What is Particle Size Distribution in Soils?

by Andrew Lees, on 26-May-2022 04:58:51

The importance of particle size distribution in soils

Soils are made up of mineral particles, and the size and arrangement of these particles has a major influence on the properties of the soil. Particles in a single soil sample can range in 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. If we know the particle size distribution of a soil, we can make predictions as to its strength and properties.

In this blog, read on to learn more about:

What is particle size distribution?

When we separate a soil sample into different fractions, where each fraction represents a range of particle size, we can measure the proportion of the soil in each size range – or the particle size distribution. The results are usually plotted on a chart, where conventionally the vertical axis is the cumulative percentage (by weight) that is finer than a specific size and the horizontal axis is the particle size plotted on a log scale. The curved plot is referred to as the particle size distribution curve, or sometimes the grading curve. A soil comprising only uniform size particles would have a grading curve that is a vertical line located at the appropriate particle size. Most soils comprise a range of different particle sizes and the grading curve will slope 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 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. A well graded material is one that has a wide particle distribution with no significant size gaps. The grading 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?

The soil 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 75micron 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 size range. The finer material in the wash can be subjected to a sedimentation analysis to establish the proportions of silt and clay.

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 0.63mm
Sand – 0.063mm to 2mm
Gravel –2mm to 63mm
Cobbles - 63mm to 200mm
Boulders – greater than 200mm

Specifying particle size distribution for road construction

With the importance of particle size distribution well understood, 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 can be plotted as two grading curves on a single chart with the envelope between the two curves defining the material required. Any measured grading curve falling entirely within the envelope will meet the specification.

Geogrid aperture size compatibility

A well graded aggregate material is usually specified for use with stabilisation geogrids to maximise performance. It is important that the aperture size of the geogrid is compatible with the maximum particle size of the aggregate used. Tensar offers a range of geogrids with different aperture sizes for this reason.

Advice on geogrid selection can be obtained from info@tensar.co.uk