依铺设的位置可分上下两层，上层为顶层道渣(Top-ballast)，下层为底层道渣(Sub-ballast)。美国铁路工程协会(AREA)建议在运输频繁，在重量大的轨道，两层的厚度各应为30公分(12英？
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sub ballast 底碴
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Lower turbines should, together with clever design and material selection, help reduce the turbines' weight, and thus their need for ballast, which in turn should reduce sub-sea costs.
BBC: Floating challenge for offshore wind turbine
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