Abstract: Geomaterial,known as sand-expanded polystyrene(EPS) lightweight fill,is formed by blending EPS beads with standard fine sands at various ratios.The geomaterial has many merits,e.g.,saving cementitious materials,mitigating additional loads and settlements.Researches on the stress-strain relationships of the geomaterial become important in guiding its field uses.Based on the elliptic-parabolic yield surfaces model,the incremental stress-strain equations describing the stress-strain variations of materials are established.The equations consider the blending ratios and confining stresses of materials while depicting the mechanical behaviors of materials.Consolidated drained triaxial shear test results are used to analyze the reliability of the derived stress-strain equations.It is indicated that the equations well demonstrate the deviatoric stress-axial strain-volumetric strain variations of lightweight fills,meaning that the incremental stress-strain equations basically reflect the critical behaviors of materials,i.e.elastic-plasticity,dilatancy and contraction.Interpretation is advised from the perspective of the shear deformation of materials.