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 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, helping geotechnical engineers to make more informed decisions on their uses.

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?
• Soil classification by particle size
• Specifying particle size distribution for road construction
• Geogrid aperture size compatibility

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 analysing particle size distribution 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.

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 uniform size particles would have a grading curve that is a vertical line located at the appropriate particle size. Most soils consist of a range of different particle sizes and the particle size distribution 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 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. Well graded soil material is one that has a wide particle 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 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 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 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.

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

Specifying particle size distribution for road construction

With the importance of soil 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 particle size distribution curve falling entirely within the envelope will meet the specification, and can be used within the road construction. 

Geogrid aperture size 

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 tensarinfo-intl@cmc.com. 
 

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.