Civil Engineering Reference
In-Depth Information
sound-proofing of the machine's engine enclosure, but the process is generally slower
than using a top hammer configuration and therefore is more costly.
Excessive noise presents a health hazard by inducing premature deafness, although
this is more a problem for the site operatives than for neighbouring residents (Noise
at Work Regulation, 1989). Operatives now wear ear defenders as a matter of routine
on sites.
It is widely recognized that noise and vibration, although related, are not amenable
to similar treatment. In the main, noise from site is airborne and consequently the
prediction of noise levels is relatively straightforward, given the noise characteristics
and pattern of use of the equipment. On the other hand, the transmission of vibration
is largely determined by site soil conditions and the particular nature of the struc-
tures involved. There are cases cited in which vibrations have not materially affected
immediately adjacent structures, but have affected structures at a greater distance from
the source. Some general guidance may be derived from the study of case histories of
similar situations (Construction Research and Information Association, in prepara-
tion; Attewell and Farmer, 1973). Useful references on the whole subject of ground
vibrations are provided also by Skipp (1984) and by Building Research Establishment
Digest No. 353 (1990).
Piling processes produce vibrations of two basic types, namely free and forced vibra-
tions. Free vibrations are normally associated with impact loading, such as when
a hammer strikes a pile or a steel casing, and the frequency is then determined by
the pile, soil and structure characteristics. Forced vibrations occur when a vibratory
tool, such as a heavy casing vibrator, is used to produce continuous vibrations with
well defined frequency characteristics. Continuous forced vibrations are more likely
to set up resonant conditions in structural members of certain dimensions, which
in consequence may vibrate at much greater amplitudes than other non-resonant
members.
In general human perception of vibrations occurs at levels that are low in comparison
with thresholds of risk for structural damage. The British Standards Institution has
issued a 'Guide to the evaluation of human exposure to vibration in buildings (1 Hz to
80 Hz)' (BS 6472 : BSI (1992)) which sets out acceptable criteria for vibration levels
in various different types of accommodation. The vast majority of piling processes
currently in use would give rise to vibrational energy within the stated 1 to 80 Hz
frequency range.
The International Standards Organization is working on criteria for structural
damage caused by vibration, measured at foundation level in terms of peak particle
velocities, for various categories of structure, above which there is initially a risk of
architectural damage in the form of minor cracks in plaster, etc. Compliance with
the proposed criteria can normally be achieved, and where structural damage occurs
below the relevant thresholds, it is usually because some other cause is also present.
It is frequently feared by adjacent building occupiers that vibrations caused by piling
will adversely affect computer operation on their premises. This matter is dealt with
in an article by Boyle (1990).
Apart from the direct effect of transmitted vibrations, it is also possible to initiate
settlement of the ground by reason of the compaction effect on loose soils such as
fine sands or silty sands. However other materials also deform as a result of repeated
loading.
