Civil defense shelters are often constructed below ground to provide protection against blast loading. Concrete has been used extensively as a construction material for the defense layer of the shelters. In order to achieve expected attenuation of blast-induced stress-waves, geofoam or cavity inclusions are sometimes embedded in the concrete layer. This paper studies the attenuation of stress-waves in a concrete medium behind a rectangular inclusion of expanded polystyrene (EPS) geofoam with different dimensions. The effects of EPS geofoam inclusions on the reduction of peak axial stress a, and hydrostatic pressure P are studied, respectively. One approximation formula is proposed to correlate the decay factor for peak a, to the dimensions of the EPS geofoam inclusion based on numerical simulations. This formula is applicable in defense engineering design to estimate the decay factors at different locations or the dimensions of EPS geofoam when other parameters are prescribed. Finally, the effects of multiple rectangular inclusions of EPS geofoam on stress-waves are numerically investigated. (C) 2006 Elsevier Ltd. All rights reserved.