The Atterberg Limits test is actually a two-part test that determines the plastic limit and the liquid limit of soils. The plastic limit of the soil is the minimum moisture content at which a soil will exhibit plastic behavior, specifically rolling into a thread of 1/8 inch diameter. The liquid limit of the soil is the moisture content at which a soil will exhibit liquid behavior. This is determined by smoothly spreading the soil into a calibrated brass cup (Casagrande Cup), cutting a groove down the middle and then tapping the cup from a set height at a set speed until the groove closes. The plasticity index, which is a soil index property used to estimate expansive potential, soil modulus, cohesion and even bearing capacity, is calculated by subtracting the plastic limit moisture content from the liquid limit moisture content.
