Abstract: A shell-like retainer structure is provided for forming headwalls at each extremity of a culvert disposed below a driveway. The retainer structure has two parallel spaced apart sidewalls having aligned U-shaped openings, and a transverse wall spanning the perimeters of said side walls, said side and transverse walls defining a chamber which opens at the bottom of the retainer structure and at the U-shaped openings. A filling aperture is located in the uppermost extremity of the transverse wall. When the retainer structure is seated upon the culvert, filling material is poured through the aperture, and the filled structure is left in place upon the culvert.