An investigation should include an assessment of the ground strength (bearing capacity) to well below (~3m) the anticipated foundation depth. For typical trench or strip foundations at about 1m depth, a survey to around 4m depth and for piles survey to around 6-10m depth. Boreholes to take core samples and probes, such as the Standard Penetration Test (SPT) or dynamic probe, are used to assess bearing capacity.
Historically, little regard was given to tree root effects and foundations in high shrink/swell or volume change potential soils, and foundations would be placed at depths shallower than 1m below the surface. An oak or leylandii tree near to such a foundation can cause considerable ground movement and subsequent cracking of the building. Testing the soil’s plasticity and volume change potential, as well as assessing tree type and proximity to foundations, allows an engineer to determine the foundation depth needed to avoid tree root effects. Basements which may indeed be deep enough (> 3m depth) to avoid the effects, can still be affected by lateral earth pressures against basement walls.
Even removing a tree does not, in the short-term, remove the problem as high-volume change potential soils, such as London clay, can take many years to fully hydrate during which time damaging heave can occur. One tree in the corner of a site, however, does not necessarily condemn the whole foundation to a deep trench, as stepping can be used keep costs down. Mini piles are often used in soils with very high volume change potential as large volume of soil removal and concrete import are avoided.
Sites located on sand, gravels or hard rock will avoid problems relating to tree root effects and tend to form the best building site locations. Areas with clay over chalk can experience some of the most devastating building collapses as these can be areas where sinkholes develop. Ground probing can find these sinkholes and in such places drill rods can be inserted into the ground into voids 4m-high occurring 3m below the surface.
A good site investigation will also assess aggressive ground parameters such as pH and sulphates to determine concrete specification and also any contamination risk from historical land use which can impact on human health and services such as water pipes.
A survey typically takes less than a day for a self-build-type project and involves small diameter boreholes (80mm) being placed in about three or four locations around a site. Equipment can vary from small, motorised hand-held drills to tracked drilling rigs.
Soil Environment Services specialises in geotechnical site investigation soil surveys for self-build projects and small residential developments.