Matt Hartnup manages Lucion Ground Engineering's UKAS-accredited geotechnical laboratory in Peterborough, where his team conducts thousands of soil tests each year for projects across the UK. From major infrastructure developments to residential schemes, Matt has seen first-hand how quality testing data can make or break a project. In this article, Matt explains the science behind the testing and why it matters for your project.
After years of running a geotechnical laboratory, I’ve noticed something interesting. Our lab technicians can become incredibly skilled at testing triaxial specimens, but they often don’t see what happens to that data once it leaves the lab. Similarly, many engineers and project managers understand they need triaxial testing for their projects, but the “why” and “how” behind those numbers can feel like a bit of a black box.
That gap in understanding bothers me. Because when you grasp what triaxial testing actually tells you, and just as importantly, what it doesn’t, you’re in a much stronger position to make decisions about your development. You know when to dig deeper, when to test more, and how to interpret results that don’t quite fit the pattern you were expecting. And what effect this all may have on the budget for your project.
Whether you’re designing foundations for a major infrastructure development, assessing slope stability, or planning earthworks, in this article, I aim to show you how triaxial testing works and why it matters for your project.
What is Triaxial Testing?
Triaxial testing is a lab procedure used to determine the shear strength characteristics of soil samples under controlled conditions. The test involves placing a cylindrical soil specimen in a pressurised chamber and subjecting it to varying stress conditions that simulate in-situ ground conditions.
At our UKAS-accredited geotechnical laboratory (No. 8180), we primarily conduct Quick Undrained Triaxial (QUT) testing (BS EN ISO 17892-8:2018), which determines the undrained shear strength of cohesive soils. This data is essential for:
- Foundation design – Understanding load-bearing capacity and settlement potential
- Slope stability analysis – Assessing the risk of landslides or embankment failure
- Excavation planning – Determining safe working practices and temporary support requirements
- Earthworks specification – Designing compaction requirements and material suitability
Why Sample Size and Comprehensive Testing Programmes Matter
One of the most important aspects of any geotechnical testing programme is ensuring you have sufficient data to make confident decisions. A single test result tells you about that specific sample at that specific depth, but it doesn’t reveal trends or identify anomalies.
When we work with clients on larger projects, we often test multiple samples across various depths and locations. This approach offers several advantages including identifying trends and soil pattern behaviours, isolating outliers and anomalies that may affect your decisions, and ultimately reduce risk.
A comprehensive testing programme provides engineers with the confidence to make informed decisions, reducing the likelihood of unexpected ground conditions during construction.
Triaxial Test Limitations
Whilst triaxial testing is highly effective, it’s important to understand its limitations. As engineers and developers, recognising these factors helps you interpret results appropriately:
- Sample Disturbance – The process of extracting, transporting, and preparing soil samples introduces some disturbance. Lab results are generally lower than in-situ values, which engineers account for in their designs.
- Scale Effects – A triaxial specimen is small compared to the volume of soil that will actually support a foundation or embankment. The test cannot capture larger-scale features such as discontinuities or the natural ‘fabric’ of the soil (the arrangement of particles and voids over a wider area).
- Geological Features – Fissures, joints, or other discontinuities in the ground may not be captured in small laboratory specimens but can significantly influence ground behaviours.
Triaxial Testing as Part of the Complete Picture
Larger samples help mitigate some of the aforementioned issues, but it only provides one piece of the puzzle.
Working with our on-site geotechnical and ground engineers, our integrated approach combines:
- Site investigation data – Borehole logs, trial pit descriptions, and in-situ testing (following the principles of BS5930:2015)
- Laboratory analysis – Triaxial testing alongside classification, compaction, and permeability testing (conducted in accordance to BS1377-1:2016, BS1377-2:2022 and BS EN ISO 17892)
- Geological context – Historical data and desk studies
- Engineering expertise – Interpretation by experienced geotechnical engineers
This approach ensures that triaxial testing results are interpreted within the full context of site conditions, providing you with actionable insights rather than just numbers on a page.
Understanding the science behind triaxial testing helps you appreciate why a well-designed ground investigation programme is so valuable and facilitates the decision making process.
In my next article, I’ll explore how engineers take triaxial testing data and transform it into practical construction decisions, using real project examples to demonstrate how trends are identified and outliers are investigated.
Need geotechnical testing for your project?
Our team of experienced engineers and laboratory technicians are ready to help. With over 85 years of expertise and state-of-the-art UKAS-accredited facilities, Lucion Ground Engineering provide the reliable data and expert guidance you need for project success.
Contact our team to discuss your next project.
