Geoscience Reference
In-Depth Information
12 FOOTINGS AND PILE FOUNDATIONS
Foundations form the interface through which structural loads are transmitted to
the subsoil. In this chapter, the functioning of footings and single pile foundations
will be considered in more detail. The behaviour of a foundation depends on the
interaction with the structure, which it carries, and the nature of the soil in which it
stands. A foundation should give sufficient bearing capacity and limited flexibility,
i.e. limited settlements or other movements. It should be at a suitable depth, not
vulnerable to climate changes etc., and (shear) failure should be avoided. For
normal buildings 0.04 m (sand) to 0.06 m (clay) is reasonable 48 and differential
settlements should be not more than 50% of these. A common overall safety factor
is between 2 to 3.
A BEARING CAPACITY OF FOOTINGS
Footings, also referred to as shallow or spread foundations, include isolated slabs
or pads, strip footings and rafts; their width is typically larger than their height. The
ultimate bearing capacity of footings is found when a potential slip pattern or a
continuous plastic zone has developed. Consider a simple example, where a strip
loading with width B acts on undrained subsoil (
| < c u ). An admissible
stress field, in equilibrium and everywhere at yield, is composed of a vertical zone
of width B under the strip loading with principal stresses according to (
= 0, |
v ,
h ) =
(4 c u , 2 c u ) and the soil zones outside with (
h ) = (0 , 2 c u ), see Fig 12.1a. Hence,
if the loading is q = 4 c u , this stress field satisfies the boundary conditions. The
ultimate bearing capacity is therefore 4 c u . It is a lower limit; the real bearing
capacity of the soil could be higher.
v ,
c u
c u
q
q
4c u
(2+
)c u
B
B
B
s
R
q
q
q
)
1
3
3
1
2
c u
c u
(a) admissible stress field (b) potential failure mode (c) Prandtl's solution
Figure 12.1 Ultimate bearing capacity for a strip loading on undrained soil
48
In practice, maximum settlements of 0.15 to 0.20 is used for high-rise buildings.
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