The Atterberg limits are a set of criteria that can be used to classify the behavior of soils. The limits were first established in the early 1900s by Swedish engineer Karl Atterberg. They are determined by measuring the soil’s water content and temperature and can be used to predict how soil will behave when it is subject to stress such as pressure or shearing forces.
There are three main types of Atterberg limits.
- Shrinkage Limit
- Plastic Limit
- Liquid Limit
The Atterberg limit can also be identified as the measure of the critical water content of fine-grained soils.
Further, the limits are specified based on the change in the soil states.
Soil changing from the liquid state to the plastic state is known as the Liquid Limit (LL). The change in the plastic state to the semi-solid state is named Plastic Limit (PL). The state of soil change from semi-solid to solid state is identified as the Shrinkage Limit.
The liquid limit can be defined as the water content at which fined-grained soil changes its state from plastic to liquid.
In otherworld, Liquid Limit can be defined as,
The moisture content at which fine-grained soil no longer flows like a liquid.
The shear strength of the soil also becomes zero at the liquid limit.
Casagrande’s method or cone penetrometer test can be used to determine the liquid limit of the soil.
The amount of water content at which the fine-grained soil changes its plastic state to a semi-solid state is defined as a plastic limit.
In other words, Plastic Limit can be defined as,
The moisture content at which fine-grained soil can no longer be remolded without cracking.
The amount of water content at which the fine-grained soil changes its semi-solid state to a solid state.
In other words, the Shrinkage Limit can be defined as,
The moisture content at which fine-grained soil no longer changes volume upon drying. Any loss of moisture is compensated by the entry of air into the pores.
The plastic index (PI) is a measure of the plasticity of the soil.
PI = LL – PL
This ratio can be used in the classification of soil. The soil with a high plastic index can be considered clay soil and soil with a low plastic index can be considered silt.
Liquidity Index (LI) = [ Natural Water Content – PL ] / [ LL – PL ]
Liquidity Index (LI) = [ Natural Water Content – PL ] / PI
According to Wikipedia, the liquidity index is used for scaling the natural water content of a soil sample to the limits.
Consistency Index (CI) = [ LL – Natural Water Content ] / [ LL – PI ]
Consistency Index (CI) = [ LL – Natural Water Content ] / PI
The Consistency Index indicates the consistency of the soil. It also can be understood as a measure of the firmness of soil.