Bridge & Bedrock

Before any building, infrastructure, or civil engineering project can proceed, the ground must be understood. This is the purpose of the ground investigation — a structured programme of work designed to characterise the subsurface conditions at a site. Without it, engineers are working blind, making assumptions about what lies beneath the surface that may or may not reflect reality. Ground investigation is not a luxury or an optional extra. It is a fundamental part of the design process, and the information it provides shapes every subsequent decision, from foundation type and depth to the management of groundwater and the treatment of contamination.

Yet despite its central importance, ground investigation is often undervalued and underfunded. The pressure to reduce upfront costs can lead clients and project managers to commission investigations that are inadequate in scope or depth, or to skip phases of investigation that would reveal critical information. The consequences of this false economy can be severe: unexpected ground conditions during construction, cost overruns, delays, and in the worst cases, structural failures. Understanding the ground investigation process — what it involves, why each component matters, and what happens when it goes wrong — is essential for anyone involved in infrastructure delivery.

The Staged Approach to Ground Investigation

Ground investigation is not a single event but a process that unfolds in stages, each informed by the findings of the previous one. The standard approach in UK practice, guided by Eurocode 7 and the associated UK National Annex, involves at minimum two principal stages: a desk study (sometimes called a Phase 1 assessment) and a ground investigation proper (sometimes called a Phase 2 investigation). In complex cases, further stages of investigation may be required to address specific questions raised by earlier findings.

The staged approach is important for several reasons. First, it allows the investigation to be targeted: the desk study identifies the likely ground conditions and the key uncertainties, and the ground investigation is then designed to resolve those uncertainties as efficiently as possible. Second, it allows the level of effort to be proportionate to the complexity and risk of the project: a simple residential development on a well-understood site may require a relatively modest investigation, while a major infrastructure project on a complex or contaminated site will require a much more extensive programme of work. Third, it provides a structured framework for quality control and documentation, ensuring that the investigation is fit for purpose and that its findings are properly recorded and communicated.

Phase 1: The Desk Study

The desk study is the first and in many ways the most important stage of the ground investigation process. It involves the systematic collection and review of all available information about a site and its surroundings before any fieldwork is undertaken. This information typically includes: geological maps and memoirs, historical Ordnance Survey maps, aerial photographs, borehole and trial pit records from previous investigations, mining records, landfill records, historical land use data, hydrogeological information, and any relevant planning records.

A thorough desk study can reveal a great deal about the likely ground conditions at a site. The BGS 1:50,000 geological maps, available through the BGS GeoIndex online portal, provide the basic geological framework — the rock types and superficial deposits that are likely to underlie the site. Historical maps, available through a range of commercial providers, can reveal former industrial uses that might have left contamination, former watercourses that might indicate soft or unstable ground, or former quarries and pits that might leave voids. Mining records can identify areas undermined by shallow coal workings or old metal mines.

The output of the desk study is typically a written report that summarises the available information, identifies the key ground-related risks and uncertainties, and makes recommendations for the scope and design of the subsequent ground investigation. It forms the basis for the Conceptual Site Model (CSM) — a structured description of the likely ground conditions, contamination status, and hydrogeological setting at the site — which will be refined and updated as more information is gathered.

Phase 2: The Ground Investigation

The ground investigation proper involves the physical investigation of the subsurface through a combination of intrusive and non-intrusive techniques. The choice of investigation method depends on the site conditions, the information required, the budget available, and the constraints imposed by access, existing structures, and underground services. In most cases, the investigation will include a combination of boreholes, trial pits, and in-situ tests, supplemented by laboratory testing of recovered samples.

Boreholes are the most versatile and widely used investigation method. They can reach depths of tens or even hundreds of metres, recover samples of soil and rock for laboratory testing, and accommodate a wide range of in-situ tests (such as the Standard Penetration Test and the pressuremeter test). In the UK, rotary and percussive borehole methods are both widely used, with the choice depending on the ground conditions: percussive boring is generally more efficient in soft soils, while rotary drilling is necessary to penetrate hard rock or to recover undisturbed samples from sensitive materials.

