DHARMBEARING CAPACITY 543In view of the wide variety of factors that affect bearing capacity, a systematic study of
the factors involved in a logical sequence is necessary for proper understanding.14.3 Methods of Determining Bearing Capacity
The following methods are available for the determination of bearing capacity of a foundation:
(i) Bearing capacity tables in various building codes
(ii) Analytical methods
(iii) Plate bearing tests
(iv) Penetration tests
(v) Model tests and prototype tests
(vi) Laboratory tests
Bearing capacity tables have been evolved by certain agencies and incorporated in build-
ing codes. They are mostly based on past experience and some investigations.
A number of analytical approaches, based on the work of Rankine, Fellenius, Housel,
Prandtl, Terzaghi, Meyerhof, Skempton, Hansen and Bella may be used. Some of these would
be dealt with in later sections.
Plate bearing tests are load tests conducted in the field on a plate. These involve effort
and expense. There are also certain limitations to their use.
Penetration tests are conducted with devices known as ‘Penetrometers’, which measure
the resistance of soil to penetration. This is correlated to bearing capacity.
Model and prototype tests are very cumbersome and costly and are not usually practica-
ble. Housel’s approach is based on model tests.
Laboratory tests which are simple, may be useful in arriving at bearing capacity, espe-
cially of pure clays.
14.4 Bearing Capacity from Building Codes
The traditional approach to the bearing capacity problems is illustrated by the building codes
of many large cities, such as New York and Boston. Practically, all codes give lists of soil types
and the respective safe or allowable bearing capacity. Some values may be subject to modifica-
tions under designated conditions. It is presumed that the soil can support the indicated pres-
sure with safety against shear failure and without undue settlement. This is perhaps the basis
for the widespread but incorrect notion that the bearing capacity depends mainly on the char-
acteristics of the soil in question.
Actually the bearing capacity depends on a number of factors as stated in a previous
section and this should never be lost sight of. Thus, the value stated for the bearing capacity is,
at best, a rough estimate, based on past experience of construction in the area, rather than a
sound basis for design. For the general use of buildings, perhaps the tabular values, which are
valid under a definite set of simple and easily defined conditions, may be modified for known
departures from the specified conditions.
The tabular values of bearing capacity are also known as ‘‘Presumptive bearing capaci-
ties’’ and are included in several Civil Engineering Handbooks. The ISI have specified these