Geotechnical Engineering

DHARM

SHALLOW FOUNDATIONS 619

Min. 60 cm (rock)

90 cm (soil)

Frost depth

Fig. 15.13 Edge distance for floating on sloping ground

5. The difference in elevation between footings should not be so great as to introduce
   undesirable overlapping of stresses in soil. The guideline used for this is that the maximum
   difference in elevation should be maintained equal to the clear distance between two footings
   in the case of rock and equal to half the clear distance between two footings in the case of soil
   (Fig. 15.14). This is also necessary to prevent disturbance of soil under the higher footing due
   to the excavation for the lower footing.

a

b

b a/2 for footings on soil

b a for footings on rock

Fig. 15.14 Footings at different elevations—restrictions

15.4.3Bearing Capacity of Soils Under Footings
Granular Soils
Bearing capacity of granular soils depends upon the unit weight γ and the angle of internal
friction φ of the soil, both of which vary primarily with the density index of the soil. Dense soils
have large values of γ and φ and consequently high hearing capacity. Loose soils, on the other
hand, have small γ and φ values and low bearing power.

The density index of granular soils in-situ is generally determined by standard penetra-
tion tests. The relationship between N-values and φ-values established empirically by Peck,
Hanson and Thornburn may be used, and later the relevant Terzaghi equations may be ap-
plied to get the bearing capacity.

In conventional design, the allowable bearing capacity should be taken as the smaller of
the following two values:

(i)Bearing capacity based on shear failure: This is the ultimate bearing capacity di-
vided by a suitable factor of safety; usually a value of 3 is used for normal loading and 2 for