BS 5930:2015 and BS EN ISO 17892-12 define the procedures we follow for Atterberg limits testing in Bournemouth, but the real value is interpreting what the numbers mean for a site on the local geology. The town sits on a mix of Eocene sands and clays, with the Branksome Sand and Poole Formation dominating much of the coastal plateau. When a borehole hits a clay lens within the London Clay Formation or weathered material from the Bagshot Beds, the plasticity index becomes critical. It tells us whether the material will swell, shrink, or lose strength when wet. Bournemouth’s average annual rainfall of around 800 mm, concentrated in winter months, means seasonal moisture variation is a real factor for shallow foundations. We run the liquid limit, plastic limit, and plasticity index on every cohesive sample before recommending bearing pressure or ground improvement. This data feeds directly into the shallow foundation design parameters used for residential blocks in areas like Westbourne or Charminster, where clay content varies significantly over short distances.
A plasticity index above 25% in Poole Formation clay is not just a lab number — it means the ground will move with every wet winter and dry summer.
Service characteristics in Bournemouth
Typical technical challenges in Bournemouth
We reviewed a site on the East Cliff last year where a developer had assumed stiff clay based on SPT N-values alone. The borehole log showed N=18 at 2.5 m depth, but the Atterberg results told a different story: liquid limit 72%, plastic limit 28%, giving a plasticity index of 44%. That clay was highly plastic, not stiff in the geotechnical sense. The liquidity index calculation placed it close to the plastic state, meaning any increase in water content from a leaking drain or heavy rainfall would push it towards a liquid consistency. For a three-storey apartment block with a basement, that meant switching from a spread footing solution to a piled foundation taken into the Branksome Sand at 6 m. Without the plasticity data, the original design would have carried settlement risk that no bearing capacity calculation would catch. In Bournemouth’s cliff areas, overlooking the seafront, this scenario plays out regularly. The combination of perched water tables and desiccated crusts masks the true behaviour of the underlying clay.
Our services
Every Atterberg limits test we run in Bournemouth connects directly to a design decision. Below are the core services we provide, from standard classification to project-specific interpretation.
Liquid Limit by Cone Penetrometer
BS EN ISO 17892-12 compliant testing using the 80 g, 30° cone method. We prepare the soil paste at varying water contents, measure penetration, and determine the moisture content at 20 mm penetration. This method removes the operator variability of the Casagrande cup and works reliably on the sandy clays found in Bournemouth’s Bagshot Beds and head deposits.
Plastic Limit & Plasticity Index
Thread-rolling procedure to 3 mm diameter until crumbling occurs. We run duplicates to control operator scatter. The PI is reported alongside a consistency classification per BS 5930 Table 6, giving the engineer an immediate sense of the material’s behaviour class: low, intermediate, high, or very high plasticity.
Shrinkage Limit & Volumetric Stability
For embankment fill and engineered earthworks in Bournemouth, shrinkage limit determination helps define the compaction moisture range that avoids cracking during summer desiccation. We test the soil at its liquid limit, oven-dry it, and measure the volume change using mercury displacement or wax coating.
Liquidity Index & In-Situ State Assessment
Combining Atterberg limits with natural moisture content from the field sample gives the liquidity index. A value near 1.0 means the clay is close to its liquid limit in the ground. This single parameter often triggers a design change from shallow to deep foundations, particularly in Bournemouth’s coastal valleys where groundwater is high.
Frequently asked questions
How much does Atterberg limits testing cost in Bournemouth?
A standard set of Atterberg limits tests, covering liquid limit, plastic limit, and plasticity index on one sample, ranges from £50 to £90 depending on whether you need the shrinkage limit as well and how quickly you require results. Expedited 24-hour turnaround carries a small premium. We quote per sample, not per test, so you get the complete classification without hidden add-ons.
How long does it take to get Atterberg limits results?
Standard turnaround is 3 to 5 working days from sample receipt. The liquid limit and plastic limit tests require careful moisture conditioning, and the oven-drying step alone takes 24 hours. We offer a 48-hour expedited service for an additional fee, and same-week reporting is standard for most Bournemouth projects. Larger batches of 10 samples or more may add a day.
What sample quality do I need for Atterberg limits testing?
You need a disturbed sample of at least 200 grams passing the 425 µm sieve, taken from a representative depth. The sample should be sealed in a plastic bag immediately after extraction to preserve natural moisture content. For Bournemouth sites with sandy clays, we recommend taking a larger 500 g sample so we have enough fine fraction after sieving. Jar or bag samples from trial pits work well. Core samples from boreholes are also suitable provided they have not dried out during transport.