Geoscience Reference
In-Depth Information
MPa
ground surface
20
10
40
50
0
Blows/25cm
sand
Blowcount
SPT
CPT
CPT after driving
peat
5
clay
1
0
sand
15
clay
20
¼ ½
pile toe at 21.80 m
(a) survey methods and driving data (b) dynamic energy dissipation (Hölscher)
Figure 12.6 Energy dissipation during pile driving
sand
In general dynamic measurements are not a reliable substitute for static pile load
tests. On a specific job site, however, a dynamic analysis (CAPWAP, PDA) can be
efficient, when a calibration with local static tests is possible. The STATNAMIC
method is in principle a dynamic impact pile testing method, but it succeeds in
drawing out the length of the impact far beyond other dynamic methods, and can
yield reliable estimates of pile capacity in many cases. Very large units are
available, for high pile capacities, up to 30 MN.
C
DEFORMATION OF FOUNDATIONS
(
SERVICEABILITY LIMIT STATE
)
Besides a failure criterion, the functioning of a foundation is also determined by
the deformation. The serviceability may be insufficient, if deformations exceed a
certain threshold, conditioned by the structure itself or the environment, e.g.
adjacent infrastructure and buildings. Deformations of the subsoil induced by a
loaded foundation are controlled by the corresponding induced stresses, which
involves the entire area and requires boundary conditions, equilibrium conditions
and compatibility conditions. The stiffness of the structure plays a role and the
theory of a flexible plate on elastic subsoil may provide an answer. However, their
use is limited (valid for relatively small strains), since soil behaviour is markedly
non-linear. For a realistic non-linear analysis sophisticated computational models
are available, which can simulate multi-dimensional complex situations.
A classic solution is the case of a single force
Q
on semi-infinite subsoil,
attributed to Boussinesq. The axi-symmetrical stress field due to the load is
described by the following set, with
r
the radial coordinate,
z
the vertical
coordinate,
R
2
= r
2
+ z
2
, and
the Poisson ratio
3
3
Qz
v
=
(12.21a)
5
2
R
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