A Treatise On The Principles And Practice Of Dock Engineering

84 DOCK ENGINEERING. as an ordinary crane. The rate of work claimed for a 12-ton machine, fitted with a IJ-yard bucket, is from 800 to 1,200 cubic yards per day of 11 hours, according to the nature of the ground. It has been known by the writer to maintain an average of 570 cubic yards of stiff clay per day of 10 hours, under normal conditions, with the attendance of 10 men and 2 horses. The cost is about £1,200, and the ordinary working expenses amount to about 25 shillings per day. The method of action is somewhat different from that of the Ruston navvy. The bucket is operated by a direct-acting steam cylinder, the piston of which has a stroke of 6 inches to 2 feet in any position of the face. Being lighter in build, the machine is not so well adapted to rock- getting as the Ruston machine. Very similar in design is the Whitaker Excavator, made by Messrs. Whitaker, of Horsforth, near Leeds, and its capabilities are also about the same. It requires the attendance of a dozen men, with two horses, and its daily working expenses lie between 25 and 30 shillings. The cost of a 10-ton machine with li-yard bucket is about £1,250. A very interesting application of hydraulic power to dock construction is illustrated in the Hydraulic Navvy (fig. 49), designed by Sir W. G. Armstrong & Co., and used in the formation of the Alexandra Dock at Hull. The jib is similar to that of the Ruston navvy. The lifting ram and multiplying sheaves are placed, in an inclined position, at the rear of the machine, so that their weight may exercise the greatest counter- balancing effect when the bucket is making a cut. The diameter of the ram is 14| inches and the stroke 4 feet 5 inches. The hydraulic working pressure at Hull was 700 Ibs. per square inch, which afforded a maximum cutting force, allowing for friction, of about 12 tons. The capacity of the bucket was IJ cubic yards, and the machine could excavate 600 cubic yards of suitable ground in 10J hours. Its speed of working, compared with a steam navvy, was as 13 to 10, and the ordinary repairs as 10 to 14. The cost of the machine complète was about £1,300, and its weight 30 tons. The average daily consumption of water was 17,000 gallons.* Hydraulic appliances are not generally feasible for constructive work, unless the power be pre-existent. A contractor would scarcely deem it worth while to lay down a special installation for the purpose. But,, where available, the system offers the following advantages over steam power. It is more rapid and more reliable in action, with less vibration and less noise. There are fewer repairs to be made, and, in the absence of coal and of water boilers, there is less weight to be carried over soft or uncertain ground. Grabs are also used as excavators, but their rate of working is much inferior, and they are best adapted to confined situations and to the removal of light surface soil, under which conditions an average output of 300 cubic yards per 10-hour day has been obtained. They can excavate * Vide Hurtzig on “ The Alexandra Dock, Hull,” Min. Proc. Inst. C.E., vol. xcii.