Liquid Limit of Soil

Liquid Limit is the minimum amount of water required to be added to a soil, expressed as a percentage of the dry weight of the soil, that will just make it to flow like a liquid when jarred slightly. At the Liquid Limit, soils have very small shear strength which may be overcome by the application of a little force, and cohesion is practically zero. The liquid limit serves mainly to distinguish soils with respect to the amount of moisture necessary to make them to slide.

Most clay soils have liquid limit of the order of 50 to 90 per cent. ‘Fat’ clays, which are highly plastic having a high content of colloidal particles, have high LL, which means they ‘flow” only on the addition of large amounts of water. ‘Lean’ clays, which are moderately plastic, having a low content of colloidal particles, have correspondingly low LL. Presence of organic matter in clay increases the LL and comparatively lowers the plastic limit. If sand or silt is added to clay, its LL is lowered. The commonest inorganic silts have LL less than 30 per cent and sands have about 20 per cent. A LL between 20 and 40 indicates a mixture with sand or silt predominating. Peats have a very high LL of several hundred per cent but a small plastic limit.