The apparatus has a number of edge components (5) each of which are joined by a connection (6) suitable for transmission of a tractive force to the central component (4). The space between the edge components and the central component is filled with a pourable material, preferably sand, grit or mould. The edge components are joined to the central component by flexible tractive elements of which at least two are used in conjunction with one another. Each edge component has a basic surface which coincides with a ring segment, and the edge component on its smallest vertical surface is provided with fixture devices for connection to the connecting components.
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