The cone penetrometer sits on the lab bench at a precise 30-degree angle, its 80-gram mass ready to drop into a brass cup of remoulded soil. That’s the Casagrande apparatus—the standard kit for determining liquid limit under BS 5930. In Swindon, where the Jurassic and Cretaceous clay formations shift from the Kimmeridge deposits north of the M4 to the Gault clays around Old Town, this simple mechanical test reveals more about site risk than a dozen borehole logs without classification. We run the full Atterberg suite—liquid limit, plastic limit, and the derived plasticity index—on every disturbed sample we collect across the borough. The numbers tell us how your soil will behave when it gets wet, how much it’ll shrink in summer, and whether your earthworks spec needs a lime stabilisation clause before the first excavator bucket hits the ground.
The plasticity index is the single best predictor of swell-shrink behaviour in Swindon’s Gault and Kimmeridge clays.
