We have recently undertaken a Phase 2 ground investigation for a private extension at a site in Berkshire, close to Windsor.
The ground investigation was required to satisfy a planning condition due to a protected tree on the site. There was concern that traditional trench foundations would cause potential root damage to the protected tree, and therefore a specialist foundation design was required in order to minimise root damage during the development of the extension.
Due to the limited access of the site, a window sampler rig was used to undertake boreholes in the area of the proposed extension. The underlying strata consisted of River Terrace Deposits and London Clay. From the ground conditions encountered, bored cast in situ piles terminating at depth in the London Clay were recommended for the site in order to minimise potential damage to the tree roots.
Geotechnical and chemical laboratory testing was undertaken on selected soil samples to show the volume change potential of the soils, and to give an indication of aggressivity of the ground in relation to buried concrete. London Clay strata are known to be very likely to have high sulphate concentrations, especially in the weathered zone or at the base of the tree root zone. Piles will be in contact with both of these zones and if the incorrect type of concrete is specified and used then the high sulphate levels can attack and weaken the concrete, showing the importance of undertaking sulphate tests, especially in the London Clay strata.