Civil Engineering Reference
In-Depth Information
Pile foundation
Piles are used to transfer structural loads to either the foundation soil or the bedrock underlying the site.
They are usually designed to work in groups, with the column loads they support transferred into them
via a capping slab. Pile foundations are covered in Chapter
10.
Pier foundation
This is a large column built up either from the bedrock or from a slab supported by piles. Its purpose is
to support a large load, such as that from a bridge. A pier operates in the same manner as a pile but it
is essentially a short squat column whereas a pile is relatively longer and more slender.
Shallow foundation
A foundation whose depth below the surface, z, is equal to or less than its least dimension, B. Most strip
and pad footings fall into this category.
Deep foundation
A foundation whose depth below the surface is greater than its least dimension. Piles and piers fall into
this category.
9.3 Ultimate bearing capacity of a foundation
The ultimate bearing capacity of a foundation is given the symbol q
u
and there are various analytical
methods by which it can be evaluated. As will be seen, some of these approaches are not all that suitable
but they still form a very useful introduction to the study of the bearing capacity of a foundation.
9.3.1 Earth pressure theory
Consider an element of soil under a foundation (Fig.
9.1)
. The vertical downward pressure of the footing,
q
u
, is a major principal stress causing a corresponding Rankine active pressure, p. For particles beyond
the edge of the foundation this lateral stress can be considered as a major principal stress (i.e. passive
resistance) with its corresponding vertical minor principal stress
γ
z (the weight of the soil).
Now
′
1
1
−
sin
sin
φ
φ
p
=
q
u
′
+
z
q
u
γ
z
p
p
Fig. 9.1
Earth pressure conditions immediately below a foundation.
