The water content of soil is an important parameter that controls its behaviour.

The water content of soil is determined as a routine matter in most of other tests.

The water content of soil can be determined by any one of the following methods:-

(a) oven drying method

(b) Torsion balance method

(c) Pycnometer method

(d) Sand bath method

(e) Alcohol method

(f) Calcium Carbide method

(g) Radiation method

(a)oven drying method

Minimum Quantity of soil for water content determination

(b) Torsion balance method

In this method infrared lamp and torsion balance moisture meter is used for rapid and accurate determination of water content.

The infrared radiation is provided by 250W lamp built in the torsion balance for use with an alternating current 220-230V , 50 cycles, single phase main supply.

Torsion balance method maximum size of specimen is 2mm.

It is kept in suitable container so that its water content is not affected by ambient conditions

The torque is applied to one end of the torsion wire by means of a calibrated drum to balance the loss of weight of sample as it drives out under infrared lamp

A Thermometer is provided for recording the drying temperature which is kept at $110^0 \pm 5^0 C.$ Provision is made to adjust the input voltage and infrared lamp to control the heat for drying of the specimen.

The weighing mechanism known as torsion balance. the time required for this test, it depends type of soil and quantity of water is present in the soil.

Generally it take 15-3- minutes,

Formula

$w = \dfrac{m^!}{100-m!}\times 100$

where $m^!$ = moisture content

(c) Pycnometer method

A Pycnometer is a glass jar of @ I litre capacity and fitted with a brass conical cap by means of screw type cover (As shown in Figure)

The cap has small hole about 6mm diameter at its apese

A rubber or fibre washer is placed between the cap and the jar to prevent the leakage .

A wet soil sample @ 200 to 400gm is taken into the Pycnometer and the weighted.

The water is then added to the soil in Pycnometer to make it about halh full.

The contents are throughout mixed using a glass rod to remove the entrapped air

$M_1$ - mass of empty Pycnometer

$M_2$ - mass of Pycnometer + wet soil

$M_3$ - mass of Pycnometer + wet soil+ water

$M_4$ - mass of Pycnometer filled with water oriy

$G = \dfrac{(m_2 - m_1)}{(m_2 - m_1) - (m_3 - m_4)}$

The method for determination of water current is quite suitable for coarse grained soil.

If a vacuum pump is available, the Pycnometer can be connected for about 10 to 20 minutes to remove the entrapped air.

(d) Sand bath method

This method is field method foe the determination of water content.

This method is rapid but not to much accurate.

Sand bath is large , open vessel containing sand filled to a depth of 3 cm or more

soil sample is determine by following equation,

$W = \dfrac{M_w}{M_s} = \dfrac{M_2 - M_1}{M_3 - M_1} \times 100$

(e) Alcohol method

Care shall be taken to prevent fire.

The method cannot be used to soil contains a large.

Proportion of day, organic matter , gypsum or any other calcareous material.

Method is quite rapid & not suitable

(f) Calcium Carbide method

$Ca C_2 + 2H_2 O = C_2 H_2 + Ca (OH)_2$

instrument used = moisture tester

$w = \dfrac{m^!}{1-m!}$

(g) Radiation method

Radio active isotopes are used for the determination of water content of soil

Radiation problem

Problem The mass of an empty Jar(glass) was 0.498 kg . when completely filled with water its mass became 1.528 kg.

An oven dried sample of soil of mass 0.198 kg was placed in the jar and water was added to fill the jar and its mass was found to be 1.653kg. Determine specific gravity of the particle

$G = \dfrac{M_2 - M_1}{(M_2 - M_1) - (M_3 - M_4)}$

or

$G = \dfrac{0.198}{(0.198) - (1.653-1.528)} = 2.71$