Geotechnical Engineering

DHARM

840 GEOTECHNICAL ENGINEERING

(b) Forced Vibration test. A mechanical oscillator is mounted on a concrete block in such
a manner that it induces pure vertical vibrations. The response is obtained from a pick-up
mounted on top for various frequencies of excitation till the “resonance” is passed through. A
graph is drawn between amplitude and frequency of excitation (Fig. 20.25). The frequency fn
corresponding to the peak amplitude represents the “resonant frequency”.

The damping ratio can now be obtained from the relation:

D =

∆f

2 fn

...(Eq. 20.77)

zmax

Ö—–z^12 max

Df

Df=f –f 21

f 1 fn f 2

Frequenccy

Amplitude

Fig. 20.25 Response curve under forced vibration

where ∆f is the frequency intercept between the two points on the response curve at which the

amplitude is 0.707 (or 12/ ) times the peak amplitude and fn is the resonant frequency.

It may be noted that laboratory tests inducing vibrations in a small specimen may also
be used to arrive at the dynamic soil parameters. But a fuller description of all such tests is
considered unnecessary for the present treatment.

20.4 MACHINE FOUNDATIONS—SPECIAL FEATURES

Machine foundations, being of a special kind, fall into a separate class of their own. For exam-
ple, the general criteria for ensuring stability of a machine foundation are rather different
from those for other foundations. Also the design approach and methods of analysis are totally
different in view of the dynamic nature of the forces. The types of machine foundations are
also different.
Responsibility for satisfactory performance of a machine is divided between the ma-
chine designer, who is usually a mechanical engineer, and the foundation designer, who is
usually a civil engineer, more specifically a geotechnical engineer. The latter’s task is to design
a suitable foundation consistent with the requirements and tolerance limits imposed by the
machine designer. It is therefore imperative that the machine designer and the geotechnical
engineer work in close co-ordination right from the stage of planning until the machinery is
installed and commissioned for its intended use.
Until recently, design of machine foundations has been mostly based on empirical rules,
before the evolution of Soil Dynamics as a discipline. With the developments in the fields of
structural and soil dynamics, sound principles of design were gradually established.