profiles, and to make their own assumptions to problems and this is a prime example to why we question everything, as this saved the House Builder significant money in the ground and also in their foundation solution to DPC.

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