Methods to Determine Water Content of Soil

Oven Drying Method :

The oven drying method is the standard laboratory method and it is very accurate method used to determine the water content of soil.

The soil sample is taken in small air tight container. The mass of the soil sample and that of the container are obtained using an accurate weighing balance. The quantity of the sample to be taken for the test depends upon the gradation and the maximum size of the particles and the degree of wetness of soil. The drier the soil, the more shall be the quantity of the specimen.

The soil sample in the container is then dried in an oven at a temperature of 110° ± 5° C for exactly 24 hours. The temperature range selected is suitable for most of the soils. The temperature lower than 110° ± 5° C may not cause complete evaporation of water and the temperature higher than this may cause breaking down of crystalline structure of soil particles and loss of chemically bound structural water. The water content of the soil may be calculated by using the equation given below :

Where,

M1 = Mass of container with lid.

M2= Mass of container with lid and wet soil.

M3 = Mass of container with lid and dry soil.

 Pycnometer Method :

A pycnometer is a glass jar of about 1 litre capacity and fitted with  a brass conical cap by means of screw type cover. The cap has a small hole of 6 mm diameter at its apex. A rubber or fibre washer is placed between the cap and the jar to prevent leakage. There is a mark on the cap and also on the jar. The cap is screwed down to the same mark such that the volume of the pycnometer used in calculations remains constant. The pycnometer method for the determination of the water content can be used only if the specific gravity of the solid particles is known.

A sample of wet soil about 200 to 400 g is taken in pycnometer and weighed. Water is then added to the soil in the pycnometer to make it about half full. The contents are thoroughly mixed using a glass rod to remove the entrapped air. More and more water is added and stirred continuously till the pycnometer is filled with flush with hole in conical cap. The pycnometer is wiped dry and weighed. The pycnometer is then emptied completely and it is then filled completely with water and is wiped dry and weighed again. The following equation can be used to determine the water content :

Here,

M1 = Mass of Pycnometer

M2 = Mass of Pycnometer + wet soil

M3 = Mass of pycnometer + wet soil + water

M4 = Mass of pycnometer filled only with water

Sand Bath Method :

Sand bath method is a field method for the determination of water content. The method is rapid but not very accurate. A sand bath is large, open vessel containing sand filled to a depth of 3 cm or more. The soil sample is taken in a tray. The sample is crumbled and placed in the tray. A few pieces of white paper are also placed on the sample. The tray is weighed and the mass of wet sample is obtained.

The tray is then placed on the sand bath. The sand bath is heated over stove. Drying of sand bath can take upto 20-60 minutes, depending upon the type of soil. During heating, the specimen is turned with a palette knife. Overheating of soil should be avoided. The white paper turns brown when overheating occurs. The drying should be continued til the sample attains a  constant mass. When drying is complete the tray is removed from the sand bath, cooled and weighed.

Alcohol Method :

The soil sample is taken in an evaporating dish. Large lumps of soil, if any, should be broken and crumbled. The mass of wet sample is taken. The sample is then mixed with methylated spirit ( alcohol ). The quantity of methylated spirit required should be one milliliter for every gram of soil. The methylated spirit and soil should me mixed or turned several times, with the help of palette knife, to make the mixture uniform.

The methylated spirit is ignited. The mixture is stirred with the help of spatula or knife when the ignition is going on. After the methylated spirit has burnt away completely, the dish is allowed to be cooled and the mass of the dry soil is obtained.

Calcium Carbide Method :

When water comes in contact with calcium carbide, acetylene gas is produced.

The water content of soil is determined indirectly from the pressure of the acetylene gas formed. the instrument used is known as moisture tester.

The wet soil sample is placed in a sealed container containing calcium carbide. The samples of sand require no special preparation. The soil sample is ground and pulverized. However, cohesive and plastic soils are tested after addition of steel balls in the pressure vessels. The test requires about 6 gram of soil.

The pressure of acetylene gas produced acts on the diaphragm of the moisture tester. The quantity of gas is indicated on a pressure gauge. From the calibrated scale of the pressure scale of the pressure gauge, the water content based on the total mass is determined.

Radiation Method :

Radio-active isotopes are used for the determination of water content of soils. A device containing radio active isotopes material such as cobalt 60 is placed in a capsule and it is then lowered in steel casing A which is then placed in bore hole as shown in figure. The steel casing has a small opening on its one side through which rays can come out. A detector is placed inside steel casing B, which has an opening facing that in casing A.

Neutrons are emitted by the radio-active materials. The hydrogen atoms in the water of the soil cause scattering of neutrons. As these neutrons strike with the hydrogen atoms, they loose energy. The loss of energy is proportional to the quantity of water present in soil. The detector is calibrated to give the water content.

This method is extremely useful for the determination of water content of the soil in the in-situ conditions. This method should be used carefully, as it may lead to radiation problems if proper shielding precautions are not taken.