Page 155 - Numerical Analysis and Modelling in Geomechanics

136 C.L.RAMSHAW AND A.R.SELBY

Figure 5.3 Vibrations caused by vibrodriving.

More controversially, Hiller and Crabbe (2000) found little change in ppv with
hammer energy, which is usually quoted as energy per cycle, calculated as power
divided by cyclic frequency in units of joules/cycle.
Within this context of varied observations and non-uniformity of attenuation,
it is desirable to develop a computational procedure to analyse ground
vibrations.

Special hammers

In locations which are highly sensitive to vibrations, e.g. close to hospitals or
micro-electronics laboratories, special measures are required. Hydraulic jacking
systems are available for sheet piling, which push one pile into the ground using
the uplift resistance of adjacent piles and the weight of the plant for vertical
reaction. For bearing piles, pre-augering may be sufficient, otherwise alternative
systems may be required such as continuous flight auger or bored piles.

Sensitivity of humans and of buildings
The human body is highly sensitive to vibrations, and typically can detect
vibrations as low as 0.1 mm/s in the frequency range of 1–80 Hz, see BS6472
(1992). Sensitivity varies depending upon the body position, i.e. seated, lying
horizontally, or standing. Fingertip touch is more highly sensitive. Vibrations of
some 5–10 mm/ s may cause alarm or discomfort. Continuous vibration is less
tolerable than intermittent, and long duration further reduces tolerance. BS6472
gives recommendations in terms of vibration dose value, VDV, but more easy to
apply is the table from Eurocode 3, which is reproduced in summary in
Table 5.1.