Patent ID: 8245452

Claim:
A subterranean structure having a body cylindrical about a vertical axis, made up of at least one ring, each ring consisting essentially of a plurality of horizontally adjacent segments of molded plastic, each segment comprising: a wall element cylindrically curved about a vertical axis having an inside surface and an outside surface, vertical side edges and horizontal top and bottom ends, a first of the vertical side edges including a protruding mating element that is vertically tapered, and a second of the vertical side edges including a slot that is vertically tapered, the vertical side edges including confronting surfaces brought into abutting relationship solely by an interlocking engagement between the vertically tapered mating elements and vertically tapered slots on the horizontally adjacent segments of similar construction, wherein the protruding mating element on each segment comprises a dovetail extending continuously along the first vertical side edge, the dovetail including a proximal portion adjacent to the first vertical side edge and a distal portion opposite the proximal portion, the proximal portion being narrower than the distal portion, and the distal portion having a width of continuously varying dimension in a vertical direction to achieve the vertical taper.