FOOTINGS AND PILE FOUNDATIONS - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 191

groundwater plays a specific role, i.e. some soils swell and not necessarily
uniformly, and rising groundwater decreases the bearing capacity. To establish the
allowable bearing capacity special in-situ testing methods are in use, e.g. plate
bearing test, California Bearing Ratio test (CBR), standard penetration test (SPT),
and the cone penetration test (CPT), see Chapter 3.

B BEARING CAPACITY OF PILE FOUNDATIONS ( ULTIMATE LIMIT STATE )
Piles are structural elements composed of timber, concrete and/or steel, which
transmit a load Q to the soil, by skin friction (floating pile) and/or the pile point
(end-bearing). The load can be vertical, horizontal, or inclined, either compressive
or tensile (tension pile). Buckling of an installed slender pile is prevented by
adjacent soil support, but offshore, when a pile reaches to the sea bottom, over the
free-standing part buckling has to be considered. Timber piles, already in use for
centuries, with a load capacity of 100 to 250 kN and a length of 10 to 25 m, are
relatively cheap, well preserved under groundwater level, best suited as friction
pile but vulnerable under hard driving conditions. Steel piles may carry 300 to
1000 kN with practically unlimited length, best suited as end-bearing pile, easy to
splice but vulnerable to corrosion. Steel pipe piles, sometimes filled with concrete
against bending, may carry 10 MN and more. Precast prestressed concrete piles,
with a load capacity of 300 to 8000 kN and a length of 20 to 30 m, are not easy to
splice, relatively expensive but resistant to hard driving conditions. Cast in-situ
concrete piles may reach 10 to 25 m and carry up to 1000 kN. Here, quality control
during construction is a major concern.
There are many different installation methods, e.g. driving by percussion
(hammer blows), vibrating, drilling, casing, boring, grouting, jacking, and jetting,
which all have their specific effect on the bearing capacity. In the case of pile
groups and pile rafts the mutual interaction of piles during installation and during
functioning has to be taken into account (Randolph, Katzenbach).
Pier foundations and caisson foundations are more or less similar to piles, but
their way of installation is different. The soil at the toe is removed through the
inside, and the foundation sinks gradually by its weight. This is a very old method,
used to make water wells and e.g. the famous monument for queen Mumtaz, the
Taj Mahal, situated on soft river sediments, is founded on this type of foundation
(see Chapter 1).
Methods for estimating the bearing capacity of piles can be divided into
analytical methods (stress state), on pile load testing, or on empirical methods
based on in-situ measurement such as CPT. Sophisticated numerical methods are in
development and not in use in practice, because numerical methods do not cover
the effects of pile installation to the soil condition with proper accuracy.

Analytical approach
For an estimate of the ultimate end-bearing capacity Q e of a pile wall with width
B , length L and depth H a slip mechanism is elaborated (for a two-dimensional
situation), as shown in Fig 12.3, for drained soil. Zone 1 (point 1) is an active
Rankine state with ) and a vertical stress q 1 = Q e /BL . From there

on, the boundary of the plastic zone develops with the spiral curve s , according to r

1 = (¼ + ½