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Particle Size Distribution (PSD)

Hydrometer, PSD, Particle Size, Grain Size

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The Particle Size Distribution (PSD) curve is widely used by geotechnical engineers to classify soils and assess their grading characteristics. Try out our tool in the link below:

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What is a Particle Size Distribution (PSD)?

Particle Size Distribution (PSD) testing measures the amount of different sized particles in an aggregate sample. It is typically defined in terms of discrete size ranges, usually using sieves, whereby each sieve “retains” particles above a certain size and “passes” particles below that size.

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.

Results are presented on a PSD curve, with particle size (log scale) on the x-axis and the cumulative percentage finer (by weight) on the y-axis.

Methods for Measuring PSD

PSD measurement methods depend on the expected grain size of the sample. Sieve analysis (wet or dry) is used for particles larger than about 0.075 mm, separating coarse-grained soils such as sands and gravels using a stack of sieves. For finer particles, hydrometer analysis is used, determining particle size based on the rate of settling in suspension.

1. Sieve Analysis (Wet and Dry)

  • Used for particles larger than 0.075 mm (or 0.063 mm)
  • Involves stacking sieves of decreasing size to separate particles
  • Commonly used for:
  • Coarse-grained soils
  • Sands
  • Gravels

2. Hydrometer Analysis

  • Used for particles smaller than 0.075 mm
  • Measures the rate at which particles settle in a fluid
  • Suitable for:
  • Silts
  • Clays

What Sieve Sizes Are Used?

The sieve sizes used in PSD testing depend on the soil type and the applicable standard in a given country. Sieve sizes are selected to adequately represent the particle size range of the sample, although it is not necessary to include every standard size in each test.

For coarse-grained soils, sieve analysis is performed using a stack of standardised sieve sizes, typically ranging from larger gravel sizes down to about 0.075 mm. For fine-grained soils, particles smaller than this threshold are not measured by sieving but instead assessed using sedimentation methods such as hydrometer analysis.

Key Components of a PSD Graph

A PSD curve is presented on a semi-logarithmic graph, with particle size (log scale) on the x-axis and cumulative percentage finer (passing) on the y-axis. The curve itself represents the distribution of particle sizes within the sample, showing how much material falls below each size.

Key features commonly interpreted from the graph include characteristic particle sizes (e.g. D10, D30, D60), which are used to describe grading and calculate parameters such as the coefficient of uniformity and coefficient of curvature. The overall shape of the curve also indicates whether the soil is well graded (broad range of sizes) or poorly graded (uniform or gap-graded).

How is a PSD Graph Used in Soil Classification?

A PSD graph is used to classify soils based on their particle size distribution and grading characteristics. The shape of the curve indicates how particles are distributed across different sizes:

  • Well-graded soil
  • A smooth, continuous curve representing a wide range of particle sizes, typically resulting in good interlocking and strength
  • Wide range of particle sizes

  • Good interlocking → higher strength

  • Uniformly graded (poorly graded) soil

  • A steep curve indicating most particles are of similar size
  • Particles mostly the same size

  • Poor interlocking

  • Gap-graded soil

  • A curve with flat or missing sections, indicating absent intermediate particle sizes
  • Missing intermediate particle sizes
  • Combination of distinct size groups

In addition to curve shape, soils are classified into size ranges based on particle diameter. While the exact boundaries may vary slightly between standards such as ASTM (US), BS/EN (UK), AS (Australia), and NZS (New Zealand), the following ranges are commonly adopted:

  • Boulders: > 200 mm
  • Cobbles: 63 mm to 200 mm
  • Gravel: 2 mm to 63 mm
  • Sand: 0.063 mm to 2 mm
  • Silt: 0.002 mm to 0.063 mm
  • Clay: < 0.002 mm

These classifications, combined with parameters derived from the PSD curve (such as D10, D30, D60, Cu, and Cc), form the basis of soil classification systems like the Unified Soil Classification System (USCS) and related national standards.

How are PSD used in geotechnical engineering?

PSDs are widely used to specify and control material properties in geotechnical works. In road design, for example, engineers define acceptable grading ranges for each pavement layer to achieve the desired strength, permeability, and compaction characteristics.

These specifications are typically presented as upper and lower bounds on a PSD chart, forming a grading envelope. A material is considered compliant if its PSD curve falls entirely within this envelope, indicating it meets the required particle size distribution for construction use.

References

  1. United Kingdom (UK): BS 1377 – Methods of test for soils for civil engineering purposes
  2. New Zealand (NZ): NZS 4402 – Methods of testing soils for civil engineering purposes
  3. Australia (AU): AS 1289 – Australia (AU): AS 1289 – Methods of testing soils for engineering purposesd
  4. United States (US): ASTM D6913/D6913M – Standard Test Methods for Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis; and ASTM D7928 – Hydrometer (sedimentation) analysis
  5. Das, B. M. (2010). Principles of Geotechnical Engineering (7th ed.). Cengage Learning, Stamford, CT.