Geology Reference
In-Depth Information
are compared to the FoS, as determined using a traditional approach
-
best estimate of strength divided by best estimate of loading. It is to be
noted from this example that whichever approach, there is considerable
judgement and approximation involved. Shear strength is taken as
undrained, which is conceptually questionable for the long-term; adhe-
sion factor estimates range from 0.3 to 0.9 for different soils. If an
effective stress approach was adopted
-
as would generally be done for
sand and weathered rock
then estimates would be needed of stress
conditions and shaft resistance coef
-
cients, which also requires estima-
tion and judgement. Workmanship may also play a key role in whether
or not shaft friction will be mobilised and whether the base of a bored pile
excavation is properly cleaned out prior to concreting. The use of a
partial factors approach does concentrate on where the key unknowns
are (rather than geometry and
t take away the need
for proper ground characterisation, analysis and design judgement. The
xed loads) but doesn
'
fixed nature of the partial factors might seem rather prescriptive to cover
all soil, rock and founding situations. Selection of parameters, adhesion
and shaft resistance factors are reviewed well in GEO (2006), and the use
of Eurocode 7 for design is summarised by Bond & Simpson (2010).
A site-speci
c way to obtain design parameters, especially for large
projects, is to install test piles and measure their performance at perhaps
2.5 times the design load of the working piles. Test piles are often
instrumented along their length using strain gauges so that the actual
resistance being provided by the ground can be measured throughout
the full pro
le, and these parameters can be used in the design of other
piles. Traditionally, piles are loaded from the top using kentledge of
concrete blocks or steel
(Figure 6.9)
. Jacks are used to push the pile into
the ground whilst the kentledge provides the reaction. One of the
dif
culties of this is that much of the support comes from the upper
soil at early stages of the test, and there is little idea of how the toe is
Figure 6.9
Pile test
set up with
kentledge. Donghai
Bridge, China.
Figure courtesy of
Leonard Tang,
Halcrow.
