Civil Engineering Reference
In-Depth Information
Chapter 10
Pile Foundations
10.1
Introduction
Pile foundations are used to transfer the load of the structure to the bearing soil or rock located at a significant
depth below ground surface. Piles are long and slender members that transfer the load to soil of high bearing
capacity that lie beneath shallower soils of lower bearing capacity. Piles may be made from concrete, steel or
timber, or from some composite of these materials and are installed either by driving, drilling or jacking. A pile
cap is fixed to the top of the pile and it is this cap onto which the structural loads are transmitted.
In addition to piles being used to transmit the foundation load to a solid stratum (by end bearing) or
throughout a deep mass of soil (through soil-pile friction), pile foundations are also used to resist horizontal
or uplift loads where such forces may act.
There are several types of pile and these are described in the coming sections.
10.2 Classification of piles
Piles can be classified by different criteria, such as their material (e.g. concrete, steel, timber), their method
of installation (e.g. driven or bored), the degree of soil displacement during installation, or their size (e.g.
large diameter, small diameter). However, in terms of pile design, the most appropriate classification
criterion is the behaviour of the pile once installed (e.g. end bearing pile, friction pile, combination pile).
10.2.1 End bearing
These piles transfer their load to a firm stratum located at a considerable depth below the base of the
structure and they derive most of their carrying capacity from the penetration resistance of the soil at the
toe of the pile (Fig.
10.1a
). The pile behaves as an ordinary column and should be designed as such. Even
in weak soil a pile will not fail by buckling and this effect need only be considered if part of the pile is
unsupported, i.e. if it is in either air or water.
10.2.2 Friction
Here, the carrying capacity is derived mainly from the adhesion or friction of the soil in contact with the
shaft of the pile (Fig.
10.1b
).
10.2.3 Combination
This is an extension of the end bearing pile when the bearing stratum is not hard, such as a firm clay. The
pile is driven far enough into the lower material to develop adequate frictional resistance (Fig.
10.1
c). A further
variation of the end bearing pile is piles with enlarged bearing areas. This is achieved by forcing a bulb of
concrete into the soft stratum immediately above the firm layer to give an enlarged base. A similar effect is
produced with bored piles by forming a large cone or bell at the bottom with a special reaming tool.
