Foundation Settlement and Soil Compression - Elements of Soil Mechanics - page 316

drain away. During this time the effects of secondary compression are also taking place so that, when
primary compression is complete, little, if any, secondary effect is noticeable. The terms 'primary' and
'secondary' are therefore seen to be rather arbitrary divisions of the single, continuous consolidation
process. The time relationships of these two factors will be entirely different if they are obtained from two
test samples of different thicknesses.

11.2 Immediate settlement

11.2.1 Cohesive soils

If a saturated clay is loaded rapidly, the soil will be deformed during the load application and excess
hydrostatic pore pressures are set up. This deformation occurs with virtually no volume change, and due
to the low permeability of the clay, little water is squeezed out of the voids. Vertical deformation due to
the change in shape is the immediate settlement.
This change in shape is illustrated in Fig. 11.1a , where an element of soil is subjected to a vertical major
principal stress increase Δ σ 1 , which induces an excess pore water pressure, Δ u. The lateral expansion
causes an increase in the minor principal stress, Δ σ 3 .
The formula for immediate settlement of a flexible foundation was provided by Terzaghi ( 1943) and is

(

2

)

pB 1 − ν N p

ρ

i =

E

Where

p = uniform contact pressure
B = width of foundation
E = Young's modulus of elasticity for the soil
ν = Poisson's ratio for the soil ( = 0.5 in saturated soil)
N p = an influence factor depending upon the dimensions of the flexible foundation.

This relationship gives the immediate settlement at the corners of a rectangular footing, length L and
width B. In the case of a uniformly loaded, perfectly flexible square footing, the immediate settlement
under its centre is twice that at its corners.
Various values for N p are given in Table 11.1.
By the principle of superposition it is possible to determine the immediate settlement under any point
of the base of a foundation (Example 11.2) . A spoil heap or earth embankment can be taken as flexible

Fig. 11.1 Compressive deformation.