Ground investigation

A direct ground investigation is the third stage in the site investigation. As far as low-rise housing is concerned, its main objective is to determine whether strip foundations will be suitable and, assuming they are, whether they can be designed in accordance with the simple ‘rule of thumb’ approach contained in the Building Regulations. The ground investigation will provide detailed information on:

- the nature and thickness of made-up ground/top soil above the sub-soil
- the nature, thickness and stratum depth of sub-soil
- an assessment of allowable bearing pressure
- groundwater levels, chemicals in the ground, etc.
- existing structures or hazards in the ground.

For low-rise housing on greenfield sites, machine-dug trial pits are probably the most common method of ground investigation. The pits do not normally need to be deeper than 4–5m unless specific problems are encountered. Trial pits should be excavated close to the proposed foundation, but not so close as to affect its actual construction. The number of pits is usually a matter for judgement and will depend on the size of the proposed development, the nature of the site and the consistency of the sub-soil across the site.