| 摘要 |
<p>A cover for underground structures has a large number of mutually independent projections of the same size and shape uniformly provided on its surface, while small protrusions of the same size and shape are formed on the upper surfaces of some or all of these projections. On top of these small protrusions are formed even smaller protrusions of the same size and shape as the topmost protrusions. Spacing between one large projection and another, between one small protrusion and another, or between one small protrusion and a projection having no small protrusion on its top is set at the optimum distance, and the size of the topmost protrusions is also designed in a shape to fit an optimum size to ensure good skid-prevention effect on tires of vehicles passing over the cover, especially in bad weather over a long period of time, no matter where the cover is installed. <IMAGE></p> |
