i078

THEORY AND PRACTICE OF ENGINEERING.

Book II.

fnay be ascertained, it being only necessary to multiply the force, in the last column, bythe weight of the ram ; thus a ram weighing 12 cwt. falling 20 feet, will have a force thus,12 x 35'9 = 430-8 cwt.

Withdrawing Piles .— In harbours or tide rivers this is easily effected by attaching bargesat low water to the heads of the piles, and as the tide rises, the pile must either be pulledout, or the barge pass under the water. In other cases Bramah’s hydrostatic press hasbeen used, or a framework of timber inclined so as to work a chain by means of a crab,to which blocks are attached, and draw them out: when piles have a slight hold, theymay be drawn out by a long lever, acting upon a chain attached to the head of a pile :iron shod piles driven into gravel are sometimes difficult to withdraw, in consequence ofthe great oxidising of the metal; this principally occurs in sea water; when, therefore, theyare to be withdrawn from such situations, it is advisable to omit the iron shoe, if at allpossible.

In drawing out piles, the operation is considerably assisted by striking them with heavyblows near the head, in an oblique direction, by means of levers, and some ingenious

Fig. 1648.

MACHINE POR DRAWING PILES.

methods have been adopted: when the heads of the piles are out of the water, it is advis-able to pierce a hole through them and pass a bolt, to which is attached the chain bywhich they are to be raised up. When the head is under the water, a square iron collar

Fig.1649.

MACHINE FOR DRAWING PILES.

is passed over the pile, and, having a greater diameter than the latter, is placed obliquely ;when the chain is tightened, the edges of the collar embrace the wood, and from the greatpressure which takes place, all slidingoff is prevented.

When the piles arc rounded and shodwith iron, they not only descend easier*but may be more readily detached : animprovement has been suggested uponthe last machine, which consists inmaking the block attached to the screwrise as it is turned round, and the headof the pile being secured to each sideof it by a link of iron, it can be loosenedfind drawn out with a more steadymotion, all disposition to swerve fromthe perpendicular being prevented. ^8- screw for drawing filer.

Cutting nff Piles _This operation is by no means difficult when the foundations are