Geotechnical Assessment

A desk study reviews existing published information about the geology, ground conditions, and land use history of a site without any physical investigation. A ground investigation involves visiting the site and physically investigating the ground through boreholes, trial pits, or other exploratory methods. The desk study identifies potential risks and informs the scope of the ground investigation. Both are components of a comprehensive geotechnical assessment.

For most standard house extensions, a full geotechnical assessment is not required at planning stage. However, if the site has known geotechnical issues such as subsidence history, proximity to mining, or steep slopes, the planning authority or building control may request one. Foundation design for extensions is usually addressed at the Building Regulations stage based on the builder's experience and trial pit observations during construction.

A coal mining risk assessment is required by the Coal Authority for planning applications in defined Development High Risk Areas. It evaluates the potential risks to the development from past coal mining activities, including shallow mine workings, mine entries (shafts and adits), and mine gas emissions. The assessment is based on a Coal Authority mining report and site-specific investigation where necessary, and must be submitted with the planning application.

Trial pits are excavations made by a mechanical excavator, typically 1 to 4 metres deep, allowing direct observation and sampling of shallow soils. Boreholes are drilled vertically into the ground to greater depths using specialist drilling rigs, providing continuous samples and enabling in-situ testing. Trial pits are suitable for shallow investigations, while boreholes are necessary for deeper ground information, rock sampling, or groundwater monitoring.

Geotechnical and contamination assessments often overlap but serve different purposes. A geotechnical assessment focuses on the engineering properties of the ground for foundation design and stability. However, soil samples collected during the ground investigation can also be tested for contamination. If contamination is suspected, a separate Phase 2 contamination investigation with appropriate chemical analysis is normally required alongside the geotechnical work.

Poor ground conditions do not necessarily prevent development but can significantly affect foundation design and construction costs. Solutions for challenging ground include piled foundations to bypass weak or compressible soils, ground improvement techniques such as vibro-compaction or dynamic compaction, engineered fill replacement, and raft foundations. The geotechnical assessment identifies the ground constraints and recommends appropriate engineering solutions.

A change of use that does not involve new construction or significant structural alteration typically does not require a geotechnical assessment. However, if the change of use involves increased floor loading, construction of new foundations, or works in areas with known ground instability, a geotechnical assessment may be appropriate. The planning authority will advise on a case-by-case basis.

Eurocode 7 (BS EN 1997) is the European standard for geotechnical design, covering ground investigation, foundation design, retaining structures, embankments, and slope stability. It replaced the previous British Standards for geotechnical design and is used alongside the UK National Annex. All geotechnical design for new developments in the UK should comply with Eurocode 7, which sets out the methodology for assessing ground conditions and designing geotechnical structures.

Borehole depth depends on the type and scale of the proposed development and the ground conditions. For a standard residential development, boreholes typically extend to 5 to 15 metres below ground level. Basements and piled foundations may require deeper investigation to 20 metres or more. The depth should be sufficient to characterise all soil strata that could influence foundation performance and to prove adequate bearing stratum for the proposed foundation type.

An existing geotechnical report may be useful background information, but planning authorities and building control generally expect site-specific investigation for new developments. Old reports may not reflect current ground conditions, particularly if there has been subsequent ground disturbance, groundwater changes, or if the proposed development differs significantly from what was previously assessed. Reports older than five years should be treated with caution and may need supplementing with new investigation.