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What site investigation should take place before designing concrete foundations?
Sufficient information needs to be collated about the site before a decision whether to adopt a standard solution or to have the foundations designed by a qualified person can be made. There are three main routes to the information:
- the desk study
- the walk-over survey
- the direct investigation
They normally need to be taken in that order, and together constitute the site investigation. A desk study will set out to:
- Identify the materials and thickness of the strata below (or near to) the proposed construction.
- estimate the angle of slope of the ground surface
- Identify all information that may reveal a potential hazard to the new foundations or to the proposed use of the site e.g. previous use.
The walk-over survey provides information that cannot be obtained form the study of documents alone. Evidence includes:
- high water table and indications of flooding
- evidence of filled ground or tipped rubbish
- existing old foundations, basements, cellars, fuel tanks.
The direct investigation will identify:
- soil type
- bearing capacity
- soil conditions
These can normally be established by digging a trial pit, possibly using an excavator of the hydraulic back-acter type. The depth may be stipulated by the local authority to ensure sufficient material is assessed. Laboratory testing of samples may be required particularly where contamination is a possibility.
Acknowledgement: The Concrete Society
Other references:BRE Good Building Guide 39 Part 1 Simple foundations for low-rise housing: site investigation