DHARM116 GEOTECHNICAL ENGINEERINGThus the ‘Effective Stress Principle’ may be stated as follows :
(i) The effective stress is equal to the total stress minus the pore pressure.
(ii) The effective stress controls certain aspects of soil behaviour, notably compressibility
and shear strength.
[Note. Latest research on the effective stress concept indicates that the effective stress
equation has to be modified in the case of saturated clays and highly plastic,
dispersed systems such as montmorillonite by introducing the term (R – A),
where R is related to the repulsive forces between adjacent clay particles due to
electrical charges and A is related to Van der Waals’ attractive forces between
these particles. Similarly, Bishop et al. (1960) proposed a different effective stress
equation for partially saturated soils. However, these concepts are of an ad-
vanced nature and are outside the scope of the present work.]
For a situation where the water table is at the ground surface, the conditions of stress at
a depth from the surface will be as follows :
σ = γsat. z ...(Eq. 5.6)
u = γw. z ...(Eq. 5.4)
By Eq. 5.5,
σ = (σ – u) = γsat. z – γw. z = z(γsat – γw)
Since (γsat – γw) = γ′, the submerged unit weight,
σ^ = γ′. z ...(Eq. 5.7)
Therefore, the effective stress is computed with the value of the buoyant or effective
unit weight.5.4 FLOW OF WATER THROUGH SOIL-PERMEABILITY
It is necessary for a Civil Engineer to study the principles of fluid flow and the flow of water
through soil in order to solve problems involving, – (a) The rate at which water flows through
soil (for example, the determination of rate of leakage through an earth dam) ; (b) Compres-
sion (for example, the determination of the rate of settlement of a foundation ; and (c) Strength
(for example, the evaluation of factors of safety of an embankment). The emphasis in this
discussion is on the influence of the fluid on the soil through which it is flowing ; in particular
on the effective stress.
Soil, being a particulate material, has many void spaces between the grains because of
the irregular shape of the individual particles; thus, soil deposits are porous media. In general,
all voids in soils are connected to neighbouring voids. Isolated voids are impossible in an as-
semblage of spheres, regardless of the type of packing; thus, it is hard to imagine isolated voids
in coarse soils such as gravels, sands, and even silts. As clays consist of plate-shaped particles,
a small percentage of isolated voids would seem possible. Modern methods of identification
such as electron micrography suggest that even in clays all voids are interconnected.
Water can flow through the pore spaces in the soil and the soil is considered to be ‘per-
meable’ ; thus, the property of a porous medium such as soil by virtue of which water (or other
fluids) can flow through it is called its ‘permeability’. While all soils are permeable to a greater
or a smaller degree, certain clays are more or less ‘impermeable’ for all practical purposes.