篇篇学之3：The Embankment Dams

There are three mainly types of dams: gravity dams,embankment dams and arch dams.(Considering these dams are ordinarily,we just take these for example.),Here we just describe the embankment dam.
NO2.Embankment dams(We have described the gravity dam in the second chapter)
The embankment dam is a massive dam made of earth or rock.They rely on their weight to resist the flow of the water.It usually has some sort of water proof insides(called the core),which is covered with earth or rock fill.Grass may even be grown on the earth fill.Water will seep in through the earth or rock fill,but should not seep into the core.The main force on an embankment dam is the force of water.The weight of the dam is also a force,but each material has a different weight.The uplift force is also acting on the embankment dam,but some of the water seeps into the dam so the force is not the same as on a concrete dam.The embankment dam is the only dam type that is not made of concrte.It maybe made of earth or rock,both of which are pervious to water--that is, water can get into it.The water will seep into the core material and should stop at the seepage line. The core material ia usually more watertight than the rock or the earth that is on the outside of the dam,but the core material is still not totally impervious to water,concrete is not truly impervious either,but it does not allow as much seepage as these material do.

Words:
gravity dam 重力坝 embankment dam 堤坝（土石坝） arch dam 拱坝 massive 大体积的/结实的
proof 不透（水）的 core 核/心（墙） seep 渗入 uplift force 浮托力/扬压力
pervious 能被接受 seepage line 浸润线 watertight 不透水的