The Casagrande cup sits on the bench. Brass, calibrated, ready. We crank the handle at two blows per second. The soil groove closes at 12.7 mm. That is the liquid limit. In Celbridge, where Liffey alluvium mixes with glacial till, knowing this number matters. We run these tests in our ISO 17025 accredited lab. Not estimates. Not visual classifications. Real numbers for real designs. A sample arrives from a site near Castletown House. Silty clay. We dry it, sieve it, mix it. Then the brass cup does its work. The plastic limit follows. Rolling threads to 3 mm diameter. The point where the thread crumbles defines the boundary. Two numbers. One index. The plasticity index. That index dictates bearing capacity assumptions, settlement predictions, and excavation stability for every foundation in the area.
Plasticity index is not just a number. It predicts how much a soil will move when the water table rises.
