What is a Geotechnical Investigation and Why is it Essential for Construction?

Before any construction project begins, it is essential to understand the ground beneath a proposed development site. A geotechnical investigation is the process of assessing soil, rock and groundwater conditions to determine whether a site is suitable, stable and safe for construction. Various geological factors and geohazards, such as sinkholes / voids, low-strength soils, aggressive ground, un-engineered Made Ground, shrinking and swelling clays and shallow groundwater can influence the suitability and design of a construction project. Geotechnical investigations form the basis of reliable design, helping engineers overcome challenges relating to ground conditions, reduce risks and ensure compliance with construction standards.

A well-designed and executed investigation should provide the data needed to make the most appropriate engineering decisions with respect to foundation design, slope stability, road and pavement design, floor slab construction, concrete classification and drainage. By identifying variations in soil type, soil strength and groundwater conditions, a robust geotechnical investigation can prevent costly delays and ensure long-term integrity of buildings and structures.

Understanding Geotechnical Investigations

In essence, a geotechnical investigation evaluates the physical and chemical properties of the ground. Physical properties include soil texture, density, moisture content, strength and plasticity that influence construction design and stability, while chemical properties such as sulphate content, pH and organic contamination have implications for the degradation of construction materials such as concrete. The investigation will commonly combine detailed descriptions of subsurface ground conditions, in-situ testing, material sampling and laboratory testing to establish how the strata beneath the site will behave during and after construction.

By interpreting the data obtained during an investigation, engineers can provide appropriate design parameters for foundations, retaining walls, pavements, earthworks and more, in accordance with industry guidance and standards. Geotechnical engineering or engineering geology are the disciplines for which such investigations and assessments form the bedrock of appropriate engineering design recommendations.

For example, identifying unsuitable founding materials, such as soft clay, loose sand or Made Ground early in the process allows the foundation solution to be adapted or ground improvement measures to be designed, in order to avoid potential settlement or instability once the building is complete.

Groundwater conditions are also important, and may have significant implications for drainage solutions and for the stability and flooding of excavations during construction.

Geotechnical investigations are site-specific, and should consider the anticipated ground conditions at a site, based on an initial review of available information regarding the geology, hydrogeology, history, topography and environmental setting of the site, to determine the investigation objectives and the most appropriate intrusive investigation techniques.

Key Stages of a Geotechnical Investigation

Although every project is unique, most geotechnical investigations follow a structured process to ensure a comprehensive assessment of site conditions.

This approach will generally involve an initial review of available data to establish the like ground conditions (a desk study) and a site visit to establish surface conditions and any access restrictions. The main geotechnical site investigation will be designed based on the initial findings, with subsequent stages of investigation following if deemed necessary by the main ground investigation.

Desk Study and Preliminary Site Assessment

The process begins with a desk study, reviewing geological maps, mining records, previous site investigation data and historical land use. Geological mapping is widely available and is one of the key information sources, providing high level information regarding the anticipated superficial and bedrock geology beneath a site. The availability of information from other sources may be more dependant on the site location and history. This desk based review helps identify potential risks such as mine workings, potentially unstable ground and past ground disturbance.

A site walkover allows engineers to assess current surface conditions, obvious drainage issues, the presence of trees and other vegetation, and any visible signs of instability or movement. This initial appraisal informs the design of the subsequent intrusive investigation phase.

Subsurface Exploration and Sampling

In some cases, surface exploration is undertaken before, or alongside, the main intrusive ground investigation works. Such methods, such as topographical and geophysical surveys, can be used to assess the ground surface and / or sub-surface characteristics of a site before intrusive methods are applied, and can be useful in directing the scope and layout of the intrusive ground investigation.

The intrusive investigation typically includes a combination of trial pits, boreholes and probe-holes together with in-situ testing and material sampling. The investigative methods allow the collection of both disturbed samples and undisturbed samples of soil and rock, and in-situ tests such as strength tests (e.g. Standard Penetration Tests (SPTs)) and infiltration (soakaway) tests, which help build a detailed ground model.

Disturbed samples are altered during collection and are suitable for certain classification tests, while undisturbed samples retain in-situ conditions and are required for accurate assessment of properties like strength, compressibility and consolidation.

The quality and type of soil sample and the in-situ information obtained varies depending on the investigation technique:

Trial pits expose shallow ground conditions for direct visual inspection and allow disturbed and bulk samples to be collected. Trial pits can also be used for infiltration tests.
Boreholes can provide information from greater depth, allow the collection of both disturbed and undisturbed samples, and can be used for in-situ strength tests (SPTs) and infiltration tests.
Cone Penetration Tests (CPT), plate load tests and CBR (California Bearing Ratio) tests measure strength, stiffness and bearing capacity directly in the ground or at the ground surface.

The data obtained during the investigation describes the subsurface conditions, such as geological strata, groundwater depth and engineering characteristics that will have an impact on development design.

Groundwater and Hydrogeological Assessment

Groundwater can affect both ground stability and foundation performance. Shallow or variable groundwater can cause a reduction in soil strength, unstable excavations and volume change, while chemically aggressive groundwater can corrode buried structures.

