Ground improvement based on the introduction of a lattice structure by jet grouting is widely adopted to prevent seismic liquefaction phenomena. Conventional design prescribes that the allowable stress of an improved body should remain within the range of elastic behavior during an earthquake, but the characteristics of phenomena associated with improved ground after earthquake-related failure remain unclear. Against this background, Namikawa and Mihira (2007) proposed an elasto-plastic constitutive law capable of expressing strain-softening/hardening response after tensile and shear failure. In the present study, the authors simulated an unconfined compression test with a previously described elasto-plastic model for large-scale core sampling of a body improved via jet grouting to verify the model’s applicability. FEM analysis was subsequently conducted to evaluate the resistance of improvement based on a lattice structure with local failure against a strong earthquake.