为了备战,在平时就需要构筑工事。这些工事是准备在未来战争中使用的。在很长的和平时期内,如果不利用它们,不仅大量的固定资产闲置,而且还要付出可观的维护费用。特别是在城市,它们还占据了可贵的地下空间,如果闲置则更为可惜。所以,在平时尽可能地把永备工事,特别是永备人防工事充分利用起来,增加经济效益和社会效益,是非常必要的。实践证明,这也是可行的。现代城市,特别是大、中城市,为满足平时需要,要修建大量的地下工程,如地下铁 In preparation for war, in the usual need to build fortifications. These fortifications are to be used in future wars. In the long peacetime, if not using them, not only a large amount of fixed assets is idle, but also considerable maintenance costs. In urban areas in particular, they also occupy valuable underground space, which is even more unfortunate if left unused. Therefore, it is absolutely necessary for us to make the best use of permanent building work, especially permanent air defense work, to increase economic and social benefits. Practice has proved that this is feasible. Modern cities, especially the large and medium-sized cities, need to construct a large number of underground works in order to meet their usual needs. For example, the underground railway