Civil Engineering Reference
In-Depth Information
compressibility of the pile. In the extreme case, piles may be so long that no load at
all reaches the base under working conditions. Working settlements will probably be
sufficient to mobilize full shaft friction in the upper part of the pile shaft (Randolph,
1983), affecting the load settlement characteristics and bringing the possibility of
degradation of the shear transfer between pile and soil.
The following sections discuss methods of estimating the ultimate capacity of piles in
different soil types. In all cases, the capacity may be assumed to be the sum of shaft and
base capacities (see equation (4.1)). Methods of estimating values of the end-bearing
pressure,
q
b
, and the shaft friction,
τ
s
, will be broadly divided into two categories,
those based on fundamental soil properties such as shear strength or angle of friction,
and those based directly on in-situ measurements such as standard penetration tests
(SPTs) or cone penetration tests (CPTs).
It should be emphasized that estimation of pile capacity is still largely based on
empirical methods, derived from correlations of measured pile capacity with soil
data of variable quality. There is generally a wide scatter in the correlations, and
different approaches suit different soil types better than others, albeit still with a
significant margin of error. Many soils do not fit neatly into the categories discussed in
the following sections of non-cohesive (implying free-draining, granular), or cohesive
(implying less permeable, fine-grained) soil, and of rock. Pile design must thus be seen
as a compromise, adopting the most appropriate method available for the particular
soil types and site conditions prevailing.
Where a pile penetrates layers of differing soil types, the shaft capacity in each layer
is cumulative, with minimal 'interaction' effect between adjacent layers. However,
experimental evidence from instrumented piles (Lehane
et al.
, 1993; Jardine and Chow,
1996) has demonstrated significant degradation in local values of shaft friction during
the installation of jacked or driven piles. Hence the relative position along the pile
shaft can have an important bearing on the design value of shaft friction.
In a given soil, design values of shaft friction and end-bearing pressure will vary
for different types of pile. While it is possible to identify trends of these parameters
with broad categories of construction technique (driven precast piles, bored piles,
continuous flight auger (CFA) piles), the actual values will also be affected by the con-
struction parameters for any given pile. Thus, excessive hard driving will lead to greater
degradation of shaft friction, as will excessive auger-turns for a CFA pile (so-called
'overflighting' resulting in overexcavation of soil). For bored piles constructed under
bentonite, the shaft friction can be severely reduced if the quality of the bentonite is
allowed to deteriorate, or if an excessive head of bentonite leads to caking of mud
on the borehole wall. It is important, therefore, to ensure good quality control during
construction of piles.
4.1.1 Capacity in non-cohesive soils
4.1.1.1 End-bearing pressure
It might be expected that the end-bearing pressure beneath a pile in a uniform deposit
of non-cohesive soil would be directly proportional to the local vertical effective stress,
and thus would increase approximately proportionally with depth. However, extensive
research by Vesic (1977) showed that the end-bearing pressure appeared to reach a
