英語(yǔ)翻譯
At the reconnaissance stage,the engineer’s task is ,by cut and try,to determine which routes deserve further study.Terminals of the road and intermediate points through which it must pass form the primary controls .A unique bridge site or single mountain pass also may become a primary control if no alternative exists .Likewise,for scenic highways ,the positions of timbered areas,waterfalls,lakes,and other attractions may be primary controls .Small settements which would be ignored or purposely bypassed in locating principal highways may be primary controls for secondary rosds.Drainage systems,mountain passes,low points in ridges,or swamps in low country,often form secondary controls.Cost factors such as favorable or unfavorable soil conditions,the numbers and sizes of structures,and the amount of excavation and embankment required for satisfactory alignment and grade likewise can be classified as secondary contrils.
In mountainous country with well-defined summit ranges,there is usuall a suitable pass with possible approaches following along the drainage on both sides.The least expensive and frequently the straightest line may lie just above high water in the streams.Often,however,the rise of the valley or canyon may exceed the maximum permissible grade.Then,if the steam grade is to be followed,extra roadway length must be gained on adjacent mountain spurs,in side canyons,or with switchbacks.