覃国强毕业设计外文翻译 2

Transportation SystemsTransportation has always been one of the most important aspects of civil engineering .one of the great accomplishments of the Roman engineers was the highway system that made rapid communication possible between Rome and the provinces of the empire. The first school that offered training specifically in engineering was the School of Bridges and Highways, established in France in 1747. And in England, Thomas Telford, a roadbuilder, became the first president of the Institution of Civil Engineers in 1820.Modern highways are still built according to the principles laid down in the eighteenth and early nineteenth cenruries by a Frenchman, Pierre Tresaguet, the Englishman Telford, and a Scot, John L. McAdam. These men designed the first modern roads that had a firm footing, the surface on which the foundstion rested. Their roads also included good drainage and a wearing surface-the top levelthat directly receives the wear of traffic-that could not be penetratated by water. Both Tresaguet and Telford used a heavy foundation of stones, on top of which a base course of lighter crushed stones and a wearing surface of still smaller stones were built up. Their roads were also slightly curved in a crown or camber so that the water would run off. McAdam realized that the soil utself could bear the weight of the road when it was compacted or pressed down, aslong as it remained dry. He wasable therefore to eliminate the heavy cost of the stone foundation by laying a base course of crushed stone on top of a compacted footing. The iron wheels of the carriages of his day ground the stones of the top level into a continually smoother and more watertight surface.These roads were adequate during the nineteenth century when wagons and carriages had tires made of iron or steel. When the automobile appeared at the beginng of the twentieth century, however, its rubber tires broke up the smooth surfaces. Therefore,the top layer was bound together more firmly by mixing the crushed rock with tar or asphalt. Millions of kilometers of roads throughout the world today havethis kind of surface.Basically, roadbuilding has improved in only two ways in the twentieeh century. The first improvement involves the use of concrete for the wearing surface. The other is traffic engineering, the design of highways for high-speed, heavy-volume traffic, highways thathat are economical to build and safe for vehicles and their passengers. Traffic engineering has produced the modern express highway that has only limited access and maximum safety controls. The angular intersections common on older roads have been eliminated in favor of interchanges or others with even more complicated designs. Modern express highways usually have special lanes where traffic can either slow down before exiting or speed up upon entering.Extreme curves or steep slopes are minimiced so that the traffic can continue to move without slo wing down. Since monotony has proved to be a safety hacard, tiaffic engineering even includes the landscaping of the borders of the road.When construction on a new highway begins, huge earthmoving machines called bulldocers level the ground along the designated route. The amount of earth to be moved, both in leveling and filling , has been previously calculated. Wherever possible, the amount in a cat where earth is being removed shoule be equlal to the amount needed for a nearby fill. Moving earth form a distant point is extremely expensive, and economy is a critical aspect of an engineer’s work.After the earth has been moved and shaped according to the design of the road, other machines prepare the footing . The most important of these is probably a vibrating roller, which compacts the earth until it can bear the weight of the base course ang wearing surface that will rest on it. In many cases, however, the soil must be stabilized by mixing some other material with it. This may be bitumen or a grout of concrete or some other substance.The base course, which is made of either crushed stone or a layer of thinly-mixed concrete, comes next, the wearing surface, which may be a layer of asphalt or a series of reinforced concrete slabs, is then laid. A concrete surface be laid in segments separated by joints to allow for expansion and contraction under difffering weather conditions. One method of laying a reinforced concrete wearing surface is to putdown the steel rods, usually in the form of a grill or mesh, after a certain proportion of the concrete has been poured. The top level must be poured within twenty minutes of the pouring of the bottom level to assure proper bonding. Anther method is to pour the entire thickness of concrete and then force the steel mesh down into it to a predetermined level.A group of machines collectively callled a concrete train usually perform these operations at a rate of about three-fourths of a meter per minute. A single new machine called a slipform pawer can perform all the different jobs of a concrete train excep laying the reinforcing mesh. However, a technique has been worked out in which the mesh is held in place before the machine passes over it and pours concrete aroung it . The slipform paver can operate at a rate of about two meters per minute. The construction of airport runways follows similar procedures, except that the slabs in a runway are much wider then those in a highway. Airport runways must also be designed to carry the heavy load of the big modern jumbo jets, as well as to withstanf enormous blastes of heat from the engines of heat form the engines of the aircraft.The impact fatcor is also greater with airplanes than with automobiles. Highways differ form airport runways, however, insize and strength rather than design and construction methods.If huge concrete freeways are the transportation phenomenon of hort this century, then the railroads were the transportation phenomenon of the nineteenth century . Transportation by rail was not unkonw before the invention of steam locomotive in 1892, but rail lines had been limited to the short distances over which horses or other draft animals pulled the load. Most of the lines were bugreat ilt in conjunction with mining operations. After the steam locomotive became a practical machine, a surge of railroad construction rapidly spread a network of railroads across the face of the word.By 1920 the age of railroad building generally seemed to be over. As automobiles came into increasingly wider use, the emphasis shifted to highway consruction. Except for few improvements such as electrification, railroads were neglected. Now, however, there is a renewed interest in railroads. The Japanese for example, haverecently opened the Tokaido line between Tokyo and Osaka, with express trains that average 166 kilometers per hiur and reach speeds as high as 210 kilometers per hour in nirmal operation. Even in the United Stares where many of the railroads are in deplorable condition, especially in the eastern part of rhe country, there is an increased in providing better service. There is also considerable interest-not only in the United States but in many other countries as well-in urban rapid transit systems.。