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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