Geology Reference
In-Depth Information
Figure 6.7
Reinforcing cage
for bored pile with
included tubes to
allow proof drilling
through the toes of
the completed pile
and cross-hole
geophysical testing
to prove integrity,
Hong Kong.
6.4.3.2 Design
Piles are designed to suit the ground pro
le. If rockhead is at relatively
shallow depth and the overlying soil does not contain boulders that
could cause dif
culties, then driven piles might be adopted, end bear-
ing onto the rock (Figure 6.8a) . At Drax, the piles were driven to found
several metres into dense sand overlying sandstone, thereby picking up
some skin friction as well as end bearing (Figure 6.8b) . If there is no
rock, then the piles will need to gain their resistance mostly from skin
friction in the soil. For example, the Sutong Bridge across the Yangtze
River, China, which is (in 2011) the longest cable-stayed bridge in the
world, with a main span of 1,088m, is founded on bored piles taken to
117m and relying upon skin friction from alluvial sediments
(Figure 6.8c) .
Ways to estimate skin friction parameters and end-bearing resis-
tance are given in textbooks such as Tomlinson (2001) and might be
governed by standards such as AASHTO (2007), used as the basis for
design of the 2 nd Incheon crossing completed in 2009. The principles
are quite simple: skin friction is calculated as soil shear strength times
some adhesion factor multiplied by the surface area of the pile shaft.
End bearing is often calculated as an empirical value for the soil or rock
quality multiplied by the basal area of the pile. At some sites, the
bottom end of the pile is enlarged by under-reaming to increase the
end-bearing contribution, although sometimes the dif
culty of this
operation is hardly justi
ed by the increase in pile capacity that
might ensue.
A worked example of pile design to Eurocode 7, using partial
factors speci
ed uniquely for the UK (to correlate with traditional
design experience),
is presented in Box 6-3, based on one
 
 
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