During the investigation, groundwater levels can be monitored via standpipes or piezometers installed in boreholes. Understanding hydrogeological conditions enables engineers to design effective drainage, waterproofing and dewatering systems to control groundwater effects during and after construction, and to adapt the construction design to consider the local groundwater conditions.

Laboratory Testing and Data Analysis

Samples of soil are sent to a suitably accredited laboratory to determine their physical and chemical properties. The laboratory tests are generally carried out in accordance with relevant British Standards to ensure consistency and reliability.

Common tests include:

Atterberg limits (plasticity), which describes how cohesive soils behave with differing moisture contents, particularly in terms of volume change.
Particle Size Distribution (PSD) is a test that classifies a soil in terms of texture and the proportions of clay, silt, sand or gravel.
Shear strength, consolidation and permeability tests are used to assess the physical properties of soil and how it responds under load and drainage conditions.
Compaction tests are used to determine the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) to provide a benchmark for earthworks.
Chemical testing, especially pH and sulphate content, to determine conditions that could affect materials such as concrete or steel.
Rock and aggregate tests such as Point Load, Unconfined Compressive Strength and the Los Angeles Co-efficient, are used to assess strength and resistance to abrasion.

Together, the results of laboratory tests provide engineers with accurate data to inform safe and efficient design.

Analysis, Interpretation and Reporting

All field and laboratory data are compiled into a geotechnical report. This document includes the factual data relating to ground conditions obtained during the investigation, together with calculations and interpretation of this data to determine design parameters, identify risks and provide recommendations for foundations, excavations, slope stability, drainage and earthworks etc.

G&J Geoenvironmental geotechnical reports are clear, evidence-based and aligned with industry guidance and standards. They form an important component of engineering design packages to ensure the site preparation and construction phases fully consider and mitigate ground risks.

Understanding Ground Conditions and Soil Behaviour

Ground conditions can be complex and vary significantly from site to site, or even within the same site. Cohesive soils like clay may expand and contract in response to variations in moisture, while granular soils such as sand and gravel may settle under load. Fill or Made Ground introduces further variability which may require significant testing and / or remediation (ground improvement) works to make it stable enough for construction.

Geotechnical engineers interpret data using established principles of soil mechanics, together with experience and professional judgement to ensure the determination of appropriate design parameters and recommendations for structures such as foundations, roads, floor slabs, retaining walls etc. This information is then used by structural engineers to incorporate into their designs.

Comprehensive soil investigations on construction sites are essential to provide sufficient information to identify and address ground risks before development begins.

Applications of Geotechnical Investigation

Geotechnical investigations form the basis of a variety of construction aspects for projects of all scales, such as:

Building design - Geotechnical data is essential for structural design.
Infrastructure projects such as highways, railways and tunnels - Understanding the underlying geological conditions and how they change along the route of a project which may cover several miles.
Slope stability analysis and retaining wall design - Often an essential element of projects on sites with challenging topography and changes in elevation.
Earthworks - Determining how the physical properties of the ground will be affected by earthworks and ground improvement techniques, and proving validation criteria to demonstrate materials have been engineered in accordance with the specification.

Geophysical surveys and geotechnical ground investigations allow engineers to evaluate subsurface conditions for a range of applications. The use of an appropriate range of techniques, which may include geophysical as well as intrusive elements, ensures that all potential ground risks and hazards that are relevant to a specific project can be evaluated and mitigated.

Managing Geotechnical Challenges

Ground investigations rarely proceed precisely to plan, with aspects such as surface ground conditions, weather, site access restrictions and unexpected sub-surface geology all representing potential complications for fieldwork. Supervising engineers must adapt in real time, which may involve modifying sampling plans, using alternative investigation techniques, extending boreholes or introducing additional testing where required.

The Importance of Early Investigation

It is important to engage a geotechnical professional early in the design process in order to identify potential risks, inform cost planning, and allow engineers to develop design solutions that fit the actual ground model, rather than relying on assumptions.

Completing this work before tendering / procuring the contract helps reduce uncertainties and avoid unexpected costs, delays and design revisions.

Why Choose G&J Geoenvironmental?

G&J Geoenvironmental have undertaken comprehensive geotechnical investigations across the UK, on sites with wildly differing geologies and ground risks. Our engineers and geologists bring extensive experience in ground investigation and geotechnical interpretation, ensuring that an accurate ground model is developed before development begins.

Our reports are structured to be clear, combining technical interpretations with practical recommendations and non-technical summaries for design and construction teams.

Summary

A geotechnical investigation is the foundation of safe and stable construction. Assessing the properties of soil, rock and groundwater allows ground risks to be mitigated through design or remediation.

Through comprehensive analysis and accurate interpretation of investigation results, G&J Geoenvironmental can reduce risks and support stable, long-term development. From small-scale residential sites to major infrastructure projects, our investigations give developers confidence that their designs rest on solid ground.

For professional support and tailored geotechnical investigation services, contact G&J Geoenvironmental to discuss your project requirements.