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4.1.10 Guidance for investigations
Site investigations shall be undertaken in accordance with BS EN 1997-2 and recognised practice. Items to be taken into account include:
- investigation technique
- sampling
- testing.
Investigation technique#
A site investigation normally comprises techniques which are classed as either indirect or direct.
Indirect investigations use geophysical techniques, including electromagnetic, resistivity, seismic, gravity and ground radar, to interpret ground conditions. Conducted from the surface, they measure variations in properties of the ground, both horizontally and vertically, to define subsurface conditions. Geophysical methods rely on contrasts in the physical properties, for example, between sand and gravel and rockhead. Contrast may also be provided by faulting, underground cables and pipelines or by cavities.
Direct investigation techniques involve intrusive activities to enable the retrieval and examination of the ground using trial pits, trenches, boreholes or probes.
Trial pits allow the detailed inspection, logging, sampling and in-situ testing of large volumes of natural soil or fill and the assessment of ground water conditions. Trenches are extended trial pits, or linked trial pits, which are excavated where greater exposure of the ground conditions is required. Trial pits and trenches should be positioned where they will not affect future foundations.
Boreholes are typically formed using the following techniques:
| Light cable percussion drilling | A shell and auger rig – typically used in the UK to drill boreholes in soils and weak rocks |
| Continuous flight auger | Exploratory boreholes may be drilled in soils by mechanical continuous flight augers of various sizes. Hollow stem methods are typically employed where sample retrieval is required |
| Rotary drilling | Either open-hole drilling or rotary coring, is used to investigate rock and sometimes stiff soils, such as boulder clay |
| Probing techniques | Used to analyse the relative density of soils and for environmental sampling and monitoring (such as chemical and physical testing of gases, liquids and solids) |
Sampling#
The number and type of samples taken should be:
- appropriate for the results of the desk study, the walkover survey and the site investigation
- appropriate for the range of ground materials encountered and the proposed development
- taken, stored and transported so that they avoid cross-contamination.
Samples are used to enable soil and rock descriptions to be made and to provide material for physical and chemical testing.
‘Undisturbed’ soil and rock samples undergo minimal disturbance, so provide a more reliable indication of the physical soil properties than ‘disturbed’ samples.
Ground water should be collected from appropriately designed monitoring wells which should be screened and sealed to ensure that the relevant stratum is being monitored.
Gas sampling should be carried out from appropriately designed monitoring wells, boreholes or window sampling holes are typically used. Identification of the probable source and the measurement of gas flow are important for risk assessments.
Testing#
Testing may be undertaken in-situ, or in a laboratory.
A wide variety of in-situ tests can be used to support the results of direct testing. These range from basic tests undertaken by geologists or engineers using simple hand-held devices or portable test kits to methods that require specialist personnel and equipment.
Testing laboratories should participate in quality assurance programmes and be accredited for relevant tests by bodies such as UKAS and MCERTS. Physical tests on soil and rock materials are carried out to provide the following information on ground:
- strength
- relative density
- deformation
- settlement
- consolidation characteristics
- permeability.
Chemical tests on soils, rocks, ground water and gases can be carried out to provide an indication of potential contamination on the site.
Last updated: 2nd January 2024