adopted for the piers of the Brooklyn, St Louis, Forth and other large bridges, and essential for forming foundations on sloping rock, such as was encountered in places under the Firth of Forth.
The methods indicated above as employed for the foundations of the piers of bridges under favourable conditions belong equally to the foundations of other structures (see Foundations); but there are some methods which, by combining bridge piers and their foundations in a single structure, appertain entirely to piers. Thus iron screw piles, sunk by turning into
Fig. 1.—Pier with Disk Piles.
the soft bed of a river till they reach a firm stratum or one sufficiently consolidated by the superincumbent layers to enable it to support the wide blades of the screws with the weight imposed on them, were formerly often arranged in converging clusters joined together at the top, so as to serve as the piers of bridges having several comparatively small spans, and intended for carrying lightly constructed railways across rivers in India and elsewhere. Hollow, cast-iron, cylindrical piles also, with a broad circular disk at the bottom to increase their bearing surface, have been used for piers founded in sandy or silty strata of considerable thickness, they are sunk to the requisite depth by lowering a pipe down the inside of the pile to the bottom and emitting a powerful jet of water which, stirring up the soft material and scouring it away from under the disk, causes the pile to descend. This system was first adopted for the piers of a railway viaduct crossing the wide, sandy Kent and Leven estuaries opening into Morecambe Bay (fig. 1). Cast-iron cylinders, consisting of a series of rings formed of segments all bolted together with a specially strong bottom ring, sometimes made of wrought iron and having a cutting edge, have been often employed for the construction of the river piers of bridges, being gradually carried down to a watertight stratum by excavating inside, and subsequently filled up solid with concrete and brickwork, the piers of the Charing Cross and Cannon Street bridges across the Thames are notable instances of the adoption of this method, which is well illustrated by the piers of the bridges across the River Chittravati in India (fig. 2). Sometimes, instead of two or more independent cylinders being sunk, the whole site of a pier is enclosed within a wrought-iron caisson, usually divided into sections by vertical partitions, which is sunk and filled up solid in the same way as cylinders, a system adopted, for instance, for the piers of the bridge across the Hawkesbury River in New South Wales.
Promenade Piers.—The term pier is often applied, to works sheltering harbours, such as the Tynemouth piers, which are strictly breakwaters. Landing stages also, whether solid or open, have for a long time been called piers, as the Admiralty Pier and the Prince of Wales's Pier at Dover; but the open promenade piers which form a common feature at seaside resorts are the type of pier best known to the general public. These piers are supported upon open pilework of timber or
Fig. 2.—Cylindrical Piers for River Bridges.
iron, and consequently expose little surface to waves in storms and do not interfere with the drift of shingle or sand along the coast (fig. 3).[1] Timber piles are best suited for withstanding the shocks of vessels at landing stages, at which places they are generally used, but since they are subject to the attacks of the teredo, and expose a considerable surface to the waves, iron piles are generally adopted for the main portion of these piers.
The pioneer of these piers was the old chain pier at Brighton, which was erected in 1822-1823. It was founded upon oak piles, was 1136 ft long, and had a timber landing-stage at the end. It consisted of four spans suspended from chains on the model of the Menai Suspension Bridge, then in course of construction, and was destroyed by a gale in December 1896. A wider and more modern type of pier was erected at the west end of Brighton in 1865-1866,
Fig. 3.—Promenade Pier.
and subsequently extended; whilst a new pier was completed in 1900 near the site of the old chain pier, 1700 ft. Long. The Southport pier, erected in 1859-1860 and afterwards prolonged, furnishes an example of an iron pier supported on disk piles sunk in sand as described above (fig. 1); whilst the much more commonly used iron screw piles, adopted as early as 1847 for an open landing-pier on the Irish coast at Courtown, which was exposed to a great littoral drift of sand, are shown as the mode of support for the pier
- ↑ The Engineer (1888), i. 380, 381 and 384.
