Abstract: The invention relates to a modular system for assembling a hollow structure. The system comprises segments for assembly into modules to assemble hollow structures such as culverts and water-retention systems. The segments comprise opposing sidewalls and a deck spanning said sidewalls having opposing sides and opposing ends which are assembled in inverted, vertically aligned relation to each other to form a hollow module. The segments engage each other with mutually engaging three dimensional structures that prevent lateral and vertical slippage between the segments. The segments may include half-columns that meet along the plane of engagement to form full-height load bearing columns in the assembled structure.