How to Determine the Water Content of Soil ?

Following are the various methods available for the determination of water content of soil: 1. Oven-Drying Method 2. Sand Bath Method 3. Alcohol Method 4. Infrared Lamp Torsion Balance Method 5. Calcium Carbide Method 6. Pycnometer Method.

1. Oven-Drying Method:

The oven-drying method is the standard method of the determination of water content in the laboratory.

Principle:

The principle of test is to determine the weight of a wet soil sample in a container, dry the sample along with the container for 24 h in an oven and then determine the weight of the dry soil sample. The sequence of steps in water content determination is illustrated in Fig. 4.8.

The water content of the soil (ω, in percentage) is obtained from the relation –

ω = [(W₂ – W₁)/(W₃ – W₁)] x 100 …(4.66)

where W₁ is the weight of the container, W₂ is the weight of container + wet soil, and W₃ is the weight of container + dry soil.

Procedure:

Water content of a soil sample is determined in the following steps:

i. A clean dry non-corrodible container with lid is taken and its weight is determined (W₁) using a balance. The balance should be of minimum sensitivity to weigh the samples to an accuracy of 0.04% of weight of soil taken. This comes to a sensitivity of 0.01 g.

ii. The required quantity of a representative undisturbed soil sample, is taken and placed loosely in the container. The weight of the container with lid and wet soil is determined (W₂).

iii. The container with wet soil is placed in the oven with its lid removed for 24 h, maintaining a temper­ature of 110 ± 5°C. The container now containing dry soil is then cooled in a desiccator with the lid closed.

iv. The weight of dry soil with the container and lid (W₃) is determined. The water content is determined from Eq. (4.66).

The oven-drying temperature of 110 ± 5°C is suitable for most of the soils. A temperature more than 110 ± 5°C should not be used as it breaks the crystal structure of the soil and causes evaporation of structural water, which has properties completely different from normal water and is considered a part of soil solids.

For soils containing gypsum or other minerals, there is loosely bound water of hydration (adsorbed water), which gets evaporated at 110°C. Hence, a lower temperature of 80°C should be used for oven-drying such soils. Similarly, for soils containing organic matter, the oven-drying temperature should not exceed 60°C to prevent oxidation of organic matter.

The oven-drying method is the most accurate method of determining water content. The only disadvantage of the method is that it takes minimum 24 h to know the test result.

2. Sand Bath Method:

The principle for determination of water content in this method is the same as in the oven-drying method except that drying of the wet soil is done using a sand bath.

Procedure:

Water content is determined by sand bath method in the following steps:

i. A clean dry non-corrodible container with lid is taken and its weight is determined (W₁).

ii. The required quantity of a representative undisturbed soil sample, is taken and placed loosely in the container. The weight of the container with lid and wet soil is determined (W₂).

iii. The container with wet soil is placed on a sand bath and is heated until all the water has evaporated. This takes about 0.5 to 1 h. The soil is mixed using a palette knife to ensure soil at the bottom is not overheated. Care should be taken to ensure that the sand bath is not too hot and does not exceed the temperature 110 ± 5°C. The container now containing dry soil is then cooled in a desiccator with the lid closed.

iv. The weight of dry soil with the container and lid (W₃) is determined. The water content is determined from Eq. (4.66).

The sand bath method allows rapid determination of water content within 0.5 to 1 h. It is more suitable as a field test. The result obtained is less accurate than that obtained by the oven-drying method because there is no control over temperature during drying in this method. This method should not be used if it is suspected that the soil con­tains significant quantities of gypsum, calcareous matter, or organic matter.

3. Alcohol Method:

The principle of water content determination in the alcohol method is the same as in the oven-drying method except that drying of wet soil is done with the help of a methylated spirit.

Procedure:

Water content is determined by alcohol method using the following steps:

i. A clean dry non-corrodible container with lid is taken and its weight is determined (W₁).

ii. The required quantity of a representative undisturbed soil sample, is taken and placed loosely in the container. The weight of the container with lid and wet soil is determined (W₂).

iii. The wet soil is mixed with a methylated spirit (1 mL/g of soil). The methylated spirit is worked well with the soil using a palette knife, and large lumps of soil, if any, are broken down.

iv. The wet soil with methylated spirit is then ignited. The contents are constantly stirred with a spatula or knife, care being taken to ensure that none of the soil is lost.

v. After methylated spirit completely burns away, the container (now with dry soil) is taken and cooled in a desiccator with the lid closed.

vi. The weight of the dry soil with the container and lid (W₃) is determined. The water content is determined from Eq. (4.66).

4. Infrared Lamp Torsion Balance Method:

This method enables rapid determination of water content of soils by employing a device providing infrared lamp for drying and torsion balance for getting percentage of water on wet basis from a scale. The results obtained are convertible to water content on dry basis.

Apparatus:

An infrared lamp torsion balance moisture meter consists of:

i. An infrared lamp for drying wet soil.

ii. A torsion balance for direct determination of water content.

The infrared lamp is a 250-W lamp built in the balance and operates under a single-phase AC voltage of 220-230 V. Provision is made to adjust the input voltage to the infrared lamp to regulate the drying heat and temperature.

