Atterberg Limits – Liquid, Plastic & Shrinkage Limit

Importance of Atterberg Limits:

  1. It has been found that both the liquid and plastic limit depends upon the type and amount of clay in a soil. However, the plasticity index depends mainly on the amount of clay. Thus, the plasticity index of a soil is a measure of the amount of clay in the soil.
  2. As the particle size decreases, both the liquid and plastic limits increase, but the liquid limit increases at a greater rate. Therefore, the plasticity index increases at a rapid rate. When some silts are added to clay, it becomes leaner. It’s the liquid limit and plastic limit both decreases, but the former at a faster rate. The net effect is that the plasticity index decreases. Plasticity index, therefore, measures of the fineness of the particles.
  3. The study of the plasticity index, in combination with liquid limit, gives information about the type of clay. Plasticity Chart, which is a plot between the plasticity index and liquid limit, is extremely useful for classification of fine-grained soils. In fact, the main use of these limits is in the classification of soils.
  4. Sand Soils change from the liquid state to the semi-solid state rather abruptly. These soils do not possess plasticity and are classified as non-plastic. Soils with the liquid limit less than 20% are generally classified as sands
  5. The plastic limit of soil increases if organic matter is added, without any significant increase in the liquid limit. Therefore soils with high organic content have a low plasticity index
  6. The liquid limit of a soil is an indicator of the compressibility of a soil. The compressibility of the soil generally increases with an increase in liquid limit
  7. The shrinkage index is directly proportional to the percentage of clay-size fraction present in the soil. It can be used as an indicator of clay.
  8. The toughness index is a measure of the shearing strength of the soil at the plastic limit. A high value of toughness index indicates a high percentage of colloidal clay containing mineral montmorillonite.
  9. When comparing the properties of two soils with equal values of the plasticity index, it is found that as the liquid limit increases, the dry strength and toughness decreases, whereas compressibility and permeability increases.
  10. When comparing the properties of two soils with equal liquid limits, it is found that as the plasticity index increase, the dry strength and toughness increase, whereas the permeability decrease. However, the compressibility remains almost the same.