Geotechnical Engineering

DHARM

858 GEOTECHNICAL ENGINEERING

Table 20.7 Minimum thickness of foundation (after Major, 1962)

Weight of hammer (kN) Minimum thickness of foundation (m)

upto 10 1.00

20 1.25

40 1.75

60 2.25

> 60 > 2.25

20.7 VIBRATION ISOLATION

If a machine is rigidly bolted to the floor, the vibration of the machine itself may be reduced,
but that transmitted to the floor and soil will be large, producing harmful effects even at large
distances. On the other hand, if an elastic support of sufficient flexibility is provided under the
machine or its foundation, the vibration transmitted to the floor and soil will be reduced, but
this may cause significant vibration to the machine itself. A judicious compromise is, there-
fore, to be struck; this is achieved usually through an appropriate frequency ratio, by adjust-
ing the natural frequency of the machine foundation to a suitable value.

To avoid excessive vibration due to the working of a machine, the following points should
be considered in the planning stage:

(i)Selection of Site: The machinery should be located far away from the area means for
precision work.
(ii)Balancing of dynamic loads: The machine should be dynamically balanced to limit
the unbalanced forces produced during its operation.

(iii)Adopting suitable foundations: The foundation for the machine should be designed
using accepted criteria, after evaluating the necessary design parameters at the site.

(iv)Providing isolation: Machine foundations should be completely separated from ad-
joining floors and other components by providing suitable isolating layers in between.

20.7.1Types of Isolation—Transmissibility

Two types of vibration problems are encountered in practice from the point of view of isolation.
The first is the one in which isolation is required against vibration caused by the machine
itself, and is called “Active Isolation”. The second is the one in which the foundation for a
delicate machinery is designed in such a way that the amplitude of its motion due to floor
vibration, caused by a disturbing force in the vicinity, is reduced to an acceptable limit; this is
called “Passive Isolation”.

The schematic for active isolation and the mathematical model for it are shown in
Fig. 20.32 (a) and (b):

Active isolation is also called ‘force isolation’, since the attempt here is to reduce the
force transmitted by the machine to the foundation in order to prevent vibration of adjacent
machines and structures.

The schematic for passive isolation and the corresponding mathematical model are shown
in Fig. 20.32 (a) and (b).