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friction, with soil hanging onto the shaft. It reduces the total bearing capacity,
Q
c
=
Q
e
Q
s
, see Fig 12.4.
Q
c
Q
c
H
H
Q
s
Q
s
Q
e
Q
e
(a) positive skin resistance (b) negative skin resistance
Figure 12.4
The previous analysis, based on the theory of slip lines, is only applicable to
two-dimensional situations. A single pile is typically three-dimensional, and
resistance values may be higher than found in two-dimensional analysis. Similar
analysis in three dimensions seems not possible. Therefore, validation by field tests
and numerical simulation is inevitable.
Pile load testing
A reliable method is by static pile load tests. They are time consuming and
expensive, but the result is a reliable load settlement curve, e.g. shown in Fig 12.5a.
A common formula (hyperbolic type) approximating the curve is
49
w/
(
Rq
)
=
e
+ w
)
/R
(12.16)
Here,
w
is the settlement,
R
the equivalent radius, and
q
the ultimate vertical soil
resistance. The formula expresses that the secant modulus
w/
(
Rq
) increases linearly
with the relative settlement
w/R
. For small values of w
/R
the parameter
e
can be
obtained from the theory of elasticity, CPT or pressuremeter test. The parameter
q
u
=
1/
)
is obtained from a pile loading test. Sometimes
)
cannot be achieved and
R
2
is determined by an ultimate allowable settlement
then a limit pile load
Q = q
u
w
u
.
49
For a nice overview see: M. Jacobsen (1992)
Bearing capacity and settlements of piles
.
Aalborg University, Denmark.
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