Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 5 BEARING CAPACITY THEORY
5.1 SHALLOW FOUNDATION
5 BEARING CAPACITY THEORY
Shallow foundations, are generally more economical than deep foundations if they do not have to
be installed deep into the ground and extensive ground improvement works are not required. They
are often used to support structures at sites where ground are sufficiently strong. Unless a shallow
foundation can be founded on strong rock, some noticeable settlement will occur. Design of
shallow foundations should ensure that there is an adequate factor of safety against bearing failure
of the ground, and that the settlements, including total and differential settlement, are limited to
allowable values.
For shallow foundations founded on granular soils, the allowable load is usually dictated by the
allowable settlement, except where the ultimate bearing capacity is significantly affected by
geological or geometric features. Examples of adverse geological and geometrical features are
weak seams and sloping ground respectively. For shallow foundations founded on fine-grained soils,
both the ultimate bearing capacity and settlements are important design considerations.
High-rise structures or the presence of weak ground bearing materials do not necessarily stopping
the design engineer from adopting shallow foundation system. Suitable design provision or ground
improvement could be considered to overcome the difficulties. Some examples are given below:
a. Design the foundations, structures and building services to accommodate the expected
differential and total settlements.
b. Excavate weak materials and replace them with compacted fill materials.
c. Carry out in-situ ground improvement works to improve the properties of the bearing materials.
Some of these methods are discussed in Chapter 9.
d. Adopt specially designed shallow foundations, such as compensated rafts, to limit the net
foundation loads or reduce differential settlement.
5.1.1 Bearing Capacity of Shallow Foundation
5.1.1.1 General
There are a many of methods for determining the bearing capacity of shallow foundations on soils.
A preliminary estimate of allowable bearing pressure may be obtained on the basis of soil
descriptions. Other methods include correlating bearing pressures with results of in-situ field tests,
such as SPT N value and tip resistance of CPT. For example, Terzaghi & Peck (1917) proposed
allowable bearing pressure of 10 N (kPa) and 5N (kPa) for non-cohesive soils in dry and submerged
conditions respectively. This was based on limiting the settlement of footings of up to about 1 m
wide to less than 25 mm, even if it is founded on soils with compressible sand pockets.
Methods based on engineering principles can be used to compute the bearing capacity of soils and
estimate the foundation settlement. This would require carrying out adequate ground investigation
to characterize the site, obtaining samples for laboratory tests to obtain parameters and establishing
a reliable model. Designs following this approach normally result in bearing pressures higher than
the presumed allowable bearing pressures given in codes of practice.
March 2009 5-1