Bridge & Bedrock

Before a single borehole is drilled or a trial pit dug, the competent geotechnical practitioner will have spent considerable time at their desk. The desk study — sometimes called a Phase 1 site assessment — is the essential first step in any ground investigation. It costs a fraction of fieldwork, yet can reveal the most significant ground risks at a site and provide the framework within which all subsequent investigation is designed. Skipping or skimping on the desk study is a false economy that almost invariably leads to a less efficient and less effective investigation programme.

A desk study is fundamentally an exercise in information gathering and critical analysis. The objective is to build up a picture of the site’s geological setting, land use history, hydrogeological context, and potential hazards from all available secondary sources — before committing to the cost of intrusive investigation. Done well, a desk study transforms a blank piece of ground into a site with a known (or at least partially known) history, enabling the investigation to be targeted at the specific uncertainties that matter most for the proposed development.

The Purpose and Value of a Desk Study

The desk study serves several interrelated purposes. First, it establishes the regional and local geological context: what rock types and superficial deposits are likely to underlie the site, what the broad stratigraphy is, and what major geological events — glaciation, marine transgression, river activity — have shaped the ground conditions. This context is essential for interpreting the results of any subsequent fieldwork: without knowing the regional setting, borehole logs are data points without meaning.

Second, the desk study identifies ground-related hazards that may be present at the site. These hazards might include: made ground or fill from former development, contamination from industrial or agricultural use, mining subsidence, natural cavities in soluble rock such as limestone or chalk, shrink-swell behaviour in clay soils, landslide risk on sloped ground, flooding from rivers or the sea, or aggressive ground chemistry that could attack concrete or steel. Some of these hazards are obvious from a glance at a geological map; others require careful detective work through historical records.

Third, the desk study informs the design of the ground investigation. By the time the desk study is complete, the practitioner should have a clear picture of the likely ground conditions, the principal uncertainties, and the specific questions that the ground investigation needs to answer. This allows the investigation to be designed efficiently: boreholes can be located where the ground conditions are most uncertain, test types can be selected to deliver the parameters required for design, and the depth of investigation can be set to reach the relevant stratum.

Sources of Information

The geological map is the starting point for any desk study. The British Geological Survey (BGS) produces 1:50,000 scale geological maps for the whole of Great Britain, showing the bedrock geology and superficial deposits at the surface or immediately beneath made ground. These maps are available through the BGS GeoIndex online portal, along with associated memoirs and reports that provide detailed descriptions of the geology. For areas where the surface geology has been significantly modified by urban development or agricultural activity, the BGS also produces a range of more specialised datasets including borehole records from the National Geoscientific Data Centre.

Historical Ordnance Survey maps are among the most valuable tools available to the desk study practitioner. A systematic review of maps from different historical periods — typically from the 1870s onwards, when detailed large-scale mapping began — can reveal a remarkable amount about the history of a site. Former industrial uses such as gasworks, tanneries, chemical works, and railway yards are all associated with potential contamination. Former landfill sites, pits, and quarries indicate the likelihood of made ground, voids, and gas generation. Former watercourses and ponds suggest soft, waterlogged ground. Mill ponds and millraces indicate the presence of water management features that may have altered natural drainage patterns.

Aerial photographs provide a complementary perspective to maps, often revealing features that are not recorded in any documentary source. Crop marks can indicate buried features such as ditches, pits, and structures. Soil marks reveal areas of disturbed or contaminated ground. Topographic anomalies may suggest mining subsidence, buried quarries, or other ground disturbance. Historic aerial photographs, available through the National Collection of Aerial Photography and various commercial sources, can extend this analysis back to the early twentieth century.

Mining records are an essential source of information for sites in former coal-mining or metal-mining areas. The Coal Authority maintains records of all known mine workings and shafts in England, Scotland, and Wales, and provides a search service that can reveal whether a site is underlain by known workings. Similarly, records of former mine shafts — which can be hazardous even when they have been capped — are available through the Coal Authority and through local authority planning records.

The Conceptual Site Model

The key output of a desk study is the Conceptual Site Model (CSM) — a structured description of the site’s likely ground conditions, contamination status, and hydrogeological setting. The CSM is not a final answer; it is a hypothesis that will be tested and refined by the subsequent ground investigation. But it is an essential framework for organising the information gathered during the desk study and for communicating the key ground risks to clients, designers, and other project stakeholders.

A CSM typically includes: a description of the geological setting and likely stratigraphy, a summary of the principal ground-related hazards identified, an assessment of the likely hydrogeological conditions including groundwater depth and flow direction, and a description of the potential contaminant sources, pathways, and receptors if contamination is a concern. The CSM is usually presented as a combination of text, figures, and tables, often including geological cross-sections and a site location map.

For sites where contamination is a significant concern — former industrial land, sites adjacent to landfills, or sites with a history of chemical use — the CSM forms the basis for a formal risk assessment under the contaminated land framework established by Part IIA of the Environmental Protection Act 1990. This requires the identification of plausible source-pathway-receptor linkages: contaminants that are present or likely to be present, pathways by which those contaminants could travel through the ground or groundwater, and receptors — people, property, or the environment — that could be harmed if the pathway is complete.

The Walkover Survey

Most Phase 1 assessments include not just a desk study but also a walkover survey — a physical inspection of the site and its surroundings carried out by a competent geotechnical or environmental professional. The walkover survey allows the practitioner to observe site conditions directly: the topography, drainage, vegetation, visible geology, existing structures, and any surface evidence of contamination or ground instability.

The walkover survey often reveals features that are not recorded in any available documentary source. Differential settlement in existing buildings may indicate problem ground conditions beneath. Springs or wet areas may reveal the position of the water table or the existence of perched water. Anomalous vegetation — the presence of species associated with contaminated ground, or the absence of vegetation on areas where it would normally be expected to grow — can be a valuable indicator of subsurface conditions. Visible made ground, the presence of rubble or fill at the surface, and odours indicative of contamination are all significant observations.

Reporting and Recommendations

The Phase 1 desk study report should present the information gathered during the desk study and walkover survey in a structured and accessible format, develop and present the Conceptual Site Model, identify and characterise the principal ground-related risks and uncertainties, and make clear and specific recommendations for the subsequent ground investigation. The recommendations should specify the type, number, location, and depth of proposed investigation points, the in-situ tests and monitoring required, and the laboratory testing programme.

The quality of a Phase 1 assessment is ultimately judged by the quality of its recommendations: whether the subsequent ground investigation, designed on the basis of those recommendations, successfully resolves the key uncertainties and provides the information needed for design. A Phase 1 assessment that fails to identify a significant ground hazard — because the relevant information source was not consulted, or because the practitioner lacked the experience to recognise the significance of what they found — can have serious consequences for a project.

The desk study and Phase 1 assessment are in many ways the intellectual core of the ground investigation process. They require broad knowledge, careful analysis, and sound professional judgement. They are also, relative to the cost of fieldwork and laboratory testing, extraordinarily good value — which is why every ground investigation, no matter how simple or complex the site, should begin with one.