橋梁畢業(yè)設(shè)計(jì)文獻(xiàn)綜述開(kāi)題報(bào)告外文翻譯-橋梁設(shè)計(jì)(已改無(wú)錯(cuò)字)
2023-03-03 05:47:11 本頁(yè)面
【正文】 vantages of the both tunneling and the alternative method of either passing around or over the obstruction. In practice, it is often found that tunneling, although costly, proves to be less expensive in the long run than any alternative system. There are instances, however, where the engineer has no choice but to pass around or over the obstacle. For example, intensely hard rock might well make tunneling extremely difficult and far too costly to even contemplate. The inability of existing road systems of many large towns to copy modern traffic requirements has made tunnel construction a proposition well worth serious consideration. Up to a point, flyover and underpasses, being the first really decisive steps to speed up and divert traffic , have eased the situation, but a considerable contribution toward a satisfactory solution can be made by underground railway work. Many of the larger cities of the world have been successfully served by underground railways for years, and most of them are still extending their works. In the meantime, 12 other cities have introduced them for the first time in an effort to reduce traffic congestion. In the broadest sense, there are two fundamentally different types of construction employed for underground systems, they are subpavement tunnels and “ tubes” , the latter being situated at a far greater depth. The technique in both forms has improved dramatically over the years. The decision of city planning authorities as to whether a subpavement or “ tube” system should be adopted, depends a great deal on what type of public transport system already exists. If a perfectly efficient surface tramway system is functioning at the time, it is often found advantageous to extend it as a subpavement system. Normally, it is relatively straightforward to provide a ramped connection between the existing street level system and the underground extension. On the other hand, it may prove more satisfactory to reanize the public transport system and provide an entirely separate tube system, working independently form the service at steel level. Modern equipment and ingenuity have enabled the construction of tunnels to be far less laborious than hitherto. In applying the method known as the “ Berlin Building System” , firstly, the open excavation is taken down to a depth of between 13 12 and 25 meters. The sides of the excavation are supported by lines of driven steel, piles, as used during the excavation for a fourtrack underground station in Hamburg. Generally, this type of preliminary construction is also required to carry temporary bridges for both vehicles and pedestrians. Alternative methods to that show in , for supporting open excavations, are often employed. For example, steel sheet piling and the more modern methods of either “ BenotoVeder” pile walling or “ ” diaphragm reinforced concrete walling are all in mon use today, because in each no lateral supports are necessary. The “ BenotoVeder” pile walling consists of concrete piles accurately interlocked one with the other to form a plete vertical support the excavation. The use of bentonite mud is the basis of the “ ” p
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