Water Content of Soil by Oven Drying Method

 Water Content of soil is the quantity of soil contained in a sample of soil. Generally this is expressed in ratio.

Here : s-soil (dry), v-void (pores filled with water or air), w-water, a-air. V is volume, M is mass.

Volumetric Water Content is defined by

where V_w is the volume of water and V_T = V_s + V_v = V_s + V_w + V_a is the total volume (that is soil volume + water volume + air space).

Gravimetric water content is expressed by mass (weight) as follows:

where m_w is the mass of water and m_t is the bulk mass. The bulk mass is taken as the total mass, except for geotechnical and soil science applications where oven-dried soil (m_s, see the diagram) is conventionally used as m_t

By Oven Drying Method  we can determine the Gravimetric water content in soil  as per IS: 2720 (Part II) - 1973.

PRINCIPLE
The water content (w) of a soil sample is equal to the mass of water divided by the mass of solids.

APPARATUS

i) Thermostatically controlled oven maintained at a temperature of 110 ± 5^oC
ii) Weighing balance, with an accuracy of 0.04% of the weight of the soil taken
iii) Air-tight container made of non-corrodible material with lid
iv) Tongs

SAMPLE
The soil specimen should be representative of the soil mass. The quantity of the specimen taken would depend upon the gradation and the maximum size of particles as under:

Size of particles 90 percent passing through IS Sieve	Minimum quantity of soil specimen to be taken for test (g)
425μm 2.0mm 4.75mm 9.50mm 19mm 37.5mm	25 50 200 300 500 1000

PROCEDURE
i) Clean the container, dry it and weigh it with the lid (Weight 'W₁').
ii) Take the required quantity of the wet soil specimen in the container and weigh it with the lid (Weight 'W₂').
iii) Place the container, with its lid removed, in the oven till its weight becomes constant (Normally for 24hrs.).
iv) When the soil has dried, remove the container from the oven, using tongs.
v) Find the weight 'W₃' of the container with the lid and the dry soil sample.

REPORTING OF RESULTS
The water content,  

 

An average of three determinations should be taken.
A sample proforma for the record of the test results is given below