5.1 Introduction
Natural soil deposits invariably include water. Under certain conditions soil moisture or water
in the soil is not stationary but is capable of moving through the soil. Movement of water
through soil affects the properties and behaviour of the soil, rather in a significant way. Con-
struction operations and the performance of completed construction could be influenced by soil
water. Ground water is frequently encountered during construction operations; the manner in
which movement of water through soil can occur and its effects are, therefore, of considerable
interest in the practice of geotechnical engineering.5.2 Soil Moisture and Modes of Occurrence
Water present in the void spaces of a soil mass is called ‘Soil water’. Specifically, the term ‘soil
moisture’ is used to denote that part of the sub-surface water which occupies the voids in the
soil above the ground water table.
Soil water may be in the forms of ‘free water’ or ‘gravitational water’ and ‘held water’,
broadly speaking. The first type is free to move through the pore space of the soil mass under
the influence of gravity; the second type is that which is held in the proximity of the surface of
the soil grains by certain forces of attraction.5.2.1 Gravitational Water
‘Gravitational water’ is the water in excess of the moisture that can be retained by the soil. It
translocates as a liquid and can be drained by the gravitational force. It is capable of transmit-
ting hydraulic pressure.
Gravitational water can be subdivided into (a) free water (bulk water) and (b) Capillary
water. Free water may be further distinguished as (i) Free surface water and (ii) Ground
water.
(a) Free water (bulk water). It has the usual properties of liquid water. It moves at all
times under the influence of gravity, or because of a difference in hydrostatic pressure head.
112Chapter 5