In this article we will discuss about the types and uses of consistency limits of soil.
Types of Consistency Limits:
In 1911, Atterberg, a Swedish agricultural engineer, stated that a fine-grained soil can exist in four states, namely, liquid, plastic, semi-solid, or solid state, depending on its water content. The water contents at which the soil changes from one state to the other are known as consistency limits or Atterberg limits.
From geotechnical engineering view point, the following three consistency limits are significant:
1. Liquid limit (LL).
2. Plastic limit (PL).
3. Shrinkage limit (SL).
The water content at which the soil changes from the liquid state to the plastic state is known as liquid limit and the water content at which the soil changes from the plastic state to the semi-solid state is known as plastic limit. Similarly, the water content at which the soil changes from the semi-solid state to the solid state is known as shrinkage limit.
Thus, when the water content of a given soil is more than its liquid limit, the soil will be in liquid state and has negligible shear strength. When the water content is between the liquid limit and the plastic limit, the soil exhibits plasticity, that is, the soil can be molded to any shape without rupture. Plastic limit is the minimum water content at which the soil exhibits plasticity.
The shrinkage limit is the water content below which the soil does not undergo any shrinkage and is in solid state. The three consistency limits are shown in Fig. 5.1.
The liquid limit and plastic limit form the basis for classification of fine-grained soils and for classification of coarse-grained soils with fine fraction. They are also used directly in specifications for controlling compaction used for the construction of embankments and Earth dams. The consistency limits have also been related to various other properties of soils.
1. Liquid Limit:
Liquid limit is the water content at which a soil changes from the liquid state to the plastic state. It is the minimum water content at which the soil is still in the liquid state but possesses small shear strength against flow.
The liquid limit is not the same for all soils. The magnitude of liquid limit for a given soil depends on the type and proportion of clay minerals present in the soil. The liquid limit is high for soils containing montmorillonite clay mineral, medium for soils containing illite, and minimum for soils containing kaolinite clay mineral. In general, the higher the clay content present in a given soil, the higher is the liquid limit.
The shear strength of all soils is found to be the same, equal to about 27 g/cm2 or 0. 027 kgf/cm2, when their water content is equal to liquid limit. The liquid limit is an important index property of fine-grained soils and helps predict its swelling and compressibility behavior under loads. In general, the higher the liquid limit, the higher will be the swelling of the soil on wetting.
The compressibility, which is the decrease in volume under loads and which determines the settlement of structures, also increases with the increase in the liquid limit of the soil. Skempton gave an equation for compression index in the terms of liquid limit.
The liquid limit of soils can be determined by any one of the following two methods:
i. Casagrande’s mechanical method.
ii. Uppal’s cone penetration method.
2. Plastic Limit:
Plastic limit is the water content at which a soil changes from the plastic state to the semi-solid state. It is the minimum water content at which the soil remains in plastic state and can be molded to any shape without rupture.
Experimentally the plastic limit is defined as the water content at which a soil begins to crumble (forms cracks) when rolled into a thread of 3-mm diameter.
The apparatus consists of a flat square glass plate of minimum 45-cm size and 10-mm thickness, a rod of 3-mm diameter, oven, and containers for water content determination.
IS – 2720 (Part 5) – 1985 describes the procedure for the determination of plastic limit.
Following is the procedure for the determination of plastic limit of the soil:
i. About 60 g of air-dried soil passing through 425 pm IS sieve is taken and mixed with sufficient water such that its water content is more than the estimated plastic limit and such that the soil becomes plastic enough to be easily molded with fingers.
ii. About 20 g of the thoroughly mixed soil is taken. A ball is made with about 8 g of this soil and rolled on the glass plate with fingers with just sufficient pressure to roll the mass into a thread of uniform diameter, throughout its length.
iii. The rate of rolling with fingers shall be at the rate of 80-90 strokes per minute, counting a stroke as one complete forward and backward motion of the fingers.
iv. When the diameter of the soil thread reaches 3 mm, the soil thread is worked back to form a ball.
v. The procedure of rolling into a thread of uniform diameter of 3 mm and kneading back into a ball is repeated until cracks appear on the surface of the soil thread, which begins to crumble. When this condition is reached, the water content of the pieces of soil thread is determined.
vi. The test is repeated taking another portion of the soil paste; a total of three trials are made and the corresponding water contents are determined. The average water content out of three trials to the nearest whole number is reported as the plastic limit.
3. Shrinkage Limit:
Shrinkage limit is the water content at which the soil changes from the semi-solid state to the solid state. For fine-grained soils, it was observed that a decrease in the water content causes a corresponding decrease in the volume of the soil, when the soil is in plastic or semi-solid state. At some water content, a further reduction of the water content does not cause any decrease in the volume of the soil, as shown in Fig. 5.10. The decrease in the volume of the soil with the decrease in the water content due to drying/evaporation is called as shrinkage.
Thus, the minimum water content above which the decrease in the water content causes shrinkage of the soil is known as shrinkage limit. It is also the maximum water content below which the volume of the soil remains constant irrespective of the change in the water content.
Uses of Consistency Limits:
Consistency limits are very significant in the study of clays and other fine-grained soils. Important deductions can be made based on the relative values of consistency limits and the index properties as follows:
1. The liquid limit and plasticity index are extremely useful for the classification of soils. The plasticity index and the plasticity chart are used to classify the coarse-grained soils having some fine fraction. The liquid limit and the plasticity chart are used to classify fine-grained soils.
2. The liquid limit is a measure of the compressibility of the soil, that is, the decrease in volume of saturated soils under loads from the structure. Skempton related compression index of soils to the liquid limit.
3. The liquid limit of a clay decreases at a faster rate compared to plastic limit with the increase in silt content in the clay. The plasticity index, therefore, decreases with the increasing silt content in a clayey soil.
4. The liquid limit and plasticity index are an indication of the type and amount of clay present in a soil. The higher the liquid limit and plasticity index, the more severe will be the anticipated problems due to compressibility, swelling, and shrinkage to the foundations and to the structure located in such soils.
5. The compressibility of the soil having higher liquid limit is more, and its shear strength is also less compared to a soil with lower liquid limit, although their plasticity index is the same.
6. Soils with higher plasticity index have a higher dry strength and lower permeability compared to soils with lower plasticity index, although their liquid limit is the same.
7. The plasticity index of the soil increases with the increase in organic content in the soil.
8. Consistency index is a measure of the shear strength of the soils. The higher the consistency index, the higher will be the unconfined compressive strength of the soil.
9. The toughness index is a measure of the shear strength of the soil at its plastic limit and for soils with same plasticity index, the toughness is inversely proportional to flow index.
10. Soils with a high flow index lose their shear strength rapidly with the increase in water content. Such soils cannot sustain heavy loads at high water content.
11. Soils with higher flow index (steeper flow curve) will have lower shear strength if the plasticity index is the same.
12. Earth work can be carried out easily with the least effort when the water content of the soil is at plastic limit.
13. The shrinkage limit is useful to estimate the expected settlements of structures due to drying of soils with change of seasons. The lower the shrinkage limit, the higher will be the possible settlements of structures. Expansive soils also have low values of shrinkage limit.
14. The plasticity index is a useful measure to verify the suitability of clay for potteries, for the construction of the clay core in an earth dam, and for the construction of a clay liner to contain a polluted material. In all these cases, a high plasticity index is desirable for better workability and low permeability.