Performance of Recycled Subgrade Filler from Unsorted Demolition Waste: Insights from Laboratory and Field Studies

China’s rapid urbanization has generated substantial low-strength demolition waste, creating significant environmental and disposal challenges. Recycling this waste as subgrade filler provides a viable solution given the subgrade’s lower strength requirements and high consumption capacity. This study systematically evaluated the feasibility of using unsorted rural demolition waste (URDW) as subgrade filler without separation to improve recycling efficiency. Three gradations (FF20, MF40, and CF80) with maximum particle sizes of 20, 40, and 80 mm were designed based on the characteristics of the raw materials. The composition, compaction behavior, particle breakage mechanisms, and mechanical properties of these fillers were analyzed. The experimental road subgrade employed FF20, with model tests assessing long-term performance under wet–dry (WD) cycles and field evaluations combining quality tests with settlement monitoring. Results showed that increased particle size reduced the combined content of bricks and mortar (CCBM) while raising that of concrete and stones (CCCS) in the raw materials, demonstrating particle size effects on material strength. This explains the decreasing CCBM and increasing CCCS from FF20 to CF80. Heavy compaction produced elevated particle breakage rates (Br) and irregular compaction curves, whereas surface vibration compaction generated standard curves with significantly reduced Br. Although filler compositions varied, particle size and coarse content primarily controlled Br variation during compaction. The FF20 subgrade model exhibited minimal compressive deformation under WD cycling, with transitional deformation progressively diminishing as cycling enhanced the model’s deformation resistance. After cycling, the saturated FF20 model maintained a resilient modulus (MR) above regulatory requirements. Field tests confirmed FF20-filled subgrade complies with specifications for compaction degree, deflection, and MR in both dry and saturated conditions. Forty-month monitoring recorded only 1.64 mm centerline compression, validating satisfactory deformation resistance. This study broadens potential applications for URDW and provides practical guidelines for similar projects.