Civil Engineering Reference
In-Depth Information
Geotechnical centrifuge modelling
27.1 Modelling in engineering
Engineers frequently use scale models in conjunction with theoretical analyses. For
example, wind tunnel modelling is used routinely by engineers to study the flow of
air past vehicles, aircraft and buildings. Hydraulic engineers frequently use models
to study the flow of water in river channels, tidal flow in estuaries and wave loading
on structures. Scale modelling is used most often when the theoretical solutions con-
tain major simplifications and approximations or when numerical solutions are very
lengthy, as is often the case in geotechnical engineering.
A geotechnical model might be tested when it would be too difficult, expensive or
dangerous to build and test a full-scale structure. For example, it would be very difficult
to test the response of a large earth-fill dam to earthquake loading and it would be very
dangerous to examine the collapse of a tunnel heading during construction. Usually a
model will be smaller than the prototype (or full-scale) structure that it represents.
The principles for modelling fluid flows are well established and so too are the
principles for geotechnical modelling. To achieve correct scaling in geotechnical models
the unit weight of the soil is increased by accelerating the model in a geotechnical
centrifuge.
At present modelling is used less frequently in geotechnical engineering than in other
branches of civil engineering but it is an important and valuable technique and one that
you should know about. Detailed discussion of geotechnical centrifuge modelling is
obviously beyond the scope of this topic and what I want to do in this chapter is simply
to set out the basic principles and to describe the principal purposes of modelling.
27.2 Scaling laws and dimensional analysis
Normally a model and the prototype will be geometrically similar so that all the linear
dimensions in a model will be scaled equally but, for various reasons, it is impossible
to construct a model that behaves exactly like a large prototype in all respects. (You
have probably noticed that the waves made by a model sailing boat are different from
the waves made by a full-sized yacht.) Instead, the model should have similarity with
the prototype in the aspect of behaviour under examination. For example, in a wind
tunnel model of an aircraft wing the relationships between lift, drag and velocity
should be similar while in a river model the relationships between water depths and
velocities should be similar, but neither model need look very much like the prototype
