Atterberg limits of soil | consistency limit of soil – Soil mechanics

Consistency of Soils

The physical state of fine-grained soil at particular water content is known as its consistency. Consistency is a term used to indicate the degree of firmness of soils. It is mostly used for fine-grained soil.(clay) for which the consistency is related to a large extent of water content.

Depending upon the water content soil can have various states of consistency i.e., liquid state, plastic state, semi-solid state and solid state. The limiting water content at which soil passes from one state of consistency to another state is called the consistency limit.

The consistency limits were determined for the first time by an Agronomist, Atterberg in 1911 and therefore these limits are also known as Atterberg’s limits.

Types of Atterberg limits of Soil

1) Liquid limit (LL)

Liquid limit (LL) is defined as the boundary of water between the liquid and plastic state of the soils. At the liquid limit, the soil possesses a small value of shear strength. The liquid limit is the minimum water content at which the soil is still in a liquid state but has a small shearing strength against flowing.

Atterberg limits of soil | consistency limit of soil - Soil mechanics

2) Plastic limit (PL)

Plastic limit (PL) It is defined as the boundary of water between the plastic states and semi-solid states of the soil. It is also defined as the minimum water content at which a soil will just bring to grumble (crack) when rolled into thread approximately 3 mm in diameter.

3) Shrinkage limits

If is defined as the boundary of water between the semi-solid state and solid-state of the soil. If is also defined as the lowest water content at which a reduction in water content will not cause a decrease in the volume of a soil mass.

Determination of consistency limit

1) Liquid Limit

a) About 120 gm of the dried soil sample passing through a 425-micron sieve is taken. It is mixed thoroughly with distilled water.

b) A portion of the soil paste is kept in the cup and levelled by means of a spatula.

c) Using a standard grooving tool a groove is cut in the soil pat formed in the cup.

Atterberg limits of soil | consistency limit of soil - Soil mechanics

d) The cup is given blows by manual operation of the handle or by the electrically operated motorised system, the rotation of the handle being at the rate of 2 revolutions per second.

e) The number of blows required to close the groove for a distance of 13 mm is e) noted down.

f) The water content of the soil is altered and the process is repeated.

g) Adjusting the water in such a way that the number of blows to close the groove may fall within the range of 5 to 40 blows carries out at least four tests.

h) The water content values are plotted as ordinate on the natural scale against numbers to obtain a best-fitting straight line which is referred as a flow curve.

i) From this plot the liquid limit is obtained as water content corresponding to 25 blows, as shown in figure 2.7.

2) Plastic limit

There are the following procedures to find out the plastic limit (PL)

a) About 30 gms of the dried soil sample passing through a 425-micron sieve is taken. It is mixed thoroughly with distilled water.

b) About 10 gm of soil paste is rolled to form a ball and then rolled on a glass plate with the hand until it is formed in form of a thread of 3 mm diameter.

c) If crumbling does not take place in the thread then the thread is remoulded into a ball.

d) This process of rolling and remoulding is repeated until the thread starts just crumbling as a diameter of 3 mm.

e) The crumbled thread is tested for water content when its weight is found out before drying say w1 and after drying say W2 (in the oven) then,

3) Shrinkage limit

There are the following procedures to find out the shrinkage limit (SL):

a) About 30 gm of air-dried soil sample passing through 425 microns. IS. sieve is placed in the evaporating dish and thoroughly mixed with distilled water to form a soil paste of a slightly flowing consistency.

b) The shrinkage dish (non-corrodible cup of 45 mm dia. and 15 mm height) is weighed after coating grease or oil to prevent the adhesion of the soil to the shrinkage dish.

c) The shrinkage cup is filled with the soil paste in their layers the cup is gently tapped on a cushioned surface after filling with each layer to ensure the expulsion of air bubbles.

d) The surface of the soil is levelled and the outer side of the cup is cleaned.

e) The mass of the shrinkage cup with wet soil pat is found and this is deducted from the mass of the shrinkage cup to get mass of wet soil pat (M1).

f) The wet soil pat is allowed to dry in the air sometime, then kept in a thermostatically controlled oven, and dried for 24 hours at 105°-110°C.

g) After oven drying the mass of dry soil pat (Md) is found.

h) The volume of dry soil pat (Vd) is found by the mercury displacement method.

i) The volume of wet soil pat (V1) is equal to the volume of the shrinkage dish which is found by filling it with mercury and finding the mass of mercury and finding the mass of mercury required to fill if after removing the convex portion at the top by pressing flat plate.

j) The volume is obtained by dividing this mass by the density of mercury.

The shrinkage limit is determined by calculation as given below:

To derive an expression for shrinkage limit we consider the two-phase diagrams shown in figure 2.8 which correspond to the three states of soil pat during the experimental procedure.

Atterberg limits of soil | consistency limit of soil - Soil mechanics

Mass of water in figure (b) = Mass of water in figure (a) minus loss in mass of water from the figure (a) to figure (b)

= (M1-Md) -(V1-V2) x Yw

Where, V2 = Vd

Therefore,

Where,

M1 = Mass of wet soil pat

V1 = Volume of wet soil pat

Md = Mass of dry soil pat

Vd = Volume of dry soil pat

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