Environmental · Best Practice · Pre-Construction

Soil and Ground Investigation Report

A report on ground conditions based on boreholes, trial pits, and laboratory testing.

Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. CDM 2015 / Contract remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.

Legal basis	Construction (Design and Management) Regulations 2015 / Contract requirements + BS 5930:2015 Code of Practice for Ground Investigations
Status	Best Practice
Prepared by	Geotechnical Engineer
Methods	Trial pits, boreholes, cone penetration tests (CPTs)
CDM requirement	Ground investigation forms part of CDM pre-construction information — essential for identifying geotechnical risks before design and construction

1. Ground Investigation — Know What You're Building On Before You Design

Ground investigation is the process of determining the geological, geotechnical, and hydrogeological conditions beneath a site so that foundations, earthworks, and below-ground structures can be designed safely and economically. Without adequate ground investigation, foundation design relies on assumptions rather than evidence, and the risk of encountering unexpected ground conditions during construction increases significantly. Unexpected ground conditions are one of the most common causes of cost overruns and programme delays on construction projects.

Under the Construction (Design and Management) Regulations 2015, the client has a duty to provide pre-construction information to designers and contractors, including information about ground conditions that could affect the health or safety of any person carrying out construction work. A ground investigation report is a key component of CDM pre-construction information. BS 5930:2015 provides the code of practice for the planning, execution, and reporting of ground investigations in the UK, and sets out the requirements for a properly conducted investigation.

The investigation eliminates foundation uncertainty by providing factual data on soil and rock types, their engineering properties, groundwater levels and behaviour, and the presence of any hazards such as made ground, voids, or contamination. This data allows the structural engineer to design foundations that are appropriate for the actual ground conditions, rather than relying on conservative assumptions that may result in over-engineered and unnecessarily expensive foundation solutions.

Timing is critical

Conduct ground investigation before design is finalised — not after foundation design is substantially complete. If the investigation reveals ground conditions that differ from assumptions, the foundation design may need to be fundamentally changed, causing significant abortive design costs and programme delays. Early investigation allows the design to be developed in response to actual ground conditions from the outset.

2. Key Components

The following table sets out the key components that a soil and ground investigation report must address. Each element contributes to providing the factual and interpretive geotechnical data needed for safe and economic foundation design.

Component	What it covers
Site description	Location, boundaries, topography, current land use, proposed development, and the geotechnical questions that the investigation is designed to answer. The description must set the investigation in the context of the proposed construction works.
Desk study	Review of published geological maps, British Geological Survey borehole records, historical Ordnance Survey maps, previous site investigation reports, mining records, and any other available information about the ground conditions beneath the site. The desk study informs the design of the intrusive investigation.
Investigation methods	Description of the investigation techniques used, including trial pits, boreholes, cone penetration tests (CPTs), and any geophysical surveys. The rationale for the investigation layout, depths, and methods must be explained and must be appropriate to the proposed development and the anticipated ground conditions.
Strata logs	Detailed logs of all exploratory holes showing the sequence, thickness, and description of each stratum encountered, following the BS 5930:2015 soil and rock description conventions. Strata logs are the factual record of what was found beneath the site and are the foundation of all subsequent geotechnical interpretation.
Groundwater	Groundwater levels recorded during and after the investigation, including response monitoring over time. Groundwater regime, permeability, and seasonal variation are critical for foundation design, excavation dewatering, waterproofing design, and the assessment of contaminant migration pathways.
Laboratory testing	Results of laboratory tests on soil and rock samples, including classification tests (moisture content, particle size distribution, Atterberg limits), strength tests (triaxial, shear vane, unconfined compressive strength), compressibility tests (oedometer), and chemical tests (pH, sulphate, chloride). Test results must be presented in a clear tabular format.
Geotechnical assessment	Interpretation of the factual data to provide geotechnical parameters for foundation design, including bearing capacity, settlement predictions, slope stability, and excavation support requirements. The assessment must identify any geotechnical hazards such as soft or compressible strata, high groundwater, made ground, or aggressive ground conditions.
Contamination	Any evidence of ground contamination identified during the investigation, including visual or olfactory evidence, elevated chemical concentrations, and the presence of asbestos-containing materials in made ground. If contamination is found, the report should cross-refer to the contaminated land risk assessment or recommend that one is carried out.
Conclusions	Summary of ground conditions, geotechnical parameters for design, groundwater regime, any geotechnical or contamination hazards identified, and recommendations for foundation type, construction methodology, and any further investigation or monitoring required.
Prepared by	Name, qualifications, and professional memberships of the geotechnical engineer who prepared the report. The engineer should be a chartered member of a relevant institution such as the Institution of Civil Engineers (ICE) or the Geological Society of London, and should have appropriate experience in ground investigation and geotechnical engineering.

3. Common Mistakes

Inadequate number of investigation points

A ground investigation with too few exploratory holes may miss critical variations in ground conditions across the site. The number and spacing of investigation points must be appropriate to the size of the site, the complexity of the anticipated geology, and the sensitivity of the proposed structures. Eurocode 7 and BS 5930:2015 provide guidance on investigation point spacing, but the investigation designer must exercise professional judgement based on the specific site conditions. Under-investigation creates a false sense of certainty and increases the risk of encountering unexpected ground conditions during construction.

Not extending to sufficient depth below foundation level

Exploratory holes must extend to a depth sufficient to identify all strata that could affect the performance of the proposed foundations. As a minimum, boreholes should extend to a depth below foundation level at which the stress increase from the proposed loading is negligible. For piled foundations, this means extending well below the anticipated pile toe level. Terminating boreholes too shallow may miss compressible strata that could cause unacceptable settlement, or fail to identify the bearing stratum on which piles are intended to found.

Not including seasonal groundwater monitoring

Groundwater levels measured during a single site visit provide only a snapshot of the groundwater regime. Groundwater levels fluctuate seasonally and can vary significantly between summer and winter, particularly in areas with shallow water tables or permeable superficial deposits. A single measurement may not represent the worst-case condition for foundation design or excavation dewatering. Installing standpipe piezometers and monitoring groundwater levels over several months, ideally capturing both a wet and dry season, provides a much more reliable basis for design.

4. Frequently Asked Questions

Is a ground investigation always required?▾

A ground investigation is not always a strict legal requirement, but it is always best practice and is strongly recommended for any construction project involving foundations, earthworks, or below-ground structures. On many projects, a ground investigation will be required by the structural engineer before they will design foundations, by the building control body as part of the building regulations approval process, or by the client as a condition of contract. For projects on brownfield sites, a contaminated land risk assessment (which includes intrusive ground investigation) will typically be required as a planning condition. Under CDM 2015, the client has a duty to provide pre-construction information about ground conditions, and a ground investigation is the most reliable way to obtain this information. Proceeding without a ground investigation is a false economy — the cost of dealing with unexpected ground conditions during construction almost always far exceeds the cost of the investigation.

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