Grain Size Analysis (Sieve + Hydrometer) in Gloucester

Gloucester’s development has always been shaped by its geology. The Romans settled here partly because the Severn Vale offered deep alluvial soils that were easy to work, but modern engineering on those same deposits demands precise characterisation. The city docks, excavated from Lias Clay and river gravels in the 1820s, demonstrated early on that grain size controls everything from dewatering rates to bearing capacity. Today, when a contractor encounters silty sand at Quedgeley or laminated clay near the Cathedral, the first step is a complete particle size distribution. Our laboratory runs combined sieve and hydrometer analysis to BS 5930, delivering the full grading curve needed for earthworks specification. For deeper investigation where disturbance is minimal, we often pair this with sondaje SPT in boreholes across the Cotswold fringe to correlate in-situ density with lab-derived gradation. The hydrometer fraction is critical here—Gloucester’s alluvium often carries 15–30% fines that control permeability and frost susceptibility.

A grading curve without hydrometer data in Gloucester’s alluvium misses the fines that govern consolidation time and drainage design.

Our service areas

How we work

The contrast between Gloucester’s eastern and western suburbs is stark. East of the M5, around Barnwood and Hucclecote, the soils sit on the Charmouth Mudstone Formation—weathered clays that break down into a silty matrix with occasional limestone bands. Sieve results here typically show 60–80% passing the 425 µm sieve, and the hydrometer reveals a clay fraction between 18 and 25%. West of the Severn, toward Highnam and the Forest of Dean, the drift geology shifts to glacial sands and gravels over Mercia Mudstone. These materials can have less than 5% fines, demanding a different approach to compaction control. A test pit campaign in these areas lets us extract bulk samples representative of the full gradation range, avoiding the bias that a small-diameter borehole can introduce in gravelly ground. The

Full wet sieve stack from 75 mm down to 63 µm
Sedimentation hydrometer per BS 1377-2
Coefficient of uniformity (Cu) and curvature (Cc) calculated from the grading curve

are standard deliverables, but the engineering value is in interpreting those numbers against local precedent.

Grain Size Analysis (Sieve + Hydrometer) in Gloucester — Technical reference — Gloucester

Local geotechnical context

On the reclaimed ground around Gloucester Quays, we often see imported fill that was placed as ‘engineered granular material’ but lacks a single particle size distribution report. The problem surfaces years later when differential settlement opens cracks in block paving or service trenches collapse after heavy rain. Missing the hydrometer step is another common shortcut—assuming everything passing the 63 µm sieve is clay when it’s actually rock flour or silt changes the entire drainage design. BS 5930 and Eurocode 7 require classification based on full grading, not just the sand and gravel fractions. We’ve also encountered site-won Lias Clay that plots near the A-line on a plasticity chart; without the hydrometer curve, the material can be misclassified as low-plasticity silt, leading to inadequate compaction specifications and later pavement failure. The cost of a proper grain size analysis is negligible compared to re-excavating a failed formation.

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Applicable standards

BS 5930:2015+A1:2020, BS 1377-2:1990, Eurocode 7 (BS EN 1997-2:2007)

Typical values

Parameter	Typical value
Sieve range (coarse)	75 mm – 2 mm
Sieve range (fine)	2 mm – 63 µm
Hydrometer range	63 µm – 0.6 µm
Standard	BS 5930 + BS 1377-2
Sample mass (sandy)	500 g typical
Sample mass (clayey)	250 g typical
Dispersant	Sodium hexametaphosphate
Reported coefficients	D10, D30, D60, Cu, Cc

Frequently asked questions

What does a grain size analysis cost in Gloucester?

A combined sieve and hydrometer test typically runs between £80 and £170 per sample, depending on whether you need the full hydrometer sedimentation series or just the fines percentage. Samples with high gravel content that require larger split masses fall at the upper end of the range. We quote per batch, so if you have 5 samples from a trial pit campaign, the unit cost drops slightly.

How long does a hydrometer test take compared to a simple sieve?

A dry sieve can be completed in 24 hours. The hydrometer sedimentation requires a minimum 24-hour settling period plus preparation and dispersant mixing. We typically report combined grading curves within 3 working days of sample receipt. Urgent projects on tight earthworks programmes can get priority scheduling—call to confirm current lab queue.

Do I need the hydrometer if the material looks sandy?

Yes, in Gloucester’s alluvial deposits that visual classification often misses 10–20% fines. If the fines content exceeds 12%, BS 5930 requires classification based on plasticity as well, and the hydrometer tells you exactly how much clay-sized material you are dealing with. Skipping it on a ‘sandy’ sample that turns out to be silty sand can lead to drainage layer failure.

Location and service area

We serve projects in Gloucester and surrounding areas.

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