8/12/2019 Consistency Limits

1/2

Consistency Limits

Consistency is the relative ease with which a soil can be deformed. In practice, the property ofconsistency is associated only with fine grained soils, especially clays whose consistency depends onwater content. Consistency is a term which is used to describe the degree of firmness of a soil in a

qualitative manner by using relative consistency descriptions such as hard, very stiff, stiff, medium stiff,firm, soft, very soft with increasing water content.

The physical properties of clays are considerably influenced by the amount of water present in them.Depending upon the water content, the following four states or stages of consistency are used todescribe the consistency of a clayey soil: (i) Liquid state, (ii) Plastic state, (iii) Semi solid state, and (iv)

Solid state.

The boundary water contents at which the soil undergoes a change from one state to another are called consistency limits . In 1911, a Swedish soil scientist, Atterberg, first demonstrated the significance of

these limits. Hence, they are also known as Atterberg Limits.

When a fine grained soil is mixed thoroughly with a large quantity of water, the resulting suspension isin a liquid state, and offers practically no resistance to flow that is the soil has virtually no shearstrength. If water content of the suspension is gradually reduced while keeping the consistency of thesample uniform, a stage comes when it just begins offering resistance to flow. This is the stage wherethe sample changes from the liquid to plastic state. The boundary water content between liquid stateand the plastic state is called the liquid limit. In the plastic state, the soil can be moulded to differentshapes without rupturing it, due to its plasticity. If the water content is further reduced, the clay sample

changes from the plastic state to the semi-solid state, at a boundary water content which is called theplastic limit. In the semi-solid state, the soil does not have plasticity; it becomes brittle.

Up to the semi-solid state, the soil remains fully saturated and any reduction in volume of water willresult in an equal reduction in the volume of the soil mass. A further reduction in the water contenthowever, brings about a stage where the volume of the soil mass does not decrease but remains thesame with a decrease in water content. The sample changes from semi-solid to the solid state. Theboundary water content is called the shrinkage limit. Below this limit, the sample begins to dry up at the

surface and the soil is no longer fully saturated.

8/12/2019 Consistency Limits

2/2

Fig: Consistency Limits in Soil

Fig: Atterberg Limits

Liquid Limit (w L), Plastic Limit (w P) and Shrinkage Limit (w S) are the atterberg limits. These limits are

most useful for engineering purpose in order to classify the soils.

a) Liquid Limit (w L) : It is the water content corresponding to an arbitrary limit between liquid and

plastic states of consistency of a soil. It is a minimum water content at which soil is still in liquidstate, but possessing a small shear strength and exhibiting some resistance to flow.

b) Plastic Limit (w P) : It is the water content corresponding to an arbitrary limit between plastic andsemi-solid states of consistency of a soil. It is a minimum water content at which soil will just beginto crumble when rolled in to a thread of approximately 3 mm diameter.

c) Shrinkage Limit (w S) : It is the water content corresponding to an arbitrary limit between semi-solidand solid states of consistency of a soil. It is the lowest water content at which soil is fully saturated.It is also the maximum water content at which any reduction in water content will not reduce

volume of the soil mass.