atterberg limits | consistency limits of soil

Introduction to atterberg limits

  • A Swedish scientist in (1911) Mr. Albert Atterberg originally defined different limits of consistency to classify fine-grained soils(sieve number #40 passing material, cohesive soil).
  • In engineering usage, the term Atterberg limits usually refer to the liquid limit, plastic limit and shrinkage limit.
  • atterberg limits are also called consistency limits of soil, atterberg limits name is due to the scientist’s name Mr. Albert atterberg who performed that test and consistency limit of soil name is because of practically name.
  • Moreover, this test is theoretically shown by the atterberg and practically shown by the Casagrande.

Definition of atterberg limits

A cohesive soil changes its behavior from one state to another state with the addition of water, or removal of water is called atterberg limits.

state of soil or consistency of soil

  • The Atterberg limits are based on the water content of the soil.
  • As a dry, clayey soil takes on increasing amounts of water, it undergoes distinct changes in behavior and consistency.
  • Depending on the water content of the soil, it may appear in four states i.e solid-state, semi-solid state, plastic state, and liquid state.
  • In each state, the consistency and behavior of soil is different. Thus, the boundary between each state can be defined based on a change in the soil’s behavior.
  • The moisture content at the boundary of these states are known as atterberg limits. the boundary Between the solid and semisolid states is called shrinkage limit, and the boundary between semi-solid state and plastic states is known as plastic limit, and the boundary between plastic states and liquid states is called liquid limit.
  • The Atterberg limits can be used to distinguish between silt and clay, and it can distinguish between different types of silts and clays.
  • These distinctions in the soil are used in assessing the soil’s behavior that is to be used as foundation soil.
  • Atterberg limit tests are mainly used on clayey or silty soils since these are the soils that expand and shrink due to moisture content.
  • Clays and silts influenced by water and thus change the value and have varying shear strengths.
  • Atterberg limit tests are used widely in the preliminary stage of designing any structure to ensure that the soil has enough shear strength to resist against failure.

atterberg limits

Shrinkage limit

The shrinkage limit is the moisture content that defines where the soil volume will not reduce further if the moisture content is reduced.

Plastic Limit

The plastic limit is the moisture content that defines where the soil changes from a semi-solid to a plastic (flexible) state.


Liquid limit

The liquid limit is the moisture content that defines where the soil changes from a plastic to a viscous fluid state.

Plasticity Index

The plasticity index (PI) is a measure of the plasticity of the soil. The plasticity index is the size of the range of water contents where the soil exhibits plastic properties. The PI is the numerical difference between the liquid limit and the plastic limit (PI = LL-PL). Soils with a high PI tend to be clayey (more cohesive). Similarly, soils with a lower PI value tend to be silt, and those with a PI of 0 (non-plastic) tend to have little or no silt or clay.

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