The Viaduct Works' Handbook

53 with advantage in a case where a bridge crosses a ravine, for the most part nearly dry, but occasionally subject to heavy and rapid floods, where also the foundation is of a soft nature. The up-stream side is protected by wrought-iron guards, against accident to the cast-iron columns that might happen by floating bodies striking them during flood times. The foundation is formed by screwing wrought-iron piles into the soil to as great depth as practicable, the heads are then cut off to the required height, and base plates are firmly attached to them. Screw Piles.—Hollow tubes of cast-iron and solid bars of wrought-iron, cast or fitted with the "Mitchell” screws, are manu- factured at the Viaduct Works for piers of bridges, foundation supports for lighthouses, landing stages, jetties, moorings, &c. The sustaining power of a well-made and well-fitted screw in a firm sand, is a load in tons equal to five or six times the square of the diameter of the screw in feet. Cast-iron tubes are generally found convenient in from 9-feet to 12-feet lengths, with their meeting surfaces planed; wrought-iron can be employed in much longer lengths, without risk of fracture in transit, and are less fiable to be broken by concussion from vessels or floating timber, or other objects, when ultimately fixed in place. Wrought-iron solid piles are generally preferred in as long lengths as can conveniently be put on board ship; but it will frequently occur that small vessels only can approach the spot where it is desirable or necessary to load or tran- ship the work. Joints, simple in adjustment, can be made; it is desirable, however, that these be avoided as much as possible in so much of the length of the pile as will be subjected to torsion in the process of screwing down. The process of screwing down is very simple. A light framing of wood is generally made so as to hold the pile to be screwed, and by driving a few temporary wood guide piles, the whole is adjusted to the exact spot; a capstan wheel, from 15 feet to 20 feet diameter, is then fitted on to the upper part of the pile, and held by tem- porary keys; a rope is passed round the circumference of this wheel and attached to a crab winch on shore, which causes the wheel and same to revolve when set in motion, and a slight pressure on top of the screw causes it to enter.