To evaluate water treatment residue (WTR) as a soil substitute material, its physical properties were investigated and compared with decomposed granite soil (DGS). For comparison purposes, relative gas diffusivity (D/D0), saturated hydraulic conductivity (Ks), water retention curve, porosity and readily available water were measured for both the WTR and the DGS. The measured D/D0, Ks, water retention ability and porosity were higher for the WTR than for the DGS. These differences may be attributable to intra-aggregate and inter-aggregate pores created in the WTR through binding of silt and clay particles in the flocculation process. The behavior of water and gasses surrounding these pores may explain the physical properties of the WTR. The characteristics measured in this evaluation indicate that WTR has good potential for reuse as a soil substitute material.