Grain size analysis for precise soil classification in Cheltenham

In Cheltenham, many site investigations hit a snag when the first trial pit reveals a mix of Lias clay and sandy brash from the Cotswold limestone. You see a soil that is neither fully cohesive nor granular. The grain size analysis (sieve and hydrometer) cuts through that ambiguity. A full particle distribution curve reveals the dominant fraction, the fines content and the D10/D30/D60 diameters needed for grading descriptors. With BS 5930:2015 as the backbone, the laboratory runs wet sieving on the coarse fraction and a sedimentation hydrometer test on material passing the 63μm sieve. The output is a single curve that defines the soil classification and feeds directly into permeability estimates and filter design. For sites near the River Chelt, where alluvial silts complicate the stratigraphy, this test becomes essential before any foundation or drainage decision.

A single grain size curve replaces guesswork with numbers: well graded, poorly graded or gap graded — each term carries precise engineering consequences.

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Process and scope

Cheltenham's expansion from a spa town into a residential and commercial hub meant building on the Charmouth Mudstone Formation and overlying pleistocene gravels. These transitions between stiff clay and loose sandy layers create grading ambiguities that a simple visual inspection cannot resolve. The grain size analysis quantifies the percentage of gravel, sand, silt and clay, and the shape of the grading curve indicates whether the material is well graded, poorly graded or gap graded. A gap-graded gravel, for instance, behaves very differently under compaction than a uniform sand. When the fines content exceeds 15%, the hydrometer stage becomes critical, because it separates the silt from the true clay fraction — a distinction that controls plasticity and shrink-swell potential. On schemes near Leckhampton Hill, where colluvium mixes angular limestone fragments with residual clay, the combined sieve and hydrometer analysis avoids misclassification that could lead to inadequate earthwork specifications. Complementing this with an Atterberg limits test refines the plasticity chart position for cohesive fines.

Grain size analysis for precise soil classification in Cheltenham — Technical reference — Cheltenham

Site-specific factors

The laboratory setup for a full grain size analysis in Cheltenham starts with a stack of BS 410 test sieves, from 75 mm down to 63 microns, mounted on a mechanical shaker. The coarser fractions are washed, dried and weighed, while the minus-63-micron material goes into a measuring cylinder with a hydrometer. The technician records density readings at timed intervals, converting them to particle diameters via Stokes' law. The main risk in clay-rich soils from the Charmouth Mudstone Formation is incomplete dispersion: if flocculation occurs, the hydrometer overestimates the silt fraction and underestimates the clay. That leads to an incorrect fines classification and potentially an unsafe assumption about drainage behaviour. Another pitfall is nesting sieves with damaged mesh; a single torn 425-micron sieve shifts the whole curve and masks a gap-graded condition. The lab runs duplicate hydrometer tests and checks the sieve stack integrity before each run, ensuring the curve that lands on the engineer's desk reflects the ground truth.

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Applicable standards

BS 5930:2015 — Code of practice for ground investigations, BS EN 1997-2:2007 — Eurocode 7: Ground investigation and testing, BS 1377-2:2022 — Methods of test for soils: Classification tests

Typical values

Parameter	Typical value
Minimum sample mass (fine soil)	150 g
Minimum sample mass (coarse soil)	3 kg
Sieve range	75 mm to 63 μm
Hydrometer range	63 μm to 0.6 μm
Hydrometer method	BS 1377-2 sedimentation
Dispersing agent	Sodium hexametaphosphate
Reporting output	Grading curve + coefficients Cu, Cc

Frequently asked questions

How much does a grain size analysis cost in Cheltenham?

A combined sieve and hydrometer test typically ranges from £80 to £170 per sample, depending on whether the full hydrometer sedimentation with duplicate readings is required or just a basic sieve analysis. Bulk rates apply for multiple samples from the same site.

How long does the hydrometer test take?

The sedimentation phase takes a minimum of 24 hours, because the finest clay particles settle slowly. The full reporting, including sieve analysis of the coarse fraction and combined curve generation, is usually ready within three working days after sample receipt.

What sample mass is needed for the test?

For soils dominated by sand and gravel, a minimum of 3 kg is required. For predominantly fine soils (silts and clays), around 150 g of material passing the 2 mm sieve is sufficient for the hydrometer portion, though a larger bulk sample is preferred to ensure representativeness.

Can you test samples containing both gravel and clay?

Yes. The sample is first wet-sieved to separate the coarse fraction retained on the 63-micron sieve. The material passing is then used for the hydrometer sedimentation test. The resulting combined curve represents the full particle size distribution from gravel down to clay.

What is the difference between the sieve and the hydrometer parts?

The sieve analysis covers particles larger than 63 microns (gravel and sand) using a stack of BS 410 sieves. The hydrometer analysis measures the sedimentation rate of particles smaller than 63 microns (silt and clay) suspended in water, using Stokes’ law to convert settling time to particle diameter.

Location and service area

We serve projects in Cheltenham and surrounding areas. More info.

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