What is Particle Size Distribution in Soils?

Particle size distribution can significantly impact the way that soils react when used in applications. Soils are made up of mineral particles, and the size of soil particles and their arrangement has a major influence on the properties of the soil. Particles in a single soil sample can range in soil particle size from very coarse (>100mm), down to very fine (<2micron).

Soils with large size particles are stronger due to the higher inter-particle friction, while finer soils are more sensitive to water content (see our guide on the permeability of soils for more information). Understanding the soil particle size distribution helps predict soil strength, bearing capacity and shear strength, aiding geotechnical engineers in making informed decisions about soil use.

In this blog, read on to learn more about:

• What is particle size distribution?
• What is the particle size distribution curve?
• The importance of particle size distribution
• How do we measure particle size distribution?
• Particle size soil classification 
• Particle size distribution for road construction
• How Tensar can help

What is particle size distribution?

With particle size distribution, we can measure the proportion of soils in each size range. Soil samples can be separated into different fractions or intervals, where each fraction represents a range of particle size.

The results of a particle size distribution test are usually plotted on a chart, where the vertical axis shows the cumulative percentage (by weight) finer than a specific size, and the horizontal axis shows the soil particle size distribution plotted on a log scale.

What is the particle size distribution curve?

Analysing particle size distribution results in a curved plot, which is referred to as the particle size distribution curve, or sometimes the grading curve. This is used to show the full range of particle sizes in the fraction, from the smallest soil particle size to the largest.

A soil comprising only one uniform size of soil particles would have a grading curve that appears as a vertical line located at the appropriate particle size. Most soils consist of a range of different particle sizes, resulting in particle size distribution curve that slopes upwards from left to right indicating the particle size distribution.

In this episode of 'Ask Andrew", Andrew Lees is back in his kitchen telling us all about particle size distribution in soils and why it's important. 

The importance of soil particle size distribution

When a soil comprises a range of particle sizes, the smaller particles partially fill the voids between the larger particles. This can be compacted into a dense material with close particle-to-particle contact and high interparticle friction, creating a soil with high strength and good engineering properties. For example, lateritic soil often has a varied particle size distribution, which can significantly influence its compaction and overall performance. Well graded soil material is one that has a wide particle size distribution with no significant size gaps. The particle size distribution curve will rise smoothly upwards from left to right in a flattened ‘S’ shape. Well graded material will compact to a dense state.

How do we measure particle size distribution?

To measure soil particle size distribution, the sample is separated into particle size fractions by passing the material through a series of sieves of different opening size. The sample is first washed over a 75 micron sieve to remove the fine particles. The remaining material is then dried and the process is repeated as it is passed through the different size sieves.

The fraction retained on each sieve is weighed to establish the proportion in each particle size range. The finer material in the wash can be subjected to a sedimentation analysis to establish the particle size proportions of silt and clay soils.

Soil classification by particle size

Soils can be classified by reference to particle size, and in the UK, BS5970 defines the particle size range for a series of principal soil types:

• Clay – less than 0.002mm
• Silt – 0.002mm to 0.063mm
• Sand – 0.063mm to 2mm
• Gravel –2mm to 63mm
• Cobbles - 63mm to 200mm
• Boulders – greater than 200mm

Particle size distribution for road construction

Road designers will specify the required grading of material for each of the pavement layers. The specification will typically give a lower and upper bound limit for each particle size. This information is plotted as two grading curves on a single particle size distribution graph. Any measured particle size distribution curve that falls within this envelope will meet the specification, and can be used within the road construction.

How Tensar can help

For optimal performance in stabilising applications, a well graded aggregate material is usually specified for use with stabilisation geogrids.. It is important that the aperture size of the geogrid is compatible with the maximum particle size of the aggregate type used. Tensar+ design software can help you determine the appropriate geogrid aperture size based on your project’s specifications. Tensar offers a range of geogrids, including Tensar TriAx and InterAx, each with different aperture sizes to meet your needs and maximise stability.

For expert advice on selecting the right geogrid for your project, feel free to contact us directly, email TensarInfo-UK@cmc.com or visit our support page.

Next steps

This guide has explained what is meant by particle size distribution, the importance of it in geotechnical engineering and how to create a particle size distribution curve. If you found this article useful, you may want to read:

• Bearing capacity of soil - types and calculations

• What is the shear strength of soil?

• The permeability of soils explained

Tensar manufactures a range of geotechnical engineering solutions – visit our geogrids and wall and slope systems pages for more information.