The Importance of Geotechnical Site Investigations

Geotechnical engineering investigates how earth materials like soil, rock and groundwater interact with man-made structures, such as buildings, bridges, dams and roads. Concrete, bricks and steel are man-made construction materials, designed to specific ‘recipes’ and specifications. In contrast, natural geological materials have evolved over time through natural processes, often hidden from view. To understand the composition, properties and behaviour of the soil and rock at a specific location, we conduct geotechnical site investigations to ensure it will be capable of supporting any planned construction.

Geotechnical site investigations involve a series of tests and analyses to assess the physical properties of the soil and rock beneath a proposed construction. These investigations help engineers determine the suitability of the site for the intended structure or development, ensuring safety, stability and cost-effectiveness.

What is included in a geotechnical site investigation?

A geotechnical site investigation will typically include:

• A desktop report: Identifies all the existing information about a site from resources such as existing maps and identifies what is likely to be found at the site. From here a testing regime is proposed.
• A geotechnical investigation phase: The testing regime is completed, if the results of the tests vary significantly from what was expected from the desktop investigation, further tests may be necessary or the test may need to be taken from deeper samples.
• Analysis and reporting: All the information is analysed, processed and reviewed by geotechnical engineers in order to identify the geological properties of the site. This information will guide other specialists such as architects and structural engineers in their analysis and design.

The geotechnical engineers will identify which tests are required in the investigation phase in order to answer any questions regarding the interaction between the existing soil or rock and the proposed structures. These tests can be carried out both at the site and in a laboratory and can identify:

• Soil composition and properties: Identifying the types of soil and rock present at the site and their respective properties, such as strength, density, permeability, and compressibility.
• Groundwater conditions: Assessing the level, flow, and pressure of groundwater, which can significantly impact construction and long-term stability.
• Soil behaviour: Evaluating how the soil will respond to the stresses imposed by the proposed structure, including potential settlement, liquefaction, and bearing capacity.
• Hazard identification: Detecting any geotechnical hazards, such as landslides, sinkholes, or seismic activity, that could affect the project.

A geotechnical site investigation digs into the ground in order to observe the soils below. How deep the tests need to be conducted and what is tested depends on the type of structure planned for the site, different structures will have different needs.

The project size, the type of structure planned and the geotechnical engineer’s knowledge of the area will determine which tests are needed. During the investigation phase different methods are used to test the subsurface, including boreholes and drilling, standard penetration tests, geophysical surveys and In-Situ testing such as plate load tests to assess the soil properties under actual conditions. 

The project size, the type of structure planned and the geotechnical engineer’s knowledge of the area will determine which tests are needed. A geotechnical site investigation will use a variety of methods and techniques, including:

• Boreholes and drilling: Extracting soil and rock samples from various depths to analyse their properties in a laboratory.
• Standard Penetration Tests (SPT): Measuring soil resistance to penetration to estimate its strength and density.
• Cone Penetration Tests (CPT): Providing continuous profiles of soil resistance to a cone pushed into the ground, offering detailed data on soil stratigraphy.
• Geophysical surveys: Using seismic waves or electrical resistivity methods to infer subsurface conditions without extensive drilling.
• In-situ testing: Performing tests directly at the site, such as plate load tests or vane shear tests, to assess soil properties under actual conditions.

After the geotechnical site investigation is completed the geotechnical engineer will analyse the data and produce a geotechnical report. The geotechnical report contains the information other engineers need to understand how their specialty will react with the soil at a site and will guide the design process.

A geotechnical investigation report will include:

• The details and scope of the project and the structure to be built.
• Brief geological description of the site and the conditions that can be expected.
• The tests that have been carried out on the site and the results.
• Analysis of the test results.
• Options for any geotechnical issues the project is likely to encounter.
• Conclusion and recommendation for best option based on the understanding of the project.

Geotechnical site investigations are a crucial part of the planning and design process for any construction project.

Imagine yourself in the middle of a plain. Can you determine what soil is below your feet? Is it gravel, sand or clay? How does it respond to water infiltration? Can it support a house, or a commercial warehouse?

Without investigating further, none of these questions can be answered. Geotechnical engineers identify the most appropriate tests to answer the questions relevant to your project.

Conducting a geotechnical site investigation early in a construction project is crucial for mitigating risks and ensuring the overall success of the development. Early investigation provides the necessary knowledge about the site’s subsurface conditions, which informs the entire planning and design process.

By understanding the soil and rock conditions of a site at the beginning of a project, potential geotechnical issues such as weak soils, high groundwater levels, or the presence of hazardous materials can be identified early. Strategies to address these issues can be developed before they become significant problems.

The geotechnical conditions of a site must be taken into consideration during the design phase of a project. Early identification of issues such as soil instability, potential for liquefaction, or seismic activity allows for appropriate safety measures to be incorporated into the design, protecting both the construction team and future occupants. Unexpected ground conditions or late discovery of adverse subsurface conditions can require design modifications, construction delays and increased costs.

The cost of a geotechnical report depends on the complexity of the project, the number of geotechnical tests needed and the complexity of the analysis.

For a simple dwelling, a geotechnical report will cost the equivalent of 1m2 to 2m2 of the finished building. Around half the cost of the report represents the investigations part, some geotechnical engineering consultants undersell how many investigations are needed in order to reduce the cost of a report. If a geotechnical engineer proposes more tests than a comparative offer, they may see a risk in the site that others don’t. Ensuring a geotechnical site investigation is through in the initial stages ensures fewer variations and additional test are required later in the process, which could increase the costs of the project.

Conducting thorough geotechnical site investigations ensures the safe and efficient design and construction of structures. By understanding the subsurface conditions, engineers can tailor designs to suit the specific soil and rock conditions, preventing excessive settlement and structural failure. Risks are mitigated through early identification of potential geotechnical hazards which can be addressed early in the project, reducing the risk of costly surprises and delays. Having accurate data on soil behaviour and properties means construction costs are optimised, avoiding over-design and unnecessary construction techniques. Structures can be built to withstand environmental and operational stresses over their intended lifespan, ensuring the longevity and stability of the development.

In summary, geotechnical site investigations are essential for any development project, providing the necessary information to design and construct safe, stable, and cost-effective structures. At Eliot Sinclair, we provide expert geotechnical site investigations using advanced techniques and expertise to ensure the success of your project from the ground up.

Speak to our experts before starting your next development project.