Particle Size Distribution (PSD) testing is a fundamental component of geotechnical and earthworks assessment, but its value is often underestimated when considered in isolation.

In practice, PSD testing plays a critical role in understanding material behaviour, informing reuse decisions and managing construction risk.

This article outlines how PSD testing is applied in a real project environment, and why it remains a key input into sound engineering judgement.

Particle Size Distribution Testing Project Context

A recent infrastructure project involved the construction of access roads, drainage features and engineered earthworks across a site underlain by variable Made Ground and cohesive natural soils.  Early investigation identified significant lateral and vertical variability in material composition.

A key consideration for the project team was whether excavated materials could be reused on site, rather than disposed of and replaced with imported aggregate.  Reuse would offer cost, programme and sustainability benefits, but only if material performance could be demonstrated with confidence.

PSD testing within the laboratory programme

PSD testing was undertaken as part of a wider laboratory testing suite, alongside moisture content, Atterberg limits and compaction testing.  All testing was completed within a UKAS-accredited laboratory operated by Igne, in accordance with relevant British Standards.

While classification testing established general soil types, PSD analysis provided essential detail on grading characteristics, including:

• fines content and grading distribution
• variability between excavation zones
• implications for permeability and drainage behaviour
• compliance with earthworks and fill specifications

Interpretation and engineering outcomes

PSD results demonstrated that material suitability varied across the site.  Some Made Ground contained excessive fines and was unsuitable for reuse without treatment, while other zones exhibited well-graded granular material capable of meeting specification requirements when placed and compacted correctly.

Rather than adopting a conservative blanket approach, the PSD data supported a zoned earthworks strategy, allowing selective reuse of suitable materials and targeted disposal where required.

The grading data also informed drainage design decisions, helping to avoid the placement of fine-rich materials adjacent to permeable features, reducing the risk of long-term performance issues.

Benefits of PSD realised

Using PSD testing as a decision-support tool resulted in:

• reduced off-site disposal volumes
• lower demand for imported materials
• improved confidence in earthworks design
• clearer technical justification for reuse decisions
• reduced programme and cost risk

Importantly, decisions were evidence-led and defensible, with UKAS-accredited PSD results forming part of the formal technical record.

Limitations and considerations

Particle Size Distribution testing does not provide answers in isolation.  Results must be interpreted alongside plasticity, moisture condition, compaction behaviour and site observations.  Variability within Made Ground in particular means representative sampling is critical.

Used appropriately, however, PSD testing provides a reliable foundation for understanding material behaviour and supporting proportionate, informed engineering decisions.

At Igne, PSD testing can be integrated with ground investigation, geotechnical assessment and wider construction support - ensuring laboratory data is applied in a way that directly supports project delivery.