A Treatise On The Principles And Practice Of Dock Engineering

302 DOCK ENGINEERING. nécessitâtes a considérable and expensive addition to the length of the side walls, especially when the lock or entrance is of great width, as often obtains at the présent day. Caissons do not occasion any increase in the length of the side walls, but, on the other hand, there must be reckoned the cost of a spécial chamber for sliding and rolling caissons. Ship caissons do not need a chamber, but, when out of use, they have to be berthed somewhere, and this leads to a certain amount of inconvenience in the appropriation of useful space. 2. Caissons are generally of stronger build and broader beam than gates, and they afford accommodation for the transmission of rail and road traffic across a waterway, thus discharging the functions of a bridge in addition to those peculiarly their own. 3. The first cost of a caisson is undoubtedly, in most cases, greater than that of a pair of gates, but if the cost of a swing bridge for vehicular traffic, which is a necessary adjunct in the case of gates, be also taken into considération, the advantage will be found to lie with the caisson. This advantage is still further emphasised where a lock or passage is fitted with double gates to alternately impound or exclude water. A caisson can be constructed to act equally in both directions. 4. Caissons obviate the necessity for pointed sills and gate platforms of large area, but those of the ship type, fitting into grooves so as to be capable of acting in two directions, call for battered side walls to allow of their floating clear when manœuvring in and out of position, and this gives the entrance an unsuitable profile for modern vessels of square amidship section and with bilge keels. 5. Floating caissons are not always manageable in boisterous weather and strong currents, and oftentimes they are only workable with difficulty. Sliding caissons, too, have to encounter the effect of wind pressure, especi- ally if there be much clearance between their keels and the sliding ways. Neither can rolling caissons be said to be altogether exempt from the abrading or wearing effect due to the action of friction on the moving parts under lateral pressure. So that, on the whole, it may be claimed that gates are easier of movement and are more completely under control during manipulation. Dock Gates. Gates are sometimes distinguished as wooden gates or iron (including steel) gates, according to the nature of the bulk of the material of which they are composed. As a matter of fact, both materials enter essentially, though in varying proportions, into the construction of all gates. It would be impossible to connect the various members of a wooden gate without the aid of metal bolts, straps, and other fastenings, while iron gates depend for their watertightness (except in rare instances) on wooden posts and plates at the abutting surfaces.