Numerical analysis of the mechanical state of subgrade filled with discarded phyllite with different stone content

To address the shortage of subgrade filling materials and the land occupation caused by engineering waste in highway construction in the Longnan area, phyllite waste generated from excavation sections and tunnel blasting was reutilized as subgrade fill. Heavy compaction tests were first conducted on phyllite mixtures with stone contents of 30%, 40%, 50%, 60%, and 70% to determine their compaction characteristics. The results showed that the maximum dry density was achieved when the stone content ranged from 50% to 60%, indicating favourable compaction performance. Subsequently, California Bearing Ratio (CBR) tests were performed on phyllite fills with stone contents of 40%, 55%, and 70% to evaluate their engineering applicability. Under different compaction conditions, the 55% stone-content mixture consistently exhibited higher CBR values and lower water absorption than the 40% and 70% mixtures, demonstrating superior bearing capacity and water stability. Furthermore, finite element models of subgrades filled with phyllite waste containing 40%, 55%, and 70% stone contents were established to analyse the mechanical response of the subgrade under static vehicle loading, dynamic vehicle loading, and the combined action of water infiltration and dynamic loading. The numerical results revealed that the maximum shear stress was concentrated at the side slope of the subgrade, while the maximum tensile stress occurred at the subbase layer of the pavement structure. Among the three cases, the subgrade filled with 55% stone-content phyllite exhibited the smallest settlement and lower tensile and shear stresses. Therefore, it is recommended that the stone content of phyllite fill be controlled within 50%–60% in practical engineering applications.