01 July 1997
hammer is shown in figure 3-36.
c. Preboring. Preboring consists of drilling, auguring, or coring a hole in the ground and filling the hole
with concrete or driving a pile into the hole. This is generally done with a continuous flight auger. Filling the
hole with concrete is properly a drilled shaft, and is beyond the scope of this manual. For driven piles,
preboring is advantageous when the ground resistance is extremely high. For square concrete piles, the
diameter of the bored shaft should be approximately 125 percent of the nominal pile size. Although preboring
will generally reduce the driving resistance, it does so at the expense of shaft resistance, which decreases
during the preboring. This diminution of the pile capacity must be taken into account when determining
whether a pile can be prebored.
d. Screwing. Screw piles consist of a pile casing fitted with one or more turns of helical screw having a
larger diameter than the pile. Installation is made by screwing the casing into the ground to a predetermined
level. Torque is provided by a capstan or similar device.
e. Pull Down. This is a type of pile jacking where the pile casing is jacked into place and filled with
concrete. Where a closed-end casing is used, special equipment is limited to conventional screw or hydraulic
jacks. Where an open-end casing is used, a jet or miniature orange peel bucket is used for removing the core.
Figure 3-37 shows such a bucket; it can also be used for casings installed by vibration.
f. Concrete Pile Cutting. When the driving of a concrete pile is complete, the next step is to connect the
top of the pile with the structure it is holding up. To do this, it is frequently necessary to have the reinforcing bar
protrude above the top of the pile. Two basic ways to accomplish this are (1) to fabricate the pile with
protruding reinforcing bar or cable, or (2) to cut off the top of the pile in such a way as to leave the reinforcing
bar or cable exposed for connection. Assuming that the reinforcing bars are not damaged, piles made by
method (1) are ready for connection upon driving. For method (2), it is necessary to cut the concrete pile, and
if the reinforcing bars are needed, it is necessary to leave them protruding and undamaged.
(1) Manual Methods. Manual methods of concrete pile cutting involve the use of tools adapted for the
task. In this case, concrete crushing is performed by jackhammers. Reinforcement bars are then cut by
electric or resistance welding, gas, or flame cutting. Concrete pile saws are also used manually; these are
rotary saws which simply cut the pile.
(2) Automatic Methods. These involve the use of hydraulic devices called pile beavers, which crush
the pile by effort applied in a transverse direction perpendicular to the pile axis. As a result of this effort, the
reinforcement bars were exposed and then cut by different ways at a predetermined level. Others combine the
bond separation and the pile cracking in the same area of the pile, The cracking taking place both in the same
plane as the bond failure and also above and below it. A pile beaver is shown in figure 3-38.