which constitute the bottom the sides and the top of the lattice have fewer arms the base is additionally provided with a watertight bottom and with a watertight facade the watertight bottom is preferably constituted by a mosaic of pyramides thus enabling the bottom to penetrate as far as required into the adjacent subsoil beneath the final position of the platform fig5 is a perspective view of a pyramid component in one of the lattice tetrahedra the pyramid and the tetrahedron have a common base def but the vertex g of the tetrahedron is above the vertex h of the pyramid to construct the pyramid it is convenient to have a portion of each face of the pyramid molded integrally with the corresponding node of the lattice for example one half of the face dhe should be molded with the node d while the other half should be molded with the node e the two halves are then assembled by any suitable technique eg by a technique similar to that used to assemble two arms end to end to form a bar thus the pyramids at the bottom of the base are installed at the same time as the nodes which constitute the bottom level of the lattice the facade of the base is preferably a corrugated concrete facade to make the facade see fig6 is it convenient to prefabricate elongate concrete troughs each comprising two plane walls p1 and p2 at an angle to each other and then to fix the troughs to the outside bars of the lattice to build up the facade it is thus advantageous for the outside bars of the lattice to.