HISTORY OF ENGINEERING.
Book I.
22 0 *
After the platform was finished, the side and wing walls were carried up with all possiblesolidity.
The plan shows only one pair of gates on the side towards the harbour: the omission ofside-gates is a great defect, the vessels entering the port being frequently thrown against thejetties. The facing of the walls is carried up to the height of the iron collars which confinethe gates, and every method is adopted to fix the ties and iron keys firmly into the masonry.In the profile and plan are shown the largest, which is fixed, and has three arms, forming agoose foot; the two extremes are placed horizontally, whilst the middle one inclines tendegrees below the others, so that the arms being united with the ties, the centre mayhave the same inclination, and enable it to bear the weight of the masonry. The ties,3 inches square, are each composed of three pieces; their whole strength depending uponthe thickness of the mass in which they are embedded, the pieces should be well tiedtogether, at the same time easily separated. The ties have several holes pierced in them,to receive the keys, some of which are placed vertically in the masonry, others horizontallyin large stones introduced for the purpose. The two first are horizontal, and the other isinclined ten degrees. To give additional strength to the keys two arcs are made in thestone, one 6 feet or 7 feet radius, and the other from 14 feet to 15 feet, which ties the stoneagainst which the horizontal keys rest more solidly; grooves being cut to receive the ties,the vertical keys were fixed into eyes prepared for them.
Tlie side walls were raised to within 6 feet of the entire height.
In arranging the turning bridge, it was necessary that the surface towards the abutmentsshould be on a level with the pavement; when finished, one half therefore turned on aplatform, and the other into a recess.
The jetties had their foundations laid 12 feet below the ordinary level of the shore;and the works were carried on without cofferdams during the period of low water. Eachjetty was formed of two stone walls united by transverse walls at every 60 feet; theirintervals were filled with clay mixed with a small portion of lime. The thickness abovethe set-off was nearly 30 feet, and at the summit 21 feet; the slope or talus being 4 feet 6inches. When the walls were within 5 feet of their intended height, an arch was con-structed, over which was formed the crowning platform.
The lines which limited the width of the jetty having been traced, sheet piling wasdriven in to encase the foundations ; the earth was then taken out between, and the depth ofthe trench made as low as the heads of the sheet piles, and no more earth taken outthan would allow of its being built over in the time of low water, which was about threehours and a half. Two chapellets were employed to draw the water from the trenches,\tfiich, occupying an hour, left only two hours and a half for work. Twelve feet in lengthof sheet piling were driven at each time, tongued and grooved and maintained in their placesby two binding pieces. The thickness of the sheet piles was 6 inches, and their width13 inches, their length being proportionate to the nature of the soil, which usually re-quired about 12 feet; they were shod with iron, as the sand into which they entered wasvery compact.
As soon as the piles were driven the part of the trench which they enclosed was imme-diately filled u£, to prevent the labour of additional pumping, and the sea from deposit-ing sand or injuring the works. A dyke, 3 feet in height, was raised throughout the wholelength, formed of fascines and covered with stones. When the foundations were laid, 12feet in thickness was given to each wall, the first bed of facing wall was laid on plank, ‘Ifeet below the top of the sheet piling, and after the masonry had beer, carried up 3 feet, itsthickness was reduced to 10 feet for the first set-off, where the heads of the sheet piles wereunited to keep them in their place, and to fix the binding pieces firmly to the masonry,iron ties were introduced at every 8 feet. Two feet above the first set-oft' was another,which reduced the two walls to 9 feet in thickness, after which they were carried up withan external talus equal to one-sixth of their height, the interior face being kept perpendicular,reducing them to about 3 feet in thickness at 25 feet above the last set-off, without takinginto consideration the addition by filling in solidly the spandrils of the arches. The archof the vault, 18 inches in thickness, was formed also of stone well cut and carefullyconstructed. At the level of high water, rings and anchors were introduced into themasonry at regular distances.
Above the vault was laid a paving of a hard stone, 3 feet in length, 8 feet in thickness,and 18 feet in width ; the joints were well secured by cement, and a fall of 4 incheswas given to their surface to throw off the water. On the side towards the sea, aparapet was built, 3 feet 6 inches high, and 2 feet 6 inches in thickness, coped with stonesstrongly cramped. At every 60 feet cannons were placed for the purpose of defence.
The toe of the jetty towards the sea was protected by an additional embankment 16feet in width, which had in front a row of jointed piles, from 7 inches to 8 inches inthickness, and of a length proportioned to the nature of the soil. 'The space between thisand the jetty had other piles driven in, and their heads cut off with an inclination: onthese a platform of timber was laid; the earth was then taken out for a depth of 18 inches;