Geotechnical Engineering

DHARM

SHALLOW FOUNDATIONS 627

If there is eccentricity about both axes, the product of the two factors must be used.

0.8

0.6

0.4

0.2

Reduction factor, R

e

Cohesive soil

Granular soil

0 0.1 0.2 0.3 0.4 0.5

Eccentricity ratio e /b or e /LbL

Fig. 15.19 Reduction factor for eccentrically loaded footings

Footings with unsymmetrical shapes

The assumption till now has been that at least one axis of symmetry exists for the footing in

plan. If an unsymmetrical section is involved under eccentric loading, computation of soil pres-

sures becomes a problem, since Eq. 15.12 is not applicable even though the entire base may be

in compression. However, the errors involved in using Eq. 15.12 may not be intolerable for

design, unless the footing is greatly unsymmetrical.

15.4.7Inclined Loading
The conventional procedure of analysing the stability of footings subjected to inclined loading
consists in resolving the load into a vertical component V and a horizontal component H, and
dealing with the effect of each separately. The soil pressure due to the vertical load is consid-
ered to be uniform and the stability against ultimate failure is analysed in the usual way.

V

H

Df

Pp

F = .Vmm= 0.35 to 0.55

Pa

V

H

Df

Pp

F=c×area of base

c : Cohesion®10 to 30 kN/m^2

P is negligiblea

2c

2c+ Dg f

Factor of safety against sliding = ———————

(P –P +F)pa

H

(a) Granular soil (b) Cohesive soils

Fig. 15.20 Conventional method of analysis of footings subjected to inclined loads

The stability against the horizontal load is analysed by ensuring a minimum factor of

safety against sliding at the base, which is defined as the ratio between the total resistance to

sliding and the applied horizontal force. The total horizontal resistance usually consists of

passive resistance of the soil and a frictional resistance F at the base, which is dependent upon