Page 154 - Numerical Analysis and Modelling in Geomechanics

MODELLING OF GROUND WAVES 135
where ppv has units of mm/s, C is an empirical constant (= 0.5 in soft ground, 0.
75 in stiff soils, and 1.0 in hard ground), W is hammer energy per blow in joules,
and r (m) is horizontal distance from the source. This is not an exact estimate,
and considerable scatter of measured data is to be expected. A statistical
approach was evaluated by Attewell et al. (1992), in which curves of 0.5 and 1.0
standard deviations in excess of best fit curves on log-log scales were produced.
There is a clear need for computational analysis of this complex system of
ground vibrations, in order to clarify some of the issues observed in site data
measurements.

Vibratory hammers
Vibratory hammers or ‘vibrodrivers’ are now widely used to install steel piles
into granular soils. The strong vertical vibration of the whole pile causes
liquefaction of the soils close to the pile shaft and toe, which causes the pile to
sink into the ground under its self-weight plus the weight of the vibrodriver unit.
Vibrodrivers are much less effective in advancing a pile through cohesive soil
strata, which do not liquefy, but slow progress is possible as a result of the
hammering action. They are very effective in pile extraction when the support
crane applies an upward static force.
Vibrodrivers consist of eccentric contra-rotating masses which impose a
vertical cyclic force onto the pile head, but horizontal effects are self-cancelling.
The vibrator unit must be firmly clamped to the pile head, which is done by
hydraulic gripper, with adapter where necessary for steel tube or box piles.
Standard vibrodrivers cause the whole pile to oscillate vertically at some 25 Hz,
but higher frequency equipment (c. 40 Hz) is often preferred in an urban location,
to reduce the disturbance to buildings and occupants. A ‘resonant free’ system
has been developed recently to avoid the problem of pile-soil resonance at about
5–10 Hz during run-up to operating frequency and run-down to rest. The system
uses rotating discs which are in balance during run-up, and when operating speed
is reached, then eccentric masses are deployed.
Many site records of ground surface vibrations have been taken (e.g.
Uromeihy, 1990; Oliver and Selby, 1991; Hiller, 2000), all of which show a
characteristic pattern of strongly sinusoidal radial transverse and vertical
vibrations as a function of time, see Figure 5.3. As before, the three components
can be combined at instants in time to produce a vector trace using equation 5.2,
and a true peak particle velocity.
The signal generally attenuates with distance from the source, but with some
anomalies observed during site measurements. Ramshaw (2001) proposed that
the signal may include a standing wave component. Attewell et al. (1992)
produced curves based on log-log plots of amplitude/distance. Equation 5.3 gives
practical estimates of ppv, as quoted in BS5228 and Eurocode 3, but for all soils,
BS5228 recommends C=1.0, while Eurocode 3 proposes C=0.7.