In civil engineering, soil is a basic element that serves as the foundation for all construction projects. The type of soil and its properties such as water content determination of soil are directly responsible for the stability and durability of the structure constructed above it. Sandy soil can be beneficial for proper drainage of water, while, on the other hand, clayey soil can be properly compacted, offering strong load-bearing capacity. Thus, soil characteristics such as compaction, shear strength, and water content are very important for ensuring a proper and stable foundation.
Water content in soil refers to the total amount of moisture present in the soil relative to its dry weight. Water content is a critical factor as it affects soil behavior, including its strength, compaction, and drainage characteristics.
Determining the water content in soil is crucial in fields such as agriculture and civil engineering. In the construction industry, the soil on which the structure is being constructed must have the desired soil strength. Proper compaction of the soil is required to achieve this strength, which can only be obtained with the correct water content.
Incorrect moisture content (high or low) in the soil can lead to over or under compaction, resulting in compromised structural integrity, soil settlement, and a risk of slope failure. Therefore, determining the moisture content of the soil becomes very important.
What are the Methods of Determining Water Content in Soil?
There are various methods to find the water content of soil. Each method is capable of finding the water content of soil, but only a few of them can calculate the accuracy, but that does not mean they are not worth considering. Each has its benefits and drawbacks. Some of them can be performed in the laboratory, while others are performed on site itself. Let’s have a look at these methods one by one.
1. 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.
Procedure:
- 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 :
Formula used in Oven Drying Method:
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.
2. 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.
Procedure:
- 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 :
Formula used in Pycnometer Method:
Where,
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
3. 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.
Procedure:
- 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.
Formula used in Sand Bath Method:
Where,
M1 = Mass of container
M2 = Mass of container + wet soil
M3 = Mass of container + dry soil
4. Alcohol Method
The alcohol method is one of the fastest method of finding the moisture content of soil. This method uses alcohol or methylated spirit to evaporate the moisture content present in the soil sample.
Procedure:
- 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.
5. 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.
Procedure:
- 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.
6. Radiation Method
In radiation method, radio-active isotopes are used for the determination of water content of soils.
Procedure:
- 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.
Radiation method is extremely useful for the determination of water content of the soil in the in-situ conditions. Radiation method should be used carefully, as it may lead to radiation problems if proper shielding precautions are not taken.
What is the Most Accurate Method for Determining Water Content of Soil?
The oven drying method is considered to be the most reliable and accurate method for determining the water content of soil. Oven drying method is the standard method and is performed in laboratory conditions for precision.
Why Oven Drying Method is Considered to be Most Accurate?
In Oven drying method, the soil sample is heated in an oven at controlled temperatures, so there is no chance of moisture being present after heating, leading to precise measurements.
Oven drying method is a standardized method by various organizations such as ASTM (American Society for Testing and Materials) and IS (Indian Standard), which makes this a globally recognized procedure for determining the moisture content of soil. This method can be used on different types of soils.
What are the Drawbacks of the Oven Drying Method?
If you need the results of the procedure quickly then oven drying method is not for you. Although you might get the most precise reading but this method is time consuming. Oven drying method requires 24 hours or sometimes even more for complete process.
Oven drying method is not reliable for soils containing gypsum, minerals with loosely bound water of hydration, or significant amounts of organic material. For these soil types, a lower temperature (60-80°C) is recommended.
Which is the Quickest Method to Determine Soil Water Content?
The quickest method to determine the soil water content is calcium carbide method. Calcium carbide method for water content determination can be completed in 3- 5 minutes to get the results. Due to this, it is also termed as the ‘Rapid Determination Method’. Although it is the quickest method to find the water content of soil, it also produces less accurate results as compared to standard oven-drying method.
How Long Does It Take to Determine Soil Water Content?
The exact timing of the procedure is not completely standardized, as the time required to complete each soil testing process may vary depending on factors such as the specific soil sample and environmental weathering conditions.
We have listed the average timings for each test procedure, these timings include everything from arranging the instruments to obtaining the final results.
| Methods | Time Required |
|---|---|
| Oven-Drying (Standard) | 24 hours |
| Sand-Bath | faster than oven drying method |
| Alcohol | less than 30 minutes |
| Radiation | 30 – 60 minutes |
| Calcium Carbide Pressure | 3- 5 minutes |
| Pycnometre | less than 30 minutes |
Note: The timings provided above are just averages. Actual times may vary depending on factors like the specific conditions and the quality of the equipment used.