a rib 20 projects radially outwardly from the edge portion of the offset section 16 the connector strips 14 are of generally t shape in cross section having an enlarged base 22 the inner surface 24 which essentially forms a continuation of the inner surface of the container the body of the strip 14 is formed with surfaces 26 28 30 32 and 34 which are adapted to engage with a close sliding fit the corresponding surfaces of the flange and rib structures in a typical case the width of the base is 2 375 34 the height of the connector is 1 1 34 and the ribs 20 are 0 375 34 wide and project outwardly from the flanges 16 0 215 34 the connector strips may be of a length corresponding to the height of the module so that only two strips are required per module however to facilitate handling and installation it may be preferable to provide the strips in shorter lengths for example one half or one third of the height of the module in which case four or six strips are required per module the cylinder halves 10 are preferably molded of polypropylene using a structural foam process which has sufficient strength to contain the sand and yet is sufficiently frangible to break into small pieces when struck by an impacting vehicle connector strips 14 are preferably extruded from.