In this article, Matt Hartnup, Geotechnical Laboratory Manager at Lucion Ground Engineering, explores one of geotechnical engineering's most fundamental tests. With 18 years of experience in laboratory testing and soil classification, Matt explains how the Atterberg Limits test has remained essential to understanding soil behaviour for over a century, and why the skill and experience of laboratory technicians is crucial to delivering accurate results.
One of the most common classification tests carried out in geotechnical laboratories is the Atterberg Limits test. Comprised of the liquid limit, the plastic limit, and the shrinkage limit, this test provides crucial information about how clay soils will behave under different moisture conditions. Knowledge that’s fundamental to almost every geotechnical project.
A Century of Soil Science
The test was first devised by Dr Albert Atterberg, a Swedish chemist and soil scientist, in 1911 as a way of easily classifying agricultural soils based on their plasticity. Atterberg’s contribution to soil science extended beyond this test. He is also believed to have originally defined much of the particle size system we still use today to classify soils as clay, silt, or sand.
These testing procedures were further developed by Professor Arthur Casagrande in 1932, when seeking a way to quickly determine soil characteristics for the emerging modern engineering industry of the early 20th century. He built a simple mechanical device which is still in use today (in fact, we still have one in the lab which we keep for sentimental reasons!), although it has been largely superseded by the more accurate cone penetrometer method in recent years.
The Casagrande apparatus allowed the civil engineers of the US Bureau of Public Roads to rapidly determine soil characteristics during the major infrastructure push in the USA during the 1920s and 1930s. This period in history saw the development of many other geotechnical theories and tests, aside from classification testing, which remain in use today.
What Do Atterberg Tests Measure?
The Atterberg Limits test involves determining the precise percentage of moisture needed within a clay soil to reach the liquid limit and the plastic limit. These are the two critical points in a moisture profile where a clay transitions between different states:
- Plastic Limit – The moisture content at which the soil transitions from a plastic state to a drier, semi-solid state
- Liquid Limit – The moisture content at which the soil transitions from a plastic state to a liquid state
Once we have determined these two precise percentage limits, we can measure the moisture content range between them. This is called the Plasticity Index, which quantifies the plasticity of a clay as a percentage.
Classifying Soils With The Plasticity Chart
Our technicians use a standardised chart which plots the soil’s Plasticity Index on the vertical axis (example below) against the Liquid Limit on the horizontal axis to give a definitive classification. This chart is bisected diagonally by the ‘A-Line’ – above this line, a soil plots as an inorganic clay, whilst below it lie organic clays and silts. The chart can further classify soils as having low to extremely high plasticity and low to very high shrinkage potential.
This classification system provides engineers with crucial information about how a soil will behave in different conditions, for example:
- Will it expand when wet?
- How much will it shrink when it dries?
- How stable will foundations be?
These are questions that the Atterberg Limits help answer.
The Skill Behind the Science
Whilst the test itself is relatively straightforward in terms of procedure, it requires an experienced technician (like our brilliant team in our Peterborough-based Geotechnical Lab) to carry it out accurately, as there is considerable skill involved.
To determine the Plastic Limit our team takes a specimen slightly wetter than the plastic limit and use a combination of rolling techniques on a glass plate and in the palm of their hand to finally arrive at the precise point of the plastic limit. The soil is rolled into threads approximately 3mm in diameter, and the plastic limit is reached when these threads begin to crumble. It can take many years to become truly adept at this technique, knowing exactly the right pressure to apply and recognising the precise moment when the limit is reached.
The Liquid Limit is determined using a calibrated cone penetrometer to measure the depth of penetration of a weighted metal cone point into a specimen in a state slightly drier than the liquid limit. Three further specimens are prepared and tested at incrementally higher moisture contents, forming a linear relationship that definitively highlights the liquid limit value. Again, it can take years for a technician to become fully proficient at this, understanding how to prepare specimens consistently and interpret results accurately.
Once these points are determined, the individual specimens for both sets of tests are dried and weighed before results are reported to the client. Additional information, such as a full soil description and the ‘as received’ moisture content, is often also reported, as this can sometimes be an indicator of other factors at play, such as saturation or desiccation. All of which are essential to understand how soils will act under different situations, and how this may affect the foundational integrity of developments.
Trusted Geotechnical Testing Experience
At Lucion Ground Engineering, we are constantly validating our results and practices against other technicians, equipment, and external laboratories (such as PICTS) to maintain total confidence in our testing.
Our technicians have decades of experience between them and can often determine a soil classification just by looking at and handling the sample. This kind of experience and insight is invaluable, and it is something we are very proud of.
You can be certain that when you request an Atterberg Limits test from us, your testing will be carried out in our UKAS-accredited laboratory by highly experienced and dedicated technicians, giving you complete confidence in your project results.
Contact our team to start your foundations, right. Email us at [email protected].
