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Water Content Determination of Soil

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Water content significantly affects the engineering behavior of fine-grained soil.

Water content determination is the first test that is done in the lab after a sample is brought from the site because it can change on storage.

The various methods used for water content determination are:

1. Oven Drying Method.

2. Pycnometer Method

3. Sand Bath Method.

4. Torsion Balance Method.

5. Alcohol Method.

6. Calcium Carbide Method.

7. Radiation Method.

1. Oven Drying Method:

W1= wt. of an empty container.

W2= wt. of moist soil + container.

W3= wt. of dry soil + container.

$\large w=\frac{W_{w}}{W_{S}}$

$\large w=\frac{W_{2}-W_{3}}{W_{3}-W_{1}}$

Air-dry soil does have hygroscopic water, whereas oven-dry soil does not have hygroscopic water.

It is a Lab Method.

The wet moist sample of soil is dried in an oven at a controlled temperature of 105° C to 110° C for a period of 24 hrs.

Temp. above 110° C may result in the loss of chemically bound water of crystallization.

The water of crystallization is the water in the molecule structure, its loss may cause a change in the soil structure.

For organic soil, the temperature of drying in an oven is controlled at 60° to 80° C to avoid oxidation of organic matter present in the sample.

2. Pycnometer Method:

Pycnometer Method for water content of soil

$\large w=\frac{W_{w}}{W_{s}}=\frac{W_{2}-W_{1}-W_{s}}{W_{s}}$ ..........................................1

If from 'W₃', the wt. of solids is removed and replaced by wt. of the water having the same volume of solid then 'W₄' can be obtained.

$\large {\color{DarkBlue} W_{3}-W_{s}+\frac{W_{s}}{G_{s}\times\gamma_{w}}\times\gamma_{w}=W_{4}}$

$\large {\color{DarkBlue} W_{s}\times(1-\frac{1}{G_{s}})=W_{3}-W_{4}}$

$\large {\color{DarkBlue} W_{s}=(W_{3}-W_{4})\times\frac{G_{s}}{G_{s}-1}}$ .........................................2

From 1 and 2

$\large {\color{DarkBlue} w=\frac{(W_{2}-W_{1})}{(W_{3}-W_{4})}\times\frac{(G_{s}-1)}{G_{s}}\;-1}$

In the pycnometer method, we get a quick result, but it is important that all the entrapped air is removed.

Removal of entrapped air is difficult in clayey soil or cohesive soil. Hence this method is suitable for cohesionless soil.

Before calculating w using pycnometer method we must know the specific gravity of soil.

$\large {\color{DarkBlue} G_{s}=\frac{\gamma_{s}}{\gamma_{w}}}$

$\large {\color{DarkBlue} = \frac{W_{s}}{V_{s}\times\gamma_{w}}}$

$\large {\color{DarkBlue} G_{s}=\frac{(W_{2}-W_{1})}{(W_{4}-W_{1})-(W_{3}-W_{2})}}$

Normally the value of Gs is reported at 27° C.

3. Torsion Balance Method:

Used for soil which quickly absorbs moisture from the atmosphere.

It is a Lab method.

The Infrared lamp and torsion balance moisture meter are used for rapid and accurate determination of water content.

4. Sand Bath Method:

Sand bath method for soil moisture content

This method is Rapid, but not very accurate.

Dry takes about 20 to 60 min depending upon the type of soil.

It is a Field method.

The white paper turned brown indicates overheating of soil.

5. Calcium Carbide Method:

CaC₂ +2H₂O = C₂H₂+ Ca(OH)₂

C₂H₂=Acetylene gas

Water Content w.r.t total weight.

${\color{DarkBlue} w_{t}=\frac{W_{w}}{W}}$

${\color{DarkBlue} w_{t}=\frac{W_{w}}{W_{s}+W_{w}}}$

$\color{DarkBlue} w_{t}=\frac{w}{1+w}$

$\color{DarkBlue} w=\frac{w_{t}}{1-w_{t}}$

In this method, we measure the pressure exerted by the acetylene gas when CaC₂ is added to the soil.

The pressure produced can be correlated with the amount of water present. The instrument used is known as Moisture Tester.

In this method, we get the moisture content in terms of the total weight of the soil.

6. Radiation Method

Radioactive isotopes are used, neutrons are emitted which are scattered by the moisture in the soil.

More is the scattering, more is the moisture present.

CE Masters-Civil Engineering Study Materials