Strength and Deformation Behavior of Bottom Ash–Crumb Rubber Mixtures for Lightweight Embankments with Geocell Confinement

The construction of embankments on soft ground often results in excessive total and differential settlements. To address these challenges, it is essential to reduce the self-weight of the embankment by utilizing lightweight fill materials with adequate strength and stiffness. This study investigated the feasibility of using pure bottom ash (BA) and crumb rubber (CR)-bottom ash (CR-BA) mixtures as lightweight embankment materials based on their mechanical properties. The shear and deviatoric responses of BA and CR-BA mixtures were analyzed to evaluate their strength and stiffness characteristics. Due to their lower density, these materials exhibit reduced stiffness compared with conventional fills, and stiffness decreases further as CR content increases. To enhance their mechanical performance, geocell confinement was incorporated. The influence of geocell inclusion on stiffness was examined by varying the geocell aspect ratio (1, 2, and 3) in specimens of pure BA and CR-BA mixtures. The optimal crumb rubber content was determined to be 5%, because it yielded a slightly higher friction angle, 40.6°, than pure BA and other CR-BA mixtures. The confinement of CR-BA mixtures using a geocell with an aspect ratio of 1 improved stiffness by approximately 15% compared with unconfined cases, whereas aspect ratios of 2 and 3 produced an enhancement of less than 2%. The energy absorption capacity of unconfined CR-BA mixtures and those confined using geocells with aspect ratios of 1, 2, and 3 increased by approximately 18–27%, 11–18%, 24–34%, and 27–31%, respectively, compared with pure BA. Furthermore, the deviatoric stress—axial strain responses of pure BA and CR-BA mixtures for unconfined and confined using geocells were best fitted with the improved Duncan–Chang model and aligned with the strain softening regions.