face 12 of 100 millimeters of course in the dual block unit depicted in fig1 and 2 the total length of the unit from one end face 14 to the other end face 14 will be 600 millimeters double the length of an individual block and the grooves 20 will be disposed in the central transverse plane at a distance of 300 millimeters from either end with the grooves 21 being disposed midway the grooves 20 and each end face 14 at a distance of 150 millimeters from each end face the ridges have a width at the base at the first bearing surface 11 of 44 millimeters and a height of 16 millimeters the bevels 16 have a depth of 37 millimeters i e the distance from the imaginary corner of the block formed by the plane containing end face 14 and the plane containing the second bearing face 12 to the respective bevel edge is 37 millimeters the larger blocks 30 used for base construction have a total length of 900 millimeters and a half length when split along grove 20 of 450 millimeters the grooves 35 have a depth of 18 millimeters and a width at face 12 of 75 millimeters the foregoing dimensions provide sufficient tolerance as between grooves 35 and ridges 15 to permit the construction of retaining walls with a degree of curvature either convex or concave further these dimensions result in a 7 millimeter setback or inset as between.