To provide a lightweight landfill improved so that damage to a retaining wall formed by foaming a foam resin can be effectively prevented in the lightweight landfill formed by filling the foam resin foamed at site in a clearance between a slope and the retaining wall, and a construction method for the lightweight landfill.

This lightweight landfill is formed by filling the foam resin (4) foamed at site in the clearance between the retaining wall (2) constructed by opposing the slope (1) and the slope. A sheet-like damping material (3) damping an expansion force when the foam resin (4) foams is disposed on the inner surface of the retaining wall (2) opposed to the slope (1).