DHARM542 GEOTECHNICAL ENGINEERINGSafe bearing capacity: Ultimate bearing capacity divided by the factor of safety. The
factor of safety in foundation may range from 2 to 5, depending upon the importance of the
structure, and the soil profile at the site. The factor of safety should be applied to the net
ultimate bearing capacity and the surcharge pressure due to depth of the foundation should
then be added to get the safe bearing capacity.
It is thus the maximum intensity of loading which can be transmitted to the soil without
the risk of shear failure, irrespective of the settlement that may occur.
Allowable bearing pressure: The maximum allowable net loading intensity on the soil at
which the soil neither fails in shear nor undergoes excessive or intolerable settlement, detri-
mental to the structure.14 .2 Bearing Capacity
The conventional design of a foundation is based on the concept of bearing capacity or allow-
able bearing pressure.14.2.1Criteria for the Determination of Bearing Capacity
The criteria for the determination of bearing capacity of a foundation are based on the require-
ments for the stability of the foundation. These are stated as follows:
(i) Shear failure of the foundation or bearing capacity failure, as it is sometimes called,
shall not occur. (This is associated with plastic flow of the soil material underneath the foun-
dation, and lateral expulsion of the soil from underneath the footing of the foundation); and,
(ii) The probable settlements, differential as well as total, of the foundation must be
limited to safe, tolerable or acceptable magnitudes. In other words, the anticipated settlement
under the applied pressure on the foundation should not be detrimental to the stability of the
structure.
These two criteria are known as the shear strength criterion, and settlement criterion,
respectively. These are independent criteria and hence require independent investigation.
The design value of the safe bearing capacity, obviously, would be the smaller of the two values,
obtained from these two criteria. This has already been defined as the allowable bearing
pressure.
14.2.2 Factors Affecting Bearing Capacity
Bearing capacity is governed by a number of factors. The following are some of the more
important ones which affect bearing capacity:
(i) Nature of soil and its physical and engineering properites;
(ii) Nature of the foundation and other details such as the size, shape, depth below the
ground surface and rigidity of the structure;
(iii) Total and differential settlements that the structure can withstand without func-
tional failure;
(iv) Location of the ground water table relative to the level of the foundation; and
(v) Initial stresses, if any.