Soil improvement and construction methods in peat - Geotechnics of Organic Soils and Peat - page 148

Figure 6.24 A pile with structural and geological defects ( after Poulos, 2005).
Structural defects can result in the size, strength and/or stiffness of the pile being
less than assumed in design. Examples of structural defects include the following:
'necking' of the shaft of bored piles, leading to a reduced cross-sectional area along
part of the pile; poor quality control during the construction of bored piles, leading to
some parts of the shaft having lower strength than assumed in design; tensile cracking
of large diameter bored piles under the influence of thermal strains; damage during
the driving of timber, precast concrete or steel piles, leading to reduced strength and
stiffness of parts of the pile, especially near the top or tip of the pile; and bending of
slender driven piles (Poulos, 2005). Geotechnical defects usually arise from either a
misassessment of the in situ conditions during design, or else from construction-related
problems, and may include reduced shaft friction and end bearing resistance arising
from localized softer or weaker geotechnical conditions in the vicinity of one or more
of the piles in the group; reduced skin friction; and end bearing resistance arising from
construction operations such as the use of bentonite without due caution, and a 'soft
base' arising from inadequate cleaning of the base of bored piles (Poulos, 2005).
Figure 6.24 illustrates an example of the usual idealization of a pile which may
have both structural and geotechnical defects.
6.5.3 Piled raft foundation
A piled raft foundation may be described as a composite structure made up of piles
connected to the raft from underneath, exercising a variety of responsibilities jointly
and independently in providing safe transfer of bearing pressure to the underlying sub-
surface or sub-base structure. The raft and pile method (Figure 6.25) is an economical
ground improvement technique that has technical advantages derived from both raft
and pile foundations. The raft carries the embankment loading by distributing it partly