Trial pits offer a different kind of information: they are shallow (typically no deeper than 4–5 metres for safety reasons), but they expose a large area of the ground profile that can be inspected visually. This is particularly valuable for identifying geological features — such as made ground, buried structures, discontinuities in rock, or inhomogeneous fill — that might not be apparent from borehole samples alone. Trial pits are also well suited to collecting bulk samples for laboratory testing, particularly for the assessment of contamination.

In-Situ Testing

In-situ tests measure the engineering properties of the ground directly, in its natural state, without the need to recover and transport samples to a laboratory. This is important because many soils — particularly soft clays, loose sands, and granular materials — are difficult to sample without significant disturbance, and the process of sampling and transportation can alter their properties in ways that make laboratory test results unreliable.

The Standard Penetration Test (SPT) is the most widely used in-situ test in the UK. It involves driving a standard split-spoon sampler into the ground at the base of a borehole using a standard hammer, and counting the number of blows required to advance the sampler by 300 mm. The SPT blow count (N-value) is an index of soil strength and stiffness that can be correlated with a wide range of engineering parameters. It is a simple, robust, and relatively cheap test, though it has well-known limitations: it is sensitive to operator technique, borehole conditions, and the presence of gravel or obstructions.

The Cone Penetration Test (CPT) provides a continuous record of ground resistance with depth by pushing a cone-tipped probe into the ground at a constant rate. It is particularly valuable in soft soils where the SPT is less reliable, and it provides a much more detailed picture of stratigraphy than borehole methods. The CPT can also be equipped with sensors to measure pore water pressure (the piezocone, or CPTU), temperature, electrical resistivity, and seismic wave velocity, greatly extending its utility.

Laboratory Testing

Laboratory testing of soil and rock samples recovered during the ground investigation provides the quantitative engineering parameters needed for design: shear strength, stiffness, compressibility, permeability, and so on. The nature and extent of the laboratory testing programme should be dictated by the design requirements: what parameters are needed, to what precision, and for which soil units.

Classification tests — particle size distribution, Atterberg limits, moisture content, and bulk density — are carried out on virtually all samples to establish the nature and variability of the materials encountered. More specialised tests — triaxial compression tests for shear strength, oedometer tests for compressibility and consolidation, permeameter tests for permeability — are carried out on selected samples where the design requires specific parameters.

The quality of laboratory testing is critically dependent on sample quality. Disturbed samples — those that have been broken up, compressed, or contaminated with drilling fluids during recovery — cannot provide reliable measurements of in-situ properties. Obtaining good-quality undisturbed samples from soft or sensitive soils requires care, skill, and the use of appropriate sampling techniques such as thin-walled tube sampling or block sampling.

Reporting and the Ground Model

The findings of the ground investigation are compiled into a factual report, which presents the raw data from the investigation: borehole and trial pit logs, in-situ test results, laboratory test results, and groundwater monitoring data. The factual report is a permanent record of what was found and should be archived in the AGS digital format to ensure that it remains accessible for future use.

The factual data is then interpreted in a geotechnical interpretive report (or ground investigation report), which develops the ground model, assigns geotechnical parameters to identified soil and rock units, assesses geotechnical risks, and provides design recommendations. The ground model is the conceptual framework within which all subsequent design takes place. It must be not just accurate but also clearly communicated: engineers and other members of the project team who were not involved in the investigation need to understand the ground conditions well enough to make sound design decisions.

The ground investigation process, done well, transforms an unknown and potentially hazardous quantity into a managed engineering challenge. It is the foundation — in every sense — on which good geotechnical design is built. Every pound spent on a thorough and well-targeted ground investigation is an investment in the safety, quality, and cost-efficiency of the project that follows.