Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 35

The knowledge of water content is necessary in soil compaction control, in determining
consistency limits of soil, and for the calculation of stability of all kinds of earth works and
foundations.

The method for the determination of water content, recommended by the Indian Stand-
ards Institution (I.S.I.), is set out in “IS: 2720 (Part-II)–1973, Methods of Test for soils-Part II
Determination of Moisture content”, and is based on oven-drying of the soil sample.

The following methods will be given here:

(i) Over-drying method

(ii) Pycnometer method

(iii) Rapid moisture Tester method.

3.5.1 Oven-drying Method

The most accurate approach is that of oven-drying the soil sample and is adopted in the labo-
ratory.

A clean container of non-corrodible material is taken and its empty weight along-with
the lid is taken. A small quantity of moist soil is placed in the container, the lid is replaced, and
the weight is taken.

The lid is taken removed and the container with the soil is placed in a thermostatically-
controlled oven for 24 hours, the temperature being maintained between 105-110°C. After
drying, the container is cooled in a desiccator, the lid is replaced and the weight is taken. For
weighing a balance with an accuracy of 0.0001 N (0.01 g) is used.

Thus, the observations are:

Weight of an empty container with lid = W 1

Weight of container with lid + wet soil = W 2

Weight of container with lid + dry soil = W 3

The calculations are as follows:

Weight of dry soil = W 3 – W 1

Weight of water in the soil = W 2 – W 3

Water content, w =

Wt of water

Wt of dry soil

× 100%

∴ w =

()

(W 3 )

WW

W

23

1

−

−

× 100% ...(Eq. 3.5)

Sandy soils need only about four hours of drying, while clays need at least 15 hours. To
ensure complete drying, 24 hours of oven drying is recommended. A temperature of more than
110°C may result in the loss of chemically bound water around clay particles and hence should
not be used. A low value such as 60°C is preferred in the case of organic soils such as peat to
prevent oxidation of the organic matter. If gypsum is suspected to be present in the soil, drying
at 80°C for longer time is preferred to prevent the loss of water of crystallisation of gypsum.

To obtain quick results in the field, sometimes heating on a sand-bath for about one
hour is resorted to instead of oven-drying. This is considered to be a crude method since there
is no temperature control.