Page:The New International Encyclopædia 1st ed. v. 04.djvu/152

CANAL. material through which the channel is cut, and by the location of the channel under certain circumstances. The bottom of the channel is always made flat; in soft ground the sides are made sloping, the angle of slope depending upon the stability of the material, being quite steep in firm materials and quite flat in unstable materials; and in rock the sides are made vertical or nearly so. The attempt is always made for the sake of economy of excavation to approach as nearly to a rectangular cross-section as the conditions will permit. When the canal passes through towns the sides are made vertical to save space and provide quays, retaining walls being used in soft ground to form vertical sides.

Canal construction consists chiefly of open-cut excavation, but embankments, aqueducts, tunnels, culverts, bridges, and a variety of other construction work are involved. The plant used and methods adopted in excavating canals depend very largely upon the size of the canal section and the material encountered. In rock the practice is the same everywhere, and consists in the use of power drills and explosives for breaking up the rock, and derricks, conveyors, and cars hauled by animal or mechanical power for removing it. In a boat canal of small section, the plant required is small and simple, but in large ship-canal sections very large and powerful machinery and elaborate power plants supplying compressed air and electricity arc employed. In small canals soft-ground excavation is commonly performed by means of shovels and plows for loosening the material, and scrapers and carts for carrying it from the excavation. In larger canals this plant is increased by the addition of grading and excavating machines and steam-shovels loading into carts or cars hauled by horses or light locomotives. In ship canals of the largest section this plant is still further enlarged by the employment of special excavating and conveying machines and powerful dredges. Aqueducts are usually built in the form of masonry-arch bridges with the top formed into a channel for the water. Sometimes, however, masonry piers carry a wooden trough, or, in later years, one of steel. In embankments the channel is formed by building up the sides and lining the bottom and slopes with concrete or a layer of clay or other impervious material. Tunnels for canals are built in the same manner as tunnels for other purposes. (See .) Culverts are provided for carrying streams underneath the canal and bridges for carrying highways and roadways over it. See ; ;

The usual methods of transferring vessels from one level or reach of a canal to another one are by locks, inclines, or lifts. Of these three devices, the lock is the one most extensively employed. A lock is a masonry chamber built at the junction of the two reaches, the bottom of which is a continuation of the bottom of the lower reach and the top of which is at the same level as the banks of the upper reach. Structurally this chamber consists of two parallel masonry side walls, closed near each end by a pair of folding gates. When a vessel is passing from the lower reach to the upper reach through a lock, the sequence of operations is as follows: The lower gates being open and the water in the lock being at the same level as the water in the down reach, the vessel is floated into the lock-chamber and the down gates are closed. By means of valves in the upper gates or culverts in the side walls or floor of the chamber, water from the upper reach is slowly admitted until the water-levels in the chamber and in the upper reach are the same. The upper gates are then opened and the boat floated out into the upper reach to continue its journey. To lock a vessel from the upper reach to the lower reach, the operations described are merely reversed. The gates are usually made of wood or iron, and each leaf consists structurally of two vertical posts called the quoin-post and the miter-post, connected by horizontal frames, which serve as a framework for carrying the water-tight boarding or plating. The quoin-post has pivots at top and bottom which work in suitable fittings in the side wall, so that each gate-leaf swings open and shuts like a door.

A gate consists of two leaves, the swinging edges of which meet on the centre line of the chamber, but as each leaf is somewhat wider than half the width of the chamber, they do not form a straight diaphragm across the chamber when closed, but are shaped like a very flat letter V with its point projecting toward the upper level or reach. This construction gives greater strength to resist the pressure of the water. The height between the bottom of the down reach and the bottom of the upper reach is called the lift of the lock. The practicable height of lift in lock construction is limited, and where great differences in level have to be overcome, a series or flight of locks built end to end is employed. The dimensions and main structural features of the locks of several canals are given in succeeding sections.

Where a vessel passes through a lock from one level to another, a lockful of water is lost from the upper level to the lower level for each pair of boats passed. Where water is scarce and the total lift is large, therefore, resort is sometimes had to inclined planes up and down which the boats are transported in cradles or tanks running on wheels and hauled by cables or other power. Inclined planes for canals are of very early origin, being at one time quite extensively used, and some of these old inclines are described in the following section. A more important system of transferring canal-boats from one level to another is the vertical lift or lift-lock system, which has been installed in a number of places and is proposed for several other places where very high and important differences of level occur. In the vertical lift-lock system, the boat is floated into a movable trough, the ends of which are closed by gates, while similar gates close the ends of the canal approaches. When the gates are closed behind the boat the trough is raised or lowered, as the case may be, until it coincides with the other level of the canal, when the front gates are opened and the boat proceeds upon its way. The trough is raised and lowered by means of hydraulic or other power aided sometimes by counterweights or flotation tanks. The first vertical lift on a large scale was that built at Anderton, England, in 1875; a second was built at Les Fontinettes, France, in 1885; a third at La Louvière, Belgium, in 1888; and a fourth at Heinrichenberg, Germany, in 1895. In 1895 a lift lock was designed to replace the flight of locks at Lockport,