Atterberg Limits: Significance and Factors Affecting it

After reading this article you will learn about:- 1. Meaning and Values of Atterberg Limits 2. Significance of the Atterberg Limits 3. Factors Affecting.

Meaning and Values of Atterberg Limits:

Atterberg limits are used extensively by soil scientists. Atterberg, a well-known scientist, suggested three values namely upper plastic limit, lower plastic limit and plasticity number or plasticity index and so they are called Atterberg limits.

i. Upper Plastic Limit:

It is also called liquid limit. It represents the moisture content of soil at a point where the soil-water mass just flows under an applied force and fails to retain its shape.

The upper plastic was determined originally by placing a small amount of soil in a round bottomed dish, working it into a stiff paste, pressing it tightly against the bottom, cutting a ‘V’ shaped groove in the plastic mass, and jarring the dish to make the two segments flow together.

If the flow was not produced, some extra amount of water was added and the process repeated. If too much flow was obtained, dry soil was mixed with the plastic mass. This cut and dry process was repeated until the correct flow was obtained. The moisture content of the plastic soil was then determined.

ii. Lower Plastic Limit:

Lower plastic limit refers to the moisture content of a soil at a point where its consistence changes from plastic to friable and the soil-water mass is unable to change shape continuously under the influence of an applied force, and ultimately the mass breaks into fragments.

This limit is determined by mixing the soil with water in a round-bottomed dish until it starts to loss its crumbly feel and shows a tendency to become plastic. The mass is then kneaded in the hands. A small portion is rolled between the fingers and a glass plate, or a piece of glazed paper, until a wire is formed.

The process of adding water or soil is repeated until that moisture content is reached when the plastic mass will just barely roll out into a wire that breaks into pieces. In this method clayey soils are the most difficult to study.

iii. Plasticity Number (Index):

If refers to the difference between moisture contents of a soil at its upper plastic limit and lower plastic limit. Different soils are characterized by a specific plastic number or index of plasticity.

Significance of the Atterberg Limits:

The different Atterberg limits as stated above represents the moisture content of the change from the friable to the plastic consistency. It represents the minimum moisture percentage at which the soil can be puddled. Orientation of particles and their subsequent sliding over each other takes place at this point, since sufficient water has been added to provide a film around each particle.

The moisture content of Atterberg limits depends upon the amount and nature of the soil colloids present. In the upper plastic limit the film of oriented water molecules becomes so thick that cohesion is decreased and the entire soil water mass flows freely under an applied force. The tensions of the water at the lower plastic limit and upper plastic limit are equivalent to pF values 2.8-3.3 and about 0.5 respectively.

The plasticity number or index of plasticity is an indirect measure of force required to mold the soil. It is a function of the number of the films and represents the amount of water that must be added to the soil system to increase the distance between the particles of maximum tension and the tension at which flow is produced.

So there is a direct relationship between the plasticity index and the liquid limit or upper plastic limit.

Factors Affecting the Atterberg Limits:

(i) Clay Content and Type of Clay:

Amount and nature of clay colloids greatly influence the plasticity. An increase in the percentage of clay causes plastic limits to be higher with the moisture content and increases the plasticity number or index.

With the decrease in clay content in the soil, the upper plastic limit decreases and thereby decreases the plasticity number or the index of plasticity. Atterberg limits are raised as the surface is increased due to higher amount of clay present in soil.

The clay content, therefore, determines the amount of surface that is available for water adsorption. For a particular clay mineral, the amount of absorbed water required at the plastic limit will increase with the amount and size of the particles present. Plastic limits of different types of silicate clays are given in Table 8.1.

So the type of clay mineral has a tremendous influence upon the adsorption of water by the colloidal system. The plasticity of the montmorillonite group is high because of greater surface hydration, which results from more orientation of water and the inter-lattice swelling as the thickness of the water films increases.

(ii) Nature of Exchangeable Cations:

The exchangeable cations have considerable influence upon soil plasticity (Table 8.1). The Na-saturated montmorillonite has the highest plastic limit as compared to the K, Ca and Mg-saturated montmorillonite clay.

The effect of these ions on non-expanding clays is quite different from the effect on the montmorillonite group. The higher hydration energies of the divalent cations should cause a raising of the Atterberg limits.

(iii) Organic Matter:

Organic matter exhibits an interesting effect upon soil plasticity. It has been found that the plasticity limits decreased due to oxidation of organic matter with hydrogen peroxide. Organic matter has a high adsorptive capacity for water.

Hydration of organic matter must be fairly complete before sufficient water is available for film formation around the mineral particles. Consequently the plastic limit occurs at relatively high moisture contents. Thus the addition of organic matter to soil may be expected to extend the zone of friability to fairly high moisture contents.