PILING. 63
have their uses. The drawback to timber piles is that, although extremely
durable while completely protected from atmospheric influence, they are very
susceptible to decay in air and, more particularly, “betwixt wind and water,”
and to perish from the attacks of insects.
Iron piles with pointed ends, and concrete piles on the Hennebique
system (figs. 26 and 27), (vide also Chap. vii.) should only be driven through
the interposition of a wooden dolly (fig. 28).
For untrustworthy strata of indefinite depth, piles, whether of wood or
iron, are occasionally furnished with a broad screw end to the extent of a
single turn or slightly more. This considerably increases the bearing
surface. Such piles have to be lowered by rotation, either by means of
manual, animal, or mechanical power
-ARRANGEMENT FOR
SHEET PILE DRIVING PILE
Fig. 26.—Hennebique Pile. Fig. 28.—Hennebique Pile.
Kam and Fall.—Piles may theoretically be driven at the same rate with
a light ram and a long fall as with a heavy ram and short fall, but the second
method is preferable in practice. A long fall means greater oscillation in the
ram and a consequent jar in the delivery of the blow, which tends to rupture
the pile. From extensive experience in the driving of wooden piles, the
author finds a monkey of 1 ton weight, with a fall of 8 or 10 feet, a very
suitable combination. For concrete piles on the Hennebique system, even
less fall is desirable, and a monkey of 2J tons, with 4J feet drop, has
answered very satisfactorily at Southampton.
Quiescence.—lî the driving of a pile be interrupted for a short time, it is
ound that the resistance offered to driving is materially increased. Piles
hich have been left partially driven overnight have exhibited a resistance
nearly three times as great on the resumption of work in the morning. This