A Treatise On The Principles And Practice Of Dock Engineering

472 DOCK ENGINEERING. convenient size to near the level of tlie intended platform. Upon this a layer of macadam was placed, bringing the foundation up to the required height. As a guide for the accurate execution of this work, a line of piles, A (fig- 459), was driven on each side of the foundation, clear of the sides of the timber platform, and to these piles, guide timbers, B, were affixed at the required inclination of the slipway and at the depth of the ends of the straight-edge above it. The foundation was now ready to be dressed off true by divers, who, as they frequently had to work in the dark, were provided with iron-faced straight-edges, 0, made about the weight of a similar volume of water so as to be easily moved. These were long enough to reach across the entire foundation and to slide underneath the guide timbers. With these straight-edges, the divers were able to dress the macadam face so truly that, in one case of a foundation, 360 feet long, it was found, after the platform was finished, there was only one error of ^^ inch.” In the construction of a slipway for Earle’s Shipbuilding Co., at Hull, by Mr. Godfrey, in 1882, the method of piling the foundation was adopted. “Whole timber piles, cross sleepers, and longitudinal bearers were used throughout. In the centre way, two rows of piles were driven, 18 inches apart from centre to centre, transversely and 3 feet from centre to centre, longitudinally. For the side ways single piles were driven, 6 feet from centre to centre, these coming opposite every second row of piles in the centre way, thus giving one pile for each lineal foot, or a supporting power of 10 or 12 tons per lineal foot. A sleeper, 30 feet long, was placed transversely on the four piles and one, 6 feet long, on the two intermediate piles. Upon these sleepers were fixed the longitudinal timbers or rail bearers, securely fastened with oak trenails. The centre timbers were 4 feet 6 inches wide to take a plate of the same dimensions. The ground for 4 feet below the cross sleepers was excavated and filled with rough chalk for a width of 15 feet on both sides of the slipway ; the whole was planked over with 3-inch redwood deals.” The piling was effected as follows:—A cofferdam could not be thought of, the situation being too exposed and the method too costly. The width of the slipway being 30 feet, a traveller 35 feet wide was constructed to span it transversely and placed upon a line of rails, the diameter of the wheels being made to suit the inclination of the slipway. Upon this traveller was placed a Sissons & White’s steam pile- driver, with 40 feet leaders, and a ram weighing 21 cwts. As the tide ebbed the pile-driver was allowed to go down upon the traveller by gravitation, and the piles were driven in successive rows of two and four alternately, the machine being worked across the traveller from side to side. When the tide rose, the traveller was withdrawn to the higher portion of the work. Ihe Permanent Way is usually laid at some gradient between 1 in 15 and 1 in 25. There is a slipway at Palermo with a gradient of 1 in 13'3, but this is exceptionally steep, the average being 1 in 20. Any flatter slope than 1 in 25 causes an unnecessarily great length of slipway. Occasionally curved slipways may be found with a steep inclination below the water-line,