The main piles are in two pieces, the lower end of the upper one being formed so as to fit into a socket on the top of the under length, and the joining made good by means of a strong screw-bolt ; the only object of this was to insure a supply of truer castings, and lessen the difficulty of transport ing such unwieldy masses from Northumberland and Staf fordshire to London. Each sheet-pile, is secured at the top by two bolts to the uppermost wale of the woodwork behind, and the edge of the end-ones of each bay, it will be observed, pass behind the adjoining main pile, while the other joints are overlapped by the bosses with which all the sheet-piles, except the closers, are furnished on one side. Besides adding to the perfection and security of the works, by breaking the joints, so that the water, if' it penetrate, (as, even with the best pile-driving, it cannot draw the backing from its place, these projections appear to me to relieve the appear ance of the otherwise too uniform space, and a like effect is produced by the horizontal fillets on the lower edges of the plates above, which also mask the joints. These plates filling up the space over the sheet.piling, arc bolted to the main piles and to each other, in the manner shown, and the joints stopped with iron cement. Where the mooring-rings come, the plates are cast concave, with a hole perforated in the middle, to allow a bolt to pass through, and this bolt is se cured, as well as the land-ties, (*ruin the main piles to the old wharf, which was not otherwise disturbed, or to needle-piles driven adjoining to it. The backing consists of a concrete of lime and gravel, in the proportion of about one to ten, extending down to the solid bottom. The coping. with the water-channel in its rem•, is of Devonshire granite ; the is conveyed from the channel at intervals by pipes extending f•om gratings in the bottom, in a slanting fine, to the lower most plate, and discharging themselves immediately above the sh•et-piling. The main piles were originally proposed to be hollow, but this was given up on further consideration of the uncertainty of procuring sound castings of the intended them, and of the greater liability to break afterwards from a blow sideways.
" The solid firm was therefore adopted, according to which the lower lengths weighed about 26 cwt., and that this was not too much, was shown by the circumstance of several of the piles, pa•ticularly the early ones, breaking in the testing, or driving, and showing in the fracture the danger of even a slight defect. The greater care subsequently taken at the foundry, and probably also greater experience in driving, made accidents of this kind of rare occnrrence in the later stages of the work ; and it may be mentioned as no bad proof of the care of all parties, that of upward of 600 piles. includ ing both descriptions, only 16 broke in driving-7 being of one sort, and 9 of the other: the failure was in five cases attributed to strains in driving, and to imperfections of cast ing in the other eleven. The sheet-piles, which bear a con siderable reseinblance in their general outline to those used at Downes wharf ten years before. were proposed to be an inch thick, but it was found necessary to increase tldsdimen sion, and some of them was as much as 11 inch ; the average, however, was not above inch, and weight of each pile 17 cwt. ; the length of the wharf is about 720 feet, and the whole weight of iron used, upwards of 900 tons.
" The crab engine was employed invariably, the heads of the piles being covered with a slip of three-quarter inch elm, to distribute the force of the blow equally over the iron, and so prevent jarring. The monkeys used, weighed from 13 to 15 cwt. each, and it was found necessary to limit the fall to a height of 3 feet 6 inches, and sometimes less ; when the resistance proved more than usually great, the pile showed a tendency to turn from its straightforward course. The
driving throughout was very hard, more especially at the west end, where the sheet-piles in four bays, could not be fiireed to the full depth, the space above being in two of them made up with two plates in height, and in the other two admitting only one instead of three, as in the rest of the work. Driving was the only means resorted to, or indeed practicable, in the gravelly soil that prevailed. Dad the bottom been clay, or other similar substance, the plan of boring, to receive the points, might probably have been par tially adopted in the main piles with zulvantage ; but I should say, certainly not to the extent of depending mainly upon it for getting the piles home to their places." Piles similar to the above have been employed in many situations, differing only in matters of detail, which we do not think it requisite more fully to describe.
Piling of cast-iron seems to be very well adapted for coffer dams, wharf walls, embankments, and such works, but is not to be substituted for timber when employed for foundations, for unless based upon a very firm stratum, they are not so secure against sinking when heavily loaded ; nor, on account of their small sectional area, do they compress anti consoli date the soil through which they pass to the same extent as timber pilings, and this compression greatly assists in forming a solid foundation for the superstructure. Another disadvan tage which attends the use of this material, is the deteriora tion which it undergoes by the action of water upon it, and especially of sea-water.
The serew-pile introduced by Mr. Mitchell, is admirably adapted for loose and moveable strata, and has been found very useful in Whe re all other means had failed. This pile consists of a spindle of the required length, with a broad east-iron plate or disc, in a spiral or helical form, at tached to the lower end, so as to form a screw by means of which the pile is secured in the The following ac count of them is taken from a summary of a paper read by Mr. A. Mitchell, and published in a cotemporary.
The origin of the scre•-pile was the serew-mooring, which was designed for the purpose of obtaining., for an es. pecial purpose, a greater holding power than was possessed by either the pile, or any of the usual mooring anchors ur blocks, of however large dimensions. It was proved by experiment, that if' a screw with a broad spiral flange were fixed upon a spindle, and forcibly propelled by rotary motion to a certain depth into the ground, an enormous force would be required to abstract it by direct tension, and that the power employed must be sufficient to drag up a mass of the form of a frustum of a cone reversed, the base being at the surface of the ground, and the section of the apex being equal to the diameter of the screw. The extent of the ing mass must of course depend upon the natural tenacity of the soil ; even in this reasoning it must be evident that a vertical force was calculated upon, but as practically that seldom or never occurred, the angle of tension and the curve of the buoy-cable again gave the moorings greater power. This was found to be correct in practice, and the applications of these moorings became very extensive. An arrangement was with the port of Newcastle-on-Tyne, by which, for the sum of £•,500, the right of fixing the moorings in the Tyne was given ; and Mr. Brookes, the engineer, showed that last year, (1847) whilst in the neighbouring port damage was done to the shipping to the extent of nearly £30,000, no injury was sustained in the Tyne, entirely owing to the sound holding of Mitchell's screw-pile moorings.