Civil Engineering Reference
In-Depth Information
(d) Site-specific studies
In this case the model is intended to represent a particular construction so that the
behaviour of the model is used directly to assess the behaviour of the prototype. It is
obviously not easy tomodel all the details of the ground conditions and the construction
and loading sequence; these are the most difficult type of centrifuge models to construct
and test satisfactorily.
Model studies may be carried out for more than one purpose, for example combining
validation of analyses with parametric studies. In practice, designs are vary rarely
completed on the basis of model tests alone and model tests are almost always used in
conjunction with numerical analysis.
27.5 Geotechnical centrifuges
In a geotechnical centrifuge, a model in a strong container is rotated in a horizon-
tal plane about a vertical axis, as shown in Fig. 27.3. At the model the centrifugal
acceleration
a
is
2
r
a
=
ng
=
ω
(27.12)
ω
where
r
is the radius and
is the angular velocity (in radians per second). To maintain
a reasonably constant acceleration field through the model the radius
r
should be large
compared with the size of the model.
The essential features of a geotechnical centrifuge are illustrated in Fig. 27.4. The
motor drives a vertical shaft at constant speed. The arm has an adjustable counter-
weight for balance and the model sits on a swing. At rest the swing hangs down, but
as the arm rotates it swings up to a nearly horizontal position as shown. The purpose
of the swing is so that the self-weight of the model always acts towards the base of
the container; if you put a strong bucket containing water on the swing and start the
centrifuge the water will remain level in the bucket.
The selection of the dimensions and speed for design of a geotechnical centrifuge
is a matter of optimization between a number of conflicting requirements. A given
prototype size could be represented by a small model tested at high accelerations or
by a larger model at smaller accelerations; a given acceleration, or scale factor, can be
Figure 27.3
Centrifuge acceleration.
