2. 外语
  3. 地铁施工 在地铁施工中一直采用深挖法的原因是为了避免妨碍城市底下复杂的煤气、水管、电缆、电话线路及下水道系统。为了尽可能使地面上建筑物由于震动引起的损失减少到最小程度，地铁还要尽可能建造在主要街道中间部分的地底下，这就是为什么有些地铁系统有许多急弯的

地铁施工 在地铁施工中一直采用深挖法的原因是为了避免妨碍城市底下复杂的煤气、水管、电缆、电话线路及下水道系统。为了尽可能使地面上建筑物由于震动引起的损失减少到最小程度，地铁还要尽可能建造在主要街道中间部分的地底下，这就是为什么有些地铁系统有许多急弯的

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问题    地铁施工
   在地铁施工中一直采用深挖法的原因是为了避免妨碍城市底下复杂的煤气、水管、电缆、电话线路及下水道系统。为了尽可能使地面上建筑物由于震动引起的损失减少到最小程度，地铁还要尽可能建造在主要街道中间部分的地底下，这就是为什么有些地铁系统有许多急弯的原因。有些地铁的大部分线路是靠完全临时控制有关街道建成的，从地面向下挖到铁路要求的深度，铺上轨道，然后又将所挖区域覆盖，恢复原来街道的面貌。这种方法叫做“明挖法”，当然这比挖地铁隧道对地面的干扰要大得多。
   与在粘土及类似土壤的机械化地铁挖掘截然不同的是在山区岩层挖掘隧道，这是所有挖掘工程中最花劳力和财力的。线路的每一英尺都要砸开。工人们使用风动和电动操作的凿岩机，在工作面上凿满了洞，将炸药装进去，夯实，随后他们必须撤到安全地方，直到通过电气插头把炸药引爆完毕。然后将砸开的岩石运走。接着整个过程重复进行，几乎是无休无止的，直到从岩石中开出一条线路来。岩石越硬，开凿工作就越困难，尽管如此，也很少有什么岩石坚硬到爆破后不用圬工或混凝土进行衬砌就可直接形成隧道壁。
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答案 Tube Construction The reason why deep-level boring is the invariable practice is to avoid interference with the complicated system of gas, water and electricity mains, telephone conduits, and drains and sewers, found under any great city. Also the tubes are engineered as far as possible under the center of main streets in order that there may be a minimum of possible damage by vibration to buildings above, and this explains the numerous sharp curves in some tube system. Some tubes built most of its mileage by obtaining, temporarily, complete possession of the streets affected, excavating down to the railway level and laying their tracks, and then arching over their cuttings and restoring the roadways. This method is known as " cut-and-cover" tunneling, and causes, of course, far more surface disturbance than the boring of a tube. At the opposite extreme from mechanized tube tunneling through clay and similar soils is rock tunneling through the mountains — the most laborious and costly of all tunneling operations. Every foot of the way must be blasted out. By means of pneumatically or electrically operated rock drills, the drillers bore holes all over the working face into which explosive is inserted and tamped home; they must then retreat to a safe distance until electric contacts have exploded the charges, after which the rock blasted away is removed and the whole process repeated, almost endlessly, until a way has been made through the barrier. The harder the rock, so much the more difficult is the work of drilling, though few types of rock are sufficiently hard to be allowed to remain after blasting without a lining of masonry or concrete.
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