If the peak acceleration figure is too low, a vibratory with a higher dynamic force must be

chosen and the peak acceleration recalculated. This iteration must continue until the

peak acceleration value is achieved.

(4) Next, the required amplitude of the vibratory system is computed using the equation

A = 2 K y / Wdyn

where A = total cycle displacement amplitude, inches

K = eccentric moment, inch-pounds

The preferred amplitude values are shown in table 2-4b; combinations of pile, soil, and

frequency in shaded boxes show cases where the pile should not be vibrated at the given

frequency range and soil condition. Larger amplitudes than those shown are generally

permissible.

If the amplitude is insufficient, then a new vibratory hammer with the same or greater

dynamic force as the previous one and greater eccentric moment should be chosen and

the amplitude checked again.

(5) Finally, the peak velocity should be checked; it is computed by the equation

vdyn = 307.24 n/rounds per minute

where

vdyn

=

peak dynamic velocity of vibrating system, feet per second; should

fall between 1.7 and 2.6 feet per second, but it can be higher if

necessary.

RPM =

frequency of vibrations, rev/min

It should be kept in mind that method presented here is at best a very sophisticated "rule

of thumb" and should be supplemented by local experience with both actual piles and

soils and good engineering judgment. One item not considered here is the static weight

of the system; this can be increased by mounting bias weights on top of the vibratory

hammer. This can increase the speed of pile penetration.

Figure 2-2b. (Concluded)

2-12