01 July 1997
hydraulic hammer is shown in figure 3-10.
(f) Jacking. Pile jacking machines are not true impact hammers but act by simply pushing the
pile into the ground. Such a machine is depicted in figure 3-11. They are most effective when the soil
resistance is lower than the maximum ram force and when there are neighboring piles to jack against (such
as with sheet piling). They are most advantageous when vibrations and noise must be minimized.
(2) Internal Combustion Hammers. These hammers burn the fuel that powers them inside of the
hammer, and for the most part, the diesel hammers are the only constituent of this class.
(a) Open-end Diesel Hammers. The open-end diesel hammer operates as is shown in figure 3-
12. The piston, with the assistance of the starting device driven from the winch of the pile driving rig or
hydraulically, is raised to an upper position. At this point, it is released by the starting device and falls down
under its own weight. Before the bottom of the ram passes the exhaust ports, the piston pushes the fuel
pump lever, and fuel from the pump is supplied to the spherical recess of the anvil (some models directly
inject atomized fuel into the combustion chamber). At the bottom of the stroke, the piston impacts the anvil.
The energy of impact is divided between fuel vaporization and its mixing with heated air and driving of the pile.
After a short period of time, the air-fuel mixture is ignited, and because of the pressure of the expanding
exhaust gases the piston is raised up and additional driving impulse is transmitted to the pile. A typical open-
end diesel hammer is shown in figure 3-13.
(b) Closed-end Diesel Hammers. These are similar in operating principle to the open-end type
except that a compression chamber or vacuum is employed on top of the piston to assist the ram in the
downstroke. This speeds up the blow rate of the hammer, but some of these hammers have a heavier ram
relative to the energy than the open-end type. A typical closed-end diesel hammer is shown in figure 3-14.
b. Driving Accessories. It is not possible for the striking end of the ram of an impact hammer to directly
adapt itself to all shapes of piles; therefore, it is necessary to have driving accessories of various types to be
inserted between the bottom of the hammer and the pile to both mate the two geometrically and transmit the
force of impact from hammer to pile.
(1) Hammer Cushion. Most impact hammers have some kind of cushion under the end of the ram
which receives first the striking energy of the hammer. This cushion is necessary to protect the striking parts
from damage; it also modulates the force-time curve of the striking impulse and can be used to match the
impedance of the hammer to the pile, thus increasing the efficiency of the blow. The actual material of the
cushion and its configuration will vary, depending upon the hammer configuration and the cushion material
being used. Any hammer cushion should be installed and used in accordance with the recommendations of
the hammer manufacturer. Figure 3-15 shows a variety of cushion configurations. Table 3-3 shows a
summary of the various types of cushion materials and their characteristics.
(2) Anvil. The rams of most external combustion hammers strike the cushion material or top plate
directly. With internal combustion hammers, an anvil is necessary to trap the combustible mixture and thus
allow it to build pressure. Figure 3-12 shows an anvil setup. The term " nvil"is also sometimes used to
describe the drive cap or helmet (see next paragraph).
(3) Helmet. The helmet actually mates the hammer system to the pile. In doing so, it distributes the
blow from the hammer more uniformly to the head of the pile to minimize pile damage. Figure 3-16 shows
some typical driving helmets.
(4) Pile Cushion. When driving concrete piles, it is necessary to use a cushion between the hammer
and a pile. This cushion is generally made of plywood. One cushion is made for each concrete pile to be
driven, and it is installed on top of the concrete pile before it is driven or in the cap. The depth of this cushion
can vary from 6 to 18 inches. Figure 3-1 shows a typical pile cushion configuration.