when the pile is not in line with the fall of the hammer.
Always make sure the fall of the hammer is in line with
the pile axis. Otherwise, the head of the pile and the
hammer may be severely damaged and much of the
energy of the hammer blow lost.
Excessive bouncing may be caused by a hammer
that is too light. However, it usually occurs when the
butt of the pile becomes crushed or broomed, as when
the pile meets an obstruction or penetrates to a solid
footing. When a double-acting hammer is being used,
bouncing may result from too much steam or air pres-
sure. With a closed-end diesel hammer, if the hammer
lifts on the upstroke of the ram piston, the throttle setting
is probably too high. Back off on the throttle control just
enough to avoid this lifting. If the butt of the timber pile
has been crushed or broomed more than an inch or so,
Figure 10-17.-Types of pile damage caused by overdriving
it should be cut back to sound wood before you drive it
any more.
Obstruction and Refusal
When a pile reaches a level where 6 blows of a drop
hammer or 20 blows of a steam or air hammer do not
drive it more than an average of 1/8 inch per blow, the
pile has either hit an obstruction or has been driven to
refusal. In either case, further driving is likely to break
or split the pile. Examples of typical damage are shown
if figure 10-17.
If the lack of penetration seems to be caused by an
obstruction, 10 or 15 blows of less than maximum force
may be tried. This may cause the pile to displace or
penetrate the obstruction. For obstructions that cannot
be disposed of in this manner, it is often necessary to
pull (extract) the pile and clear the obstruction.
When a pile has been driven to a depth where deeper
penetration is prevented by friction, the pile has been
driven to refusal. It is not always necessary to drive a
friction pile to refusal. Such a pile needs to be driven
only to the depth where friction develops the required
load-bearing capacity.
Straightening
Piles should be straightened when any mis-
alignment is noticed during driving. The accuracy of
timber piles.
Figure 10-18.-Realigning pile by pull on a line to a winch.
10-10
