DESIGN AND CONSTRUCTION. The two points which have mainly to he considered in canal desimi are the cross-section of the channel and its longitudinal profile. Considering the longi tudinal profile first, it will readily be understood that a canal cannot, like a road or railway, adapt itself to the irregularities of the country by means of ascending and descending grades. but must consist of one or more practically level sec tions or reaches, When two or more reaches are required at different levels, the adjoining ex tremities of two reaches cannot be, for obvious reasons, connected by a grade in the channel. The various means for making such eonnections are described in the following section: but the fact which needs to be noted here is that, since the profile of the canal must consist of a series of level reaches at different elevations, care has to be exercised to select a route which will pro vide long reaches and consequently few changes in level. As in railway work, however, depres sions in the ground may be crossed by embank ments or other structures upon which the channel is carried.
Another matter which has to be carefully pro vided for is a supply of water to the highest reach, or summit level, as it is usually called; the reason for this being that this reach is constantly losing its water to the reaches below, and this loss must be supplied by streams or reservoirs so located as to discharge into the summit level. Distances being equal, a canal which connects two points with a single reach is preferable to one with two reaches. Indeed, a considerable increase in length is allowable to permit the canal to be constructed without a change of level. The reason for this is that transferring a boat from one level to another by locks or the other usual Means is a slow opera tion. and furthermore. locks are very expensive to construct compared with a similar length of the ordinary channel. The engineer carefully in tegrates these factors of time and cost and selects the route between the various points he wishes to connect which will give the minimum time of transit at the minimum cost. In deciding upon the cross-section to be given to the chan nel. two things have to be considered, viz. its dimensions and its form. As regards dimen sions, they are determined largely by the size of the vessels with Which it is proposed to navigate the canal. The width must be at least sufficient to permit two vessels of the largest size to pas: each other without fouling. Another influencing factor is that the resistance to traction is greater in a restricted waterway.
It is generally assumed that a width of bottom equal to twice the beam of the largest vessel navigating the canal regularly is necessary, and that the depth of water should be about IC, feet greater than the draught of these vessels, if good results are to he obtained. The form of the cross-section is determined very largely by the material through NS Mt the channel is cut, and by the location of the channel under certain cir cumstances. The bottom of the elnumei is al made flat; in soft ground the sides are made ,loping, the angle of slope depending upon the stability of the material, being quite steep in firm material- and quite flat in unstable ma terials: and in rock the sides are made vertical or nearly The attempt is always made for the sake of economy of excavation to approach its nearly to a rectangular cross-section as the con ditions 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, tun nels, culverts, bridges, and a variety of other construetion 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 prac tice is the same everywhere, and consists in the use of power drills and explosives for breaking up the rock, and derricks, conveyors, and ears hauled by animal or mechanical power for removing it. In a boat canal of small section. the plant re quired is small and simple, but in large ship canal sections very large and powerful machin ery and elaborate power plants supplying com pressed air and electricity are 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 ears hauled by horses or light locomotives. fn ship canals of the largest section this plant is still further en larged by the employinent of special excavating and cowveying and powerful dredges. Aqueducts are usually built in the form of ma sonry-arch bridges with the top formed into a channel for the water. Sometimes. however, ma sonry 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 NN ELS. ) Culverts are provided for earrying streams underneath the canal and bridges for carrying highways and roadways over it. See Itinf HIE ; CABLEWAY: CEA NES : Dm LLS Quaaay, LocKs, INCLINES, AND LIFTS. The usual methods of transferring vessels from one level or reach of a canal to another one are by locks, inclines. or lifts. Of three devices, the lock is the one most extensively employed. A is a masonry chamber built at the junction of the two readies, 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 eonsists of two parallel masonry side walls. closed near each end by a pair of folding gates. When a vessel is passing from the lower !well 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 reaeh, 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 con tinue 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 con sists structurally of two vertical posts called the quoin-post and the nfiter-post, connected by horizontal frames, which serve as a framework fur carrying the water-tight boarding or plating. The quoin-post ha .s 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.