Atterberg limits are measurements of the critical water content of fine-grained soils.
Depending on the water content, fine-grained soil can be divided into four soil states. Specifically, these are the following.
- Solid
- Semi-festive
- plastic
- Fluid
The changes between these steps represent the Atterberg limits.
- Shrinkage Limit (SL)
- Plastic Limit (PL)
- Liquidity limit (LL)
Atterberg limits can be used to distinguish between different types of silts and clays.
Additionally, they can be used to classify soils. Both soil classification systems, the Unified System and the AASHTO system, use these limits in combination with other parameters to classify soil.
Let us discuss Atterberg limits in detail.
Liquidity limit (LL)
The yield point can be defined as follows.
The percentage of moisture at which soil changes from a liquid state to a plastic state.
The yield point is the moisture content at which fine-grained soil no longer flows like a liquid.
The liquidity limit is determined by the Casagrande liquid device.
The yield point is the moisture content at which 25 drops from the cup are sufficient to close the groove over a distance of 12.7 mm.
Plastic Limit (PL)
The plasticity limit can be defined as follows.
The percentage of moisture at which soil changes from a plastic state to a semisolid state.
The plasticity limit is the moisture content at which fine-grained soil can no longer be reshaped without cracking.
Shrinkage Limit (SL)
The shrinkage limit can be defined as follows.
The percentage of moisture at which soil changes from semisolid to solid.
The shrinkage limit is the moisture content at which fine-grained soils no longer change their volume during drying. Moisture loss is compensated by air penetration into the pores.
Why Atterberg limit tests are important
- The states of soil (solid, semisolid, plastic and liquid) present significant differences in resistance, consistency and behavior.
- Therefore, it is very important to know the limits.
- Based on these data/limit values, a soil classification can be made.
- Threshold values can be used to differentiate between silt and clay.
- These parameters can be used in foundation design and geotechnical recommendations.
Plastic Index (PI)
The plasticity index is defined as the difference between the yield strength and the plastic limit.
PI = LL – PL
The plasticity index is a measure of soil plasticity. Furthermore, it is the water content that gives the soil plastic properties. And PI can also be used for classification as follows.
- Soils with a high plastic index are generally clayey soils.
- Soils with a low plasticity index tend to become very muddy
- Soils with zero plasticity index (PI = 0) generally contain little silt or little clay or no silt or no clay.
| Soil condition | Plastic Index (PI) |
| Plastic-free | 0 |
| Slightly plastic | <7 |
| Medium plastic | 7-17 |
| Highly plastic | > 17 |
Liquidity Index (LI)
The liquidity index can be defined as the relative consistency of a cohesive soil in its natural state.
LI = (ω – PL) / (LL – PL)
Where ω is the moisture content of the soil at the site.
Consistency Index (CI)
The consistency index is an indicator of the consistency or resistance of a soil.
CI = (LL – W) / (LL – PL)
Where W is the water content in the soil.
The following table shows the variations in the soil plasticity index and soil consistency index.
| Constancy Index (CI) | Liquidity Index (LI) | Soil texture |
| 1.00 – 0.75 | 0.00 – 0.25 | Hard |
| 0.75 – 0.50 | 0.25 – 0.50 | Medium soft |
| 0.50 – 0.25 | 0.50 – 0.75 | Soft |
| 0.25 – 0.00 | 0.75 – 1.00 | Very soft |
In addition to the Atterberg limits, there are other related articles such as Soil Stabilization and Soil Types You can look there for more information.
