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2.3. DRIVING SHEET PILE WALLS 17
Base resistance R b (t)
Figure 2.12: Principle of vibratory driving
The acceleration and braking of the eccentric weights is critical in vibratory driving because
in doing so they pass through the low frequencies and thus excite the natural frequencies of
buildings (approx. 1–5 Hz) and suspended floors (approx. 8–15 Hz). These days, vibrators are
therefore in the position of being able to accept the maximum r.p.m. initially and then generate a
variable (from zero to maximum) imbalance moment by rotating the eccentric weights. Furthermore,
there are systems that permit online monitoring of the oscillation velocities at measuring
points close by. The vibrator operator, in conjunction with variable imbalance, is therefore in
the position of being able to react to unacceptably high oscillation velocities by changing the
imbalance amplitude or frequency.
2.3.5 Vibrations and settlement
The use of impact driving and vibratory driving causes ground vibrations that propagate in the
subsoil. Besides possibly causing damage to neighbouring buildings through vibrations, there
may be a risk of compacting the soil at some distance from the sheet pile, which can lead to
settlement. This risk is particularly problematic in the case of long-term, repetitive vibration
effects on buildings founded on loosely packed, uniform sands and silts. Liquefaction of the
soil is another risk: the build-up of pore water pressure due to dynamic actions causes the soil
to lose its shear strength briefly and hence its bearing capacity. Impact driving causes vibrations
in the ground which, however, quickly dissipate after each blow.
Vibrations in the ground propagate in the form of different types of waves. Fig. 2.13 shows