Study on the superimposed effect of cyclic impact load on subgrade settlement by centrifuge modeling

Traffic loads frequently apply a multiple wheel superimposed effect to subgrade, leading to complex characteristics of subgrade settlement that differ from those caused by the single impact load. A series of centrifuge model tests is conducted to investigate the superimposed effect of cyclic impact load on subgrade settlement. The subgrade response is analyzed based on a full-field displacement measurement via image correlation analysis. It is found that the subgrade settlement under superimposed loads extends from the center line to the impact line, contrasting with the localized settlement seen under single loads, and the settlement profile shows an increase in the depth of influence with increasing spacing of superimposed loads but requires fewer impact numbers for settlement stabilization. The analysis of the impact loading mechanism reveals that the settlement exhibits periodic changes within each cycle, with the soil initially exhibiting predominantly plastic deformation, transitioning to predominantly elastic deformation in later stages. A loading influential zone is defined to distinguish the scope of area with loading-induced settlement and the effects of the superimposed load and its spacing are clarified subsequently. Stress propagation from impact loads is observed to be conical, with vertical strain diminishing linearly with depth, indicating basically the same diffusion characteristics across various conditions. The study categorizes loading influential zones into three subzones and further illustrates the superimposed effect.