连续型拓扑优化理论英文文献和中文翻译(2)
2.2 Earthwork
Because earthmoving methods and costs change more quickly than those in any other branch of civil engineering, this is a field where there are real opportunities for the enthusiast. In 1935 most of the methods now in use for carrying and excavating earth with rubber-tyred equipment did not exist. Most earth was moved by narrow rail track, now relatively rare, and the main methods of excavation, with face shovel, backacter, or dragline or grab, though they are still widely used are only a few of the many current methods. To keep his knowledge of earthmoving equipment up to date an engineer must therefore spend tine studying modern machines. Generally the only reliable up-to-date information on excavators, loaders and transport is obtainable from the makers. Earthworks or earthmoving means cutting into ground where its surface is too high (cuts), and dumping the earth in other places where the surface is too low (fills). To reduce earthwork costs, the volume of the fills should be equal to the volume of the cuts and wherever possible the cuts should be placed near to fills of equal volume so as to reduce transport and double handling of the fill. This work of earthwork design falls on the engineer who lays out the road since it is the layout of the earthwork more than anything else which decides its cheapness. From the available maps and levels, the engineering must try to reach as many decisions as possible in the drawing office by drawing cross sections of the earthwork. On the site when further information becomes available he can make changes in jis sections and lay out, but the drawing office work will not have been lost. It will have helped him to reach the best solution in the shortest time. The cheapest way of moving earth is to take it directly out of the cut and drop it as fill with the same machine. This is not always possible, but when it can be done it is ideal, being both quick and cheap. Draglines, bulldozers and face shovels an do this. The largest radius is obtained with the dragline, and the largest tonnage of earth is moved by the bulldozer, though only over short distances. The disadvantages of the dragline are that it must dig below itself, it cannot dig with force into compacted material, it cannot dig on steep slopes, and its dumping and digging are not accurate. Face shovels are between bulldozers and draglines, having a larger radius of action than bulldozers but less than draglines. They are angle to dig into a vertical cliff face in a way which would be dangerous tor a bulldozer operator and impossible for a dragline. Each piece of equipment should be level of their tracks and for deep digs in compact material a backacter is most useful, but its dumping radius is considerably less than that of the same escalator fitted with a face shovel. Rubber-tyred bowl scrapers are indispensable for fairly level digging where the distance of transport is too much tor a dragline or face shovel. They can dig the material deeply (but only below themselves) to a fairly flat surface, carry it hundreds of meters if need be, then drop it and level it roughly during the dumping. For hard digging it is often found economical to keep a pusher tractor (wheeled or tracked) on the digging site, to push each scraper as it returns to dig. As soon as the scraper is full, the pusher tractor returns to the beginning of the dig to heap to help the nest scraper. Bowl scrapers are often extremely powerful machines; many makers build scrapers of 8 cubic meters struck capacity, which carry 10 m ³ heaped. The largest self-propelled scrapers are of 19 m ³ struck capacity (25 m ³ heaped) and they are driven by a tractor engine of 430 horse-powers. Dumpers are probably the commonest rubber-tyred transport since they can also conveniently be used for carrying concrete or other building materials. Dumpers have the earth container over the front axle on large rubber-tyred wheels, and the container tips forwards on most types, though in articulated dumpers the direction of tip can be widely varied. The smallest dumpers have a capacity of about 0.5 m ³, and the largest standard types are of about 4.5 m ³. Special types include the self-loading dumper of up to 4 m ³ and the articulated type of about 0.5 m ³. The distinction between dumpers and dump trucks must be remembered .dumpers tip forwards and the driver sits behind the load. Dump trucks are heavy, strengthened tipping lorries, the driver travels in front lf the load and the load is dumped behind him, so they are sometimes called rear-dump trucks.
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