Liquidity Index

The Atterberg limits describe what a soil could do; the liquidity index says what it is actually doing right now. LI = 0 means the natural moisture sits exactly at the plastic limit — stiff, brittle, over-consolidated. LI = 1 means it sits at the liquid limit and the clay is at the edge of flowing. A sample at w = 32%, PL = 22% and PI = 20 gives LI = 10 ÷ 20 = 0.5: a medium-stiff, normally consolidated clay.

The trap, and the reason this index earns its place, is LI greater than 1. Natural water content above the liquid limit means the clay is held together only by a fragile bonded structure, and once that structure is disturbed it turns to soup. These are the quick clays of Scandinavia and eastern Canada, deposited in salt water and later leached by fresh groundwater; the Rissa landslide in Norway in 1978 was filmed as 5–6 million cubic metres of ground flowed away from a small excavation at the shore, and the Saint-Jean-Vianney flowslide in Quebec in 1971 took 31 lives. If you compute LI above 1, stop and call someone.