Explain the standard One Dimensional Consolidation Test.

The consolidation test is conducted in a laboratory to study the compressibility of a soil.  The test is performed in the consolidation test Apparatus, known as the consolidometer or an oedometer. The  following test procedure is applied to any type of soil in the standard consolidation test:

 Loads are applied in steps in such a way that the successive load intensity is twice the proceeding on the load intensity is commonly used being `¼`,`½`,`1`,`2`,`4`,`8`, and `16 (kg)/M^2`. Each Load is allowed to stand until compression has practically ceased. The dial readings at elapsed time of `1/ 4`,` ½`, `1`,` 2`,` 4`, `8`, `15`, `30`,`60`,`120`,`240`,` 480` and `1440` minutes from the time the new increment of load is put on the sample. The Sandy samples get compressed in a relative short time as compared to Clay samples.

After the greatest load required for the test has been a.... Show More

The consolidation test is conducted in a laboratory to study the compressibility of a soil.  The test is performed in the consolidation test Apparatus, known as the consolidometer or an oedometer. The  following test procedure is applied to any type of soil in the standard consolidation test:

 Loads are applied in steps in such a way that the successive load intensity is twice the proceeding on the load intensity is commonly used being `¼`,`½`,`1`,`2`,`4`,`8`, and `16 (kg)/M^2`. Each Load is allowed to stand until compression has practically ceased. The dial readings at elapsed time of `1/ 4`,` ½`, `1`,` 2`,` 4`, `8`, `15`, `30`,`60`,`120`,`240`,` 480` and `1440` minutes from the time the new increment of load is put on the sample. The Sandy samples get compressed in a relative short time as compared to Clay samples.

After the greatest load required for the test has been applied to the soil sample, the load is removed in decrements to provide data for plotting the expansion curve of this soil in order to learn its elastic properties and magnitude of plastic or permanent deformations. The following data should also be obtained:

1. Moisture content and weight of the soil sample before the commencement of the test.

2.  Moisture content and weight of the sample after the completion of the test.

3.  The specific gravity of the solids.

4.  The temperature of the room where the test is conducted.

Fig:One dimensional consolidation test

Determination of void ratio:

The results of a consolidation test are plotted in the form of a plot between void ratio and the effective stress. It is therefore, required to determine the boiler ratio at various load increments. There are two methods:

1.  height of solids method

2.  change in void ratio method

 The first method is generally applicable to both saturated and unsaturated soils while the second method is applicable only to saturated soil. 

Height of solids Method:

In this method,The equivalent height of solid is determined from the dry mass of the soil. The height of solid is given by:

`H_s=V_s/A`

Or,`H_s=W_s/(G*gamma_w)*1/A`........(i)

Where,

`H_s=`Height of solids

`V_s=` volume of solids

`W_s=` dry mass of sample

`G=` specific gravity of solids

`A=` cross sectional area of specimen

We have,

`e=`Volume of voids/Volume of solids

Or,`e=(V-V_s)/V_s`.......(ii)

Now,

`e=((A*H)-(A*H_s))/(A*H_s)`

Or,`e=(H-H_s)/H_s`......(iii)

Where,`H=`final specimen thickness

Thus,

 `H=H_0+- sum(triangleH)`

 `  =H_1+ triangleH`

`H_0=` Initial height of the specimen

`triangleH=` change in the specimen thickness under any pressure increment

`H_1=` height of specimen at the beginning of any load increment

 Knowing the void ratio at the beginning and at the end of the test corresponding water content and degree of saturation can be calculated.

Change in Void ratio Method:

Let us assume the specimen to be fully saturated. Now, the void ratio at the end of the test is determined from the following relation:

` e_f=w_f*G`

Where,

`e_f=` final void ratio at the end of the test

`w_f=` final water content at the end of the test

 the change of void ratio `triangle e` under each pressure increment is calculated from the following relationship:

`(triangle e)/(1+e)=(triangleH)/H`

Or,`triangle e=(1+e_f)/H_f*triangleH`

Knowing `triangle e` and working backwards from the known value of `e_f`, the equilibrium void ratio corresponding to its pressure can be evaluated.