346
HISTORY OF ENGINEERING.
Book I.
the soil by boring, the nature of the land floods and tides, and drew up a report upon thesubject; his object being to provide wharfage and floating docks, and new ground for ship-building on the mud banks called the Links, to place locks and graving docks capable ofadmitting vessels on the best foundations, and to provide the means of scouring the docks,and cause the flux and reflux of the tide, as well as the land floods, to act most effectuallyon the existing bar and prevent future accumulations there, so as to preserve 4 feet addi-tional depth of water, and by this means admit large vessels at neap tides, and form a com-munication between the Aberdeenshire canal and the new harbour.
The pier on the south side, which was constructed by Smeaton , having been destroyedin 1807, Mr. Telford commenced his works of improvement by erecting another in cutgranite, and finished it with a slope of five horizontal to one perpendicular; this wascompleted in October, 1809.
In the following year the north pier was commenced, and by the aid of a railway itsentire extension of 300 feet was completed within the twelvemonth ; this was found sobeneficial to the harbour, that it was afterwards extended another 865 feet beyond Mr.Smeaton ’s head, which was performed in three seasons, in the full expanse of the GermanOcean.
The outer head having in the following winter received considerable injury, its slopewas altered, and made five to one, since which it has stood perfectly well.
The foundations resting on loose sand and gravel, it was found necessary to consolidatethe work under low water by dropping from lighters large stones, and filling the intersticesup with smaller, until it was brought to within a few feet of low water, at which level theashlar line commenced. The stones were not laid horizontally, but at an angle of 45°,and in this way was the masonry worked, until it arrived within 18 inches of the top,when it was built level.
By this means the work was more rapidly advanced, and less liable to temporary de-rangement, for while the ashlar wall was being carried up on both sides, the middlewas built, which was performed by a care-ful backing of large rubble stone to within18 inches of the top, when the whole wascoped with granite, 18 inches in depth,over which, on the north side, was addeda cut granite parapet the whole length ofthe pier. The outsides of the pier arcbuilt with roughly dressed granite ashlar,the headers being 3 feet 6 inches in length,and the hearting of large rubble, the in-terstices being filled up with smaller stones.
The rocks in the neighbourhood arechiefly a gneiss formation, and as it is difti-cult-to dress this stone into a regular shape,the masons call them heathens; some ofthese, of large dimensions, weighing from5 to 30 tons, were slung by the aid ofmachinery between the bows of two lighters,which had counterweights at the stern, andby this means floated to the pier head, anddeposited in the situations allotted them.
A railway laid down along the old pier,with a double crane at the end, movableon rollers, and advanced as the work pro-ceeded, was found of the greatest con-venience.
Besides this, six lighters of 40 tons each,with a crane mounted on each, were em-ployed to bring the stone used in the lowerpart of the work.
The pier on the south side was carriedout, to correspond in extension with theother; it was not, however, made parallelwith the north pier, but formed a solid breakwater from the south shore, in a north-eastdirection, with a space at its entrance of 250 feet. Within this breakwater there was asloping beach, that the surge might spend itself freely, and not agitate the waters withinthe harbour. The length of the breakwater is 800 feet, and it is constructed of largerubble stones, as they came from the quarry, except the head, which is formed of roughlydressed ashlar, laid to a slope of 45° where exposed most to the sea’s action, and about
Fig. 344.
TRANSVERSE SECTION.