Atterberg Limits

Atterberg limits, also called Atterberg consistency limits or Atterberg soil limits, are a set of three specific moisture content thresholds that define the physical properties of fine grained soils such as clay and silt.  These limits are used in geotechnical engineering and soil science to classify and evaluate the behavior of soils, especially with regard to their plasticity and liquidity characteristics.

Atterberg limits

Liquid Limit (LL)  -  The liquid limit is the moisture content at which a soil transitions from a plastic state to a liquid state.  It is determined by a standardized laboratory test called the Casagrande method or cone penetration test.  At the liquid limit, the soil behaves like a viscous liquid, and its shear strength is very low.
Plastic Limit (PL)  -  The plastic limit is the moisture content at which a soil transitions from a plastic state to a semi-solid state. It represents the lower limit of the plasticity range.  The plastic limit is determined by rolling a soil sample into a thread of a specific diameter.  Below this moisture content, the soil is too dry to exhibit plastic behavior.
Shrinkage Limit (SL)  -  The shrinkage limit is the moisture content at which further drying of the soil will not cause any further volume reduction.  Below this limit, the soil will start to crack and shrink as it dries.  The shrinkage limit is often close to the plastic limit but is slightly higher in most cases.

These Atterberg limits are essential for characterizing the engineering properties of soils.  They help in soil classification, determining the suitability of soils for construction purposes, and predicting how soils will behave under different moisture conditions.  For example, a soil with a high plasticity index (the difference between the liquid limit and plastic limit) is more prone to volume changes with variations in moisture content, making it challenging to work with in construction projects without proper treatment or stabilization.