Abstract: A hollow tube element of a predetermined maximal width and defining a predetermined axis adapted to be secured to an adjoining hollow element defines a prearranged axis for use as building blocks for the construction of a tunnel; either element is rotatable about its axis by an arbitrary angle relative to the other. The hollow tube element includes a plurality of interconnected slabs; the locus of intersection of an imaginary first plane tangential to a first exterior portion of the hollow tube element with an imaginary second plane, and tangential to a second exterior portion of said hollow tube element disposed opposite the first portion, both plates having the predetermined width, is an imaginary annular surface having a predetermined outer diameter when either element is rotated 360.degree. about its axis relative to the other. The outer diameter is finite when the planes are converging at a finite distance from the element, and otherwise infinite.