Civil Engineering Reference
In-Depth Information
Piled foundations
23.1 Types of piled foundations
Piles are long slender columns installed into the ground, often in groups. The principal
purpose of piling is to transfer loads to stronger and stiffer soil or rock at depth, to
increase the effective size of a foundation and to resist horizontal loads. Typically piles
are made from steel or reinforced concrete and possibly timber. They may be driven
or pushed into the ground or concrete piles may be cast
in situ
by pouring concrete
into a drilled hole.
Some typical pile types are illustrated in Fig. 23.1. Figure 23.1(a) shows an end
bearing pile where most of resistance is developed at the toe and Fig. 23.1(b) shows a
friction pile where a significant contribution to the pile capacity is developed by shear
stresses along the sides. Figure 23.1(c) shows raking piles to resist horizontal loads and
Fig. 23.1(d) is a pile group joined at the top by a pile cap. Notice that the pile on the
left in Fig. 23.1(c) is in tension and so all the resistance comes from shear stress on the
sides of the pile.
Figure 23.2 shows the loads on a single pile: the applied load
Q
is resisted by a force
at the base
Q
b
and a force
Q
s
due to the shear stresses between the soil and the pile
shaft; hence
Q
=
Q
s
+
Q
b
(23.1)
In conventonal pile analysis the weight of the pile is taken to be the same as the
weight of soil displaced by the pile and both are neglected. In any case these forces
are usually small compared with the applied loads, which are typically in the range
500 to 5000 kN and may be considerably larger. Figure 23.2(b) illustrates the increase
in base resistance and shaft friction with displacement. The shaft friction increases
more quickly than the base resistance and reaches an ultimate state at relatively small
strains.
Piles or pile groups may be loaded drained or undrained and the basic total and effec-
tive stress paths will be similar to those for shallow foundations, shown in Fig. 22.5.
Generally, piles installed in a clay soil will settle with time as the excess pore pressures
generated by undrained loading dissipate and the effective stresses and strength of the
soil increases. There may, however, be stress changes caused by installation, which
would cause swelling and softening of the soil around a bored and cast
in situ
pile or
compression and consolidation around a driven pile.
