CEMP-E
EI 02G001
01 July 1997
(5) Mandrel. A mandrel is used to install thin-wall shell piles which are subsequently filled with
concrete. The mandrel is necessary because the wall is too thin to withstand the stresses of driving.
Figure 3-17 shows examples of mandrels.
3-2. VIBRATORY DRIVERS. A vibratory pile driver is a machine that installs piling into the ground by
applying a rapidly alternating force to the pile. This is generally accomplished by rotating eccentric weights
about shafts. Each rotating eccentric produces forces acting in a single plane and directed toward the
centerline of the shaft. Figure 3-18 shows the basic setup for the rotating eccentric weights used in most
current vibratory pile driving/extracting equipment. The weights are set off center of the axis of rotation by the
eccentric arm. If only one eccentric is used, in one revolution a force will be exerted in all directions, giving
the system a good deal of lateral whip. To avoid this problem, the eccentrics are paired so the lateral forces
cancel each other, leaving us with only axial force for the pile. Machines can also have several pairs of
smaller, identical eccentrics synchronized and obtain the same effect as with one larger pair. The
classification of these machines is shown below.
a. Low-frequency Hammers. These are vibratory drivers with a vibrator frequency of 5 to 10 Hz, used
primarily with piles with high mass and toe resistance such as concrete and large steel pipe piles. They tend
to have large eccentric moments to achieve their dynamic force with high resultant amplitudes. An example
of this type of machine is shown in figure 3-19.
b. Medium-frequency Machines. These are drivers with a vibrator frequency of 10 to 30 Hz, used for
piling such as sheet piles and small pipe piles. An example of this type is shown in figure 3-20. These
machines make up the majority of vibratory pile drivers in use today, since they combine the dynamic force
necessary to excite the soil, the correct frequency to properly interact with most soils, and the sufficient
amplitude to get through the hard spots in the soil.
c. High-frequency Machines. These consist of all machines which vibrate at frequencies of more than
30 Hz. They are of two basic types. The first are machines in the 30 to 40 Hz range which are designed
primarily to minimize vibration of neighboring structures. These have been developed simultaneously both in
Europe and in the United States, and they are similar in construction to the medium-frequency machines.
The primary advantage of these machines is their lowered transmission of ground excitation to neighboring
structures. The frequencies of these machines are not high enough to improve driving. In some cases, these
machines have problems in overcoming toe resistance.
d. Sonic or Resonant Hammers. In a class by itself is the resonant pile driver, first introduced in the
early 1960's. The central principle of the resonant driver is to induce resonant response in the pile, thus
facilitating driving and extracting. The resonant driver operates at frequencies in the range of 90 to 120 Hz. In
most cases the driving took place at the half-wave frequency of the pile. The ability to achieve this response
was dependent upon properly matching the frequency range of the machine to the length of the pile. In
cases where this was not possible in a normal hammer/pile setup, a heavy wall follower connected the pile
with the hammer. When the pile is exceptionally long, second and third overtones can be achieved.
Although, in principle, this concept has held great potential. The mechanical complexity of this machine has
withheld it from extensive use.
e. Impact-vibration Hammer. The term "mpact-vibration hammer"refers to a type of vibratory pile driver
i
that imparts both vibrations and impacts to the pile during operation. Such a machine is shown in figure 3-21.
In common with more conventional vibratory hammers, it contains counter rotating eccentrics which impart
vertical vibrations; however, these are contained in a head which is free of the pile to some degree. Generally
speaking, its motion is regulated by a set of springs which link it to the frame. The frame can be connected to
the pile in numerous ways. These springs transmit their compression force, which is of a vibratory nature, to
the pile. In addition to this, depending upon the position of the head relative to the impact point and the effect
of the springs on the vertical motion of the head, (at or near the bottom of the vibratory cycle), the head
strikes the anvil and produces an impact similar to traditional impact hammers, but at a higher blow rate.
Although this can produce variations in the eccentric rotational speed of up to 40 percent (as opposed to the
3-3
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