The rest of England and Wales generally have low plasticity clays but even these still carry some risk.They also turn up in some isolated areas in the North West of England near the coast Medium plasticity clays are found in the rest of the South East, across the Midlands and up beyond the Humber Estuary towards the North East.Clays with the highest plasticity (and so the highest risk) are generally found in the South East of England, stretching up through the East Midlands to the Humber in the North and down to Bath in the West.There are three types of clay in the UK, classified by their plasticity, which is how much their volume can change due to their water content. Sand holds together reasonably well when damp, compacted and uniform, but trenches may collapse and so sheet piling is often used to retain the ground in trenches until the concrete is poured. A shallow, reinforced, wide strip foundation may be suitable. If the water table is high (for instance if the gravel is submerged), the bearing capacity will be halved, so it’s important to keep the foundations as high as possible.
Generally a depth of 700mm is acceptable, as long as the ground has adequate bearing capacity. Chalk soils can be prone to erosion, so be wary of the possible presence of hollows or caves.ĭry, compact gravel or gravel and sand subsoils are usually adequate for strip foundations. If the chalk is soft it will need to be excavated until firm chalk is reached. The depth of the foundation must be below any frost action, a minimum of 700mm. Providing the chalk is not too soft, foundation widths of 450mm for low-rise buildings are generally acceptable. The rock may simply need to be stripped back and levelled off to build from. Limestone, granite, sandstone, shale and hard solid chalk are all rocks that have a high bearing capacity. For even more advice, information and inspiration delivered straight to your door, subscribe to Homebuilding & Renovating magazine.Ī geotechnical study will reveal the type of ground on your site and will most likely be one of, or a mixture of the following:
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