Globally, disposal of waste materials like fly ash (FA) and polyethylene terephthalate (PET) bottles has reached threatening levels, considering their dumping and natural degradation rate. Considering the PET bottle's cellular shape, they can be used as unique alternative to reinforce and improve the behavior of foundation beds under applied load conditions. Existing research have established that contributing elements such as infill material, properties of the interface, and surface loading geometries all have a direct impact on the behavior of the subgrade. In the current study, the behavior of a square plate footing loaded on rock quarry dust (RQD) overlaying FA subgrade as FA + RQD system for associated boundary conditions of test model has been investigated (bearing pressure was applied till footing penetration depth of 40 mm). Moreover, the system has been assessed for three different conditions, i.e., with, without, and varying cellular reinforcement. The numerical study using finite element analysis (Plaxis 3D) were validated using laboratory model results. FA was modelled as a linear elastic–plastic material with Mohr–Coulomb failure criteria, and PET bottles as cellular reinforcement which have been characterized as an elastoplastic material. Additionally, the validated models have been used to evaluate the performance of conventional high-density polyethylene geocell. The results showed a significant reduction in the settlement accumulation rate and greater resilient settlement for reinforced conditions in comparison to the unreinforced. © 2022, The Author(s), under exclusive licence to Springer Nature Switzerland AG.