Chat. XIV.

OX PILES AND PILE-DRIVING.

Fig. 1639.

Fig IG40.

1073

Sheetinq Piles are formed of thick plank, shot or jointed on the edges, so that they mayform a close joint, and not admit the water to pass between them : when their sides aregrooved and tongued, the passage of either air or water is prevented ; the edge of oneplank is double, and the other single, rebated, so that about a third of the thickness of onepasses into the middle of the thickness of the other : to drive these piles the small orringing engine is generally made use of, and to bring the edges of the planks into closecontact with each other great care is necessary : the pointing of sheet piles should beeffected by cutting one side only of the plank.

Water and quicksands are often kept back by sheet piling, hut it sometimes happensthat both rise so rapidly as to impede the workmen’s progress; the pump or othermeans is then adopted to drain off the water, and it becomes necessary, under such cir-cumstances, to provide guides for driving, and afterwards retaining the planks or sheet-piles in their proper position : square timbers, placed horizontally, as well as parallel witheach other, are put as near the work as possible, and kept apart at each of their endsby a short pile driven firmly into the ground, into which they are secured with screwbolts. These horizontal timbers are not laid farther apart than just to allow of the thicknessof the sheet pile, which varies from 2 to 6 inches or more, according to the work it has toperform. Piles introduced between guides can be very accurately driven, and easilymaintained in their position : when the resistance they offer is considerable, both the upperand lower part of the guides may be made of whole timbers, and the lower stiffened bybracing, or by short piles driven close behind them at small intervals : care must also betaken to provide for the security of the sheet of piles when the whole is complete, forduring the progress of driving and fixing, the water and sand can rise to a common levelon both sides; but when these are cleared out on one side to form a dam, the accumulationand consequent weight which exerts its influence on one side will break down, or forceout the work, unless strutted and braced with sufficient strength to prevent it.

It is necessary, when the water is perfectly clear, to stop all the joints, or to caulk themwith oakum, but generally the particles of clay or earth settle in the joints, and renderthem water-tight.

In the formations of a cofferdam it is frequently requisite to drive two rows of verticalpiles and plank piles, and to fill in the intermediate space with well prepared clay : in othercases main piles are driven, to which are fixed strong horizontal planks, and another modeis to drive one row of gauged piles, and then fill the spaces between with pile planks drivenvertically.

In deep rivers the foundation of piers were previously made by driving piles all over thespace, so that their heads stood level with low water; the spaces between them were merelyfilled with loose stones, and the masonry, as at old London Bridge , commenced at their top ;but the piers were immense masses, and required to be protected by sterlings, leaving avery confined water-way, and created a head and velocity which ploughed up the entirebed of the river below the piers : our old bridges appear to have been constructed withoutcofferdams or caissoons, the engineers employed upon them probably considering that weightand resistance were requisite to oppose the current of a river, and rather than yield to itsforce they threw in sufficient material to almost draw it up : modern practice, on the con.trary, endeavours not to abridge the width of water-way, and its advantages are universallyadmitted, as are the employment of sheeting piles around the footings of the foundations.

The machine used by Perronet was worked by a number of ropes pulled by men ; henceit was named the ringing engine , and the pile was driven by a ram which descended per-pendicularly : the various parts of the framing were of stout timbers, put together withround iron pins, which allowed of its being taken to pieces •when necessary, and trans-ported to some other situation: attached in the rear of the supporting frame was a windlassfor the purpose of hoisting the pile, or placing it into its position for driving, the ropewhich wound it up passing over a pulley fixed in the projecting timber at the top of themachine.

This celebrated Engineer , in the foundations of the piers of his bridges, made use ofpiles of from 9 to 12 inches in diameter, placing them from 3 to 4 feet apart from centreto centre, and driving them 6 feet into the bed of the river, when composed of soft mud,

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