Ingles

Transportation Systems
Transportation has always been one most important aspects of civil engineering. One of the 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 Ecole des ponts et chausses, the School of Bridges andHighways, 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 accordina to the principles laid down in the eighteenth and early nineteenth centuries by a Frenchman, Pierre Tresaguet, the Englishman Telfort, and a Scot, John L. McAdam,whose name has passed into English in the words macadamize, and tarmac. These men designed the first modern roads that had a firm footing, the surface on which the foundation rested. Their roads also included good drainage and a wearing surface—the top level that directly receives the wear of traffic—that could not be penetrated by water. Both tresaguet and Teleford used a heavy foundation ofstones,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 curve in a crown or camber so that the water would run off. McAdam realized that the soil itself could bear the weight of the road when it was compacted or pressed down, as long as it remained dry. He was able therefore to eliminate the heavy cost ofthe 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 beginning of thetwentieth 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 have this kind of surface.
Basically, roadbuilding has improved in only two ways in the twentieth century. The first improvement involves the use of concrete forthe wearing surface. The other is traffic engineering, the design of highways for high-speed, heavy-volume traffic, highways that are economical to build and safe for vehicles and their passengers. Traffic engineering has produced the modern express highway, or freeway, that has only limited access and maximum safety controls. The angular intersections common on older roads have been eliminated infavor of cloverleaf interchanges or others with even more complicated designs. Modern freeways usually have special lanes where traffic can either slow down before exiting or speed up upon entering. Extreme curves or steep slopes are minimized so that the traffic can continue to move without slowing down. Since monotony has proved to be a safety hazard, traffic engineering even includes thelandscaping of the borders of the road.
When construction on a new highway begins, huge earth-moving machines called bulldozers 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 cut where earth is being removed should be equal to the amount needed for a nearby fill. Moving earthfrom 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 and wearing surface that will rest on it. In many...