Geotechnical Engineering

DHARM

SOIL MOISTURE–PERMEABILITY AND CAPILLARITY 115

The vertical geostatic stress, thus, varies linearly with depth in this case (Fig. 5.1).

sv

sv

Z

Fig. 5.1 Vertical geostatic stress in soil with horizontal surface

However, it is known that the unit weight of soil is seldom constant with depth. Usually

a soil becomes denser with depth owing to the compression caused by the geostatic stresses. If

the unit weight of soil varies continuously with depth, σv can be evaluated by means of the

integral :

σv = γ.dz

Z

z 0

...(Eq. 5.2)

If the soil is stratified, with different unit weights for each stratum, σv may be computed

conveniently by summation :

σv = Σγ. ∆z ...(Eq. 5.3)

5.3.2 Effective and Neutral Pressures
The total stress, either due to self-weight of the soil or due to external applied forces or due to
both, at any point inside a soil mass is resisted by the soil grains as also by water present in the
pores or void spaces in the case of a saturated soil. (By ‘stress’ here, we mean the macroscopic
stress, i.e., force/total area ; the ‘contact stresses’ at the grain-to-grain contacts will be very
high owing to a very small area of contact in relation to the area of cross-section and these are
not relevant to this context).
‘Neutral stress’ is defined as the stress carried by the pore water and it is the same in all
directions when, there is static equilibrium since water cannot take static shear stress. This is
also called ‘pore water pressure’ and is designated by u. This will be equal to γw. z at a depth z
below the water table :
u = γw. z ...(Eq. 5.4)
‘Effective stress’ is defined as the difference between the total stress and the neutral
stress ; this is also referred to as the intergranular pressure and is denoted by :

σ = σ – u ...(Eq. 5.5)

Equation 5.5 is the ‘Effective Stress Equation’.

The effective stress has influence in decreasing the void ratio of the soil and in mobilis-

ing the shear strength, while the neutral stress does not have any influence on the void ratio

and is ineffective in mobilising the shearing strength.