Geoscience Reference
In-Depth Information
q u
1/
)
e
1.0
e
q
w/Rq
s
secant
w u /R
1
)
100 k P a
w/R
w/R
c u
(a) load settlement curve (b) API-code
Figure 12.5 Pile load testing
Empirical methods
A suitable field test to determine the ultimate end bearing of a pile is the CPT
test, where the cone shows some similarity with a pile, at a smaller scale (when
pushed, not hammered!). A simple approximation is q u = 0.1q c . A practical method
using the variation of the CPT (see Fig 3.1) was developed in Delft (NEN 6743,
EC7, part 3), where the end bearing capacity Q e is determined by the following
formula 50
R 2 ) = q e = c e ( q c 1 + q c 2 + 2 q c 3 )/4
Q e / (
(12.17)
Here, q c 1 is the smallest CPT-value obtained in a zone between 1.4 R to 8 R under
the pile toe (related to the possibility of punching through the bearing layer), q c 2 is
the successive smallest CPT-value over that zone, and q c 3 is the successive smallest
CPT-value in the zone above the pile toe up to 16 R . The coefficient c e is related to
the amount of soil displacement/replacement, see Table 12.2. New insight (Van
Tol, Xu and Lehane) suggests using for c e a value of 0.6 for percussion driven
piles.
In case no negative skin resistance is expected to develop with time one may
take for the shaft resistance as a first estimate not the sleeve resistance q f but a
percentage c s (see Table 12.1) of the CPT value q c , according to
Q s / (2
RH ) =
%
i ( c s q ci h i /H )
(12.18)
Here, h i is the thickness of a specific soil layer. Takesu e.a. suggested a relation
with pore pressure u measured during the CPT, according to c s = 0.8 u + 0.75 for u
< 0.3 MPa and c s = 5.0 u
1.5 for u > 0.3 MPa.
50
Different methods are suggested in France (Bustamante, Frank), in Belgium (de Beer), in
the UK: the ICP method (Jardine et al.), and in Australia (Xu and Lehane). The results of
these methods may vary significantly and the uncertainty is at least 30%.
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