CONSISTENCY OF SOILS

R.Ravindranath,M.E(Geo.Tech)

Consistency is a term which is used to describe the degree of firmness of a soil in a qualitative manner by using descriptions such as soft medium, firm,stiff or hard.

It is relative ease with which a soil can be deformed.

In practice , the property of consistency is associated only with fine – grained soils, especially clay.

It is depends upon the water content

Four stages of consistency:

1.The liquid state2.The plastic state3.The semi solid state4.The solid state

1.The liquid statewhen a fine grained soil is mixed thoroughly with a large quantity of water , the resulting suspension is in a liquid state, and offers practically no resistance to flow.The soil has no shear strength.

2.The plastic stateThe water content of soil the suspension is gradually reduced to keep the consistency of the sample uniform, a stage comes when it just offering resistance to flow.

The soil has small shear strength and change from liquid state to plastic state.

The boundary of water content between liquid and plastic state is called “LIQUID LIMIT”.

In the plastic state, the soil can be moulded to different shapes with out rupturing it, due to its plasticity.

If the water content is further reduced , the clay sample changes from plastic state to the semi-solid state it is called as “PLASTIC LIMITS”.

When the water content of the soil mass is decrease, the volume of soil mass does not decrease any further but remains the same.

The sample changes from the semi-solid to solid state.

The boundary water content is called “SHRINKAGE LIMITS”.

DETERMINATION OF

LIQUID LIMIT

Casagrande Apparatus

Casagrande Apparatus

About 100g of soil specimen passing through 425 micron sieve is mixed thoroughly with distilled water in the evaporating dish to form a uniform paste.

A portion of the paste is placed in the cup of liquid limit device

Level the mix to have a maximum depth of 1cm. Draw the grooving tool through the sample along

the symmetrical axis of the cup, holding the tool perpendicular to the cup.

Rotate the handle at a rate of about 2 revolutions per second and the number of blows are counted till the two parts come into contact at the bottom of the groove.

By altering the water content of the soil & repeating the operation, obtain 5 readings in the range of 20-35 blows.

The liquid limit is determined by plotting the graph on the semi-log graph between the number of blows as abscissa on a logarithmic and water content as ordinate.

The water content corresponding to 25 blows shows the liquid limit.

SI.No Wt of dry

soil(gm)

Quantity of water

added (ml)

% wt

addedno of blows

1 100

2 100

3 100

4 100

From graph, Liquid limit = _____ %

DETERMINATION OF

PLASTIC LIMIT

About 15g of oven dried soil specimen passing through IS 425 micron sieve is taken and mixed thoroughly with distilled water until the soil mass becomes plastic enough to be easily moulded into a ball with fingers.

Take a portion of the ball and roll it on a glassplate to form the soil mass into a thread of uniform diameter throughout its length.

Take care that diameter should be around 3 mm. When the diameter reaches 3mm, the soil is

remoulded into a ball. The process of rolling and remoulding is repeated

until the soil just crumbling. The crumbled threads are kept for water content

determination. Repeat the test with two more samples.

Weight of dry soil taken (g) =

Weight of empty container (g) =

Weight of wet soil before drying W1 (g) =

Weight of soil after drying W2 (g) =

Water content = Plasticity index IP = Liquid limit -

Plastic limit

DETERMINATION OF

SHRINKAGE LIMIT

Mix about 50 g of soil passing through 425 micron sieve with distilled water to make a creamy paste which can be placed in the shrinkage dish without any air voids.

The required mixing water content is somewhat greater than the liquid limit.

Coat a thin layer of Vaseline inside of the shrinkage dish and weigh.

Fill the dish in three layers by soil paste. The last layer should stand above the rim.

Weigh the dish full of wet soil immediately. Dry in oven at 105 to 110 C

Remove the dry pat from the dish, clean and dry the shrinkage dish and determine its empty mass.

Weigh the empty mercury-weighing dish also. Keep the shrinkage dish in large porcelain

dish, fill it to overflowing with mercury. Transfer the contents of the shrinkage dish to the mercury weighing dish.

Place the glass cup in a large dish, fill to overflowing with mercury and remove the excess by pressing the glass plate.

Place the dry soil pat on the surface of mercury and submerge it.

Transfer the mercury displaced by the dry pat to the mercury weighing dish and weigh.

Weight of shrinkage dish w1 (g) = Weight of shrinkage dish + wet soil w2 (g)

= Weight of the wet soil in shrinkage dish (w2-

w1) (g) =  Weight of shrinkage dish + dry soil w3 (g)

= Weight of the dry soil pat (w3-w1) =ws (g)

= Volume of wet soil =Volume of shrinkage dish

= V1 (cc) = g/cc

Volumes of the dry soil pat V2 (cc) = g/cc

Shrinkage limits =

Shrinkage Ratio Ws/v2ρw = %

Volumetric shrinkage V 1-V2/V2 x 100

Volume of wet soil =Volume of shrinkage dish = V1 (cc)

Volumes of the dry soil pat = V2 (cc)

◦ Plasticity index◦ flow index◦ Toughness index◦ Consistency index◦ Liquidity index

Atterberg indices

Plasticity index: (Ip) I p = liquid limit – plastic limits

I p = Wl - Wp

Where,W1 = water content corresponding to number of

blows,N1

W2 = water content corresponding to number of blows,N2

flow index :

Toughness index is the ratio of plasticity index to flow index

Consistency index:

liquidity index:

Toughness index

ACTIVITY NUMBER:The relation between plasticity index and soil fraction less than 2µ is called activity number.

Soil type< 0.75 inactive0.75 to

1.40 normal>1.40 Active