The torsion balance has a built-in magnetic damper to reduce pan vibrations and is calibrated to give water content in the range of 1%-100% with a 0.2% least count. A thermometer graduated to read temperature from 40°C to 150°C is provided to monitor the temperature in the pan housing. A separate physical balance is not required to determine weights through this method.

Procedure:

Water content is determined by infrared moisture meter method using the following steps:

i. About 25 g of a soil sample is taken. The lamp housing is raised and the soil is evenly distributed on the sample pan.

ii. The lamp housing is then lowered and the infrared lamp is switched on.

iii. A thermometer is inserted in its socket. The variac control knob is set between 95°C and 100°C. The soil sample now begins to lose water.

iv. When the thermometer indicates a temperature of 105°C, the variac knob is adjusted in such a manner that there is no further increase in temperature.

v. The drum scale is rotated by turning the drum drive knob until the pointer returns to the index. The percentage of water content is directly read from the scale. The final reading is taken when the pointer is steady on the drum scale, which indicates that the soil has dried to a constant mass.

vi. The water content read from the scale is based on the initial wet weight of soil (ω’).

It should be converted to water content (ω) based on the dry weight as follows:

Water content based on the dry weight of soil, ω = W_w/W_d

Water content based on the wet weight of soil, ω’ = W_w/W

So we have –

Thus, the water content based on the dry weight is computed as –

ω = ω’/1 – ω’ …(4.67)

The infrared torsion balance method allows rapid and accurate determination of water content. The result can be obtained in 15-30 min. The maximum size of particles in the soil sample should not be more than 2 mm because the quantity of soil sample used for test is 25 g.

5. Calcium Carbide Method:

The calcium carbide method is a rapid and reasonably accurate method of determination of water content of soil using a portable moisture content kit.

The method is based on the principle that when the water in the soil reacts with calcium carbide, acetylene gas is produced and the pressure exerted by the acetylene gas on a diaphragm gives a measure of the water content.

The water content obtained from the calcium carbide method is based on the initial weight of wet soil. It should be converted to water content based on the dry weight of soil.

Procedure:

Water content is determined by calcium carbide method using the following steps:

i. About 6 g of a wet soil sample is taken for the test. Coarse-grained soils are ground and pulverized. Fine-grained soils are mixed with steel balls in pressure vessels to pulverize properly and to prevent formation of lumps of wet soil.

ii. The soil sample is then placed in an air-tight container and mixed with a sufficient quantity of calcium carbide powder. The mixture obtained is shaken vigorously.

iii. The acetylene gas produced by the reaction of the moisture of the soil with the calcium carbide exerts pressure on the diaphragm. The dial gauge located at the diaphragm reads the water content ω’ directly.

iv. As the dial gauge is calibrated to read the water content based on the wet weight of soil, it should be corrected using Eq. (4.67).

This method is very quick and the result can be obtained in 5-10 min. The method is more suited as a field method of water content determination for compaction control where water content is to be quickly determined. Care should be taken to ensure that the wet soil-calcium carbide mixture is not exposed to air as calcium carbide will then react with the moisture in air and as a result, the water content of the soil will be overestimated.

6. Pycnometer Method:

The pycnometer method is a rapid method of water content determination for soils whose specific gravity is accu­rately known. The method is suitable for coarse-grained soils only as the entrapped air is likely to cause a signifi­cant error in water content determination for fine-grained soils.

Pycnometer is a density bottle of 900 mL capacity. A brass conical cap is used as a cover. The conical cap is always screwed to the pycnometer up to the mark on the pycnometer to give the designated volume of 900 mL. A rubber washer is placed inside the conical cap to prevent the leakage of water through the walls of the pycnometer and the conical cap.

Procedure:

The principle of water content determination by pycnometer method is illustrated in Fig. 4.16.

i. The weight of a clean and dry pycnometer with the cap is taken and recorded (W₁).

ii. About 200-400 g of a wet soil sample is placed in the pycnometer and the weight of the pycnometer with cap and wet soil is taken (W₂).

iii. Water is added to the pycnometer in increments and the contents are mixed using a glass rod. Care should be taken to remove the entrapped air completely by mixing the contents thoroughly. A vacuum pump may also be used for this purpose.

iv. The pycnometer is then completely filled with water up to the hole in the conical cap. The outside surface of the pycnometer is wiped with a cloth. The weight of the pycnometer with wet soil and water is taken (W₃).

v. The contents of the pycnometer are then removed and the pycnometer is washed thoroughly. The pycnometer is again filled with water completely up to the hole in the conical cap. The outside surface of the pycnometer is wiped with a cloth and the weight of pycnometer with water is taken (W₄).

vi. The water content of the soil is computed using the following equation:

The advantage of the pycnometer method is that drying of the soil is not required for determination of water content. However, entrapped air can become a source of serious error in this method. Also, the accuracy of the water content obtained through this method is dependent on the accuracy of the specific gravity used in calculations. The method is not recommended by IS – 2720 (Part 2) – 1973, which is the code of practice for water content determination.

Derivation of Expression for Water Content in Pycnometer Method:

Referring to Fig. 4.16, water content will be –

ω = W_w/W_d

and the weight of water in wet soil will be –

W_w = (W₂ – W₁) – W_d

Here, W₂ and W₁ are known.

The weight of dry soil (W_d) is determined as follows:

G = γ_s/γ_w = W_d/V_sγ_w Þ V_sγ_w = W_d/G