Arching behavior and pressure distribution characteristics in calcareous sand strata: Insights from trapdoor tests

Abstract Calcareous sand exhibits distinct physical and mechanical properties, including irregular particle shapes, high porosity, and crushability, leading to arching behaviors that differ from siliceous sand. This study investigates the development of soil arching and pressure redistribution and ground deformation in calcareous sand under trapdoor excavation. A series of model tests examined the effects of particle gradation, void ratio, and burial depth ratio. Comparative tests were also performed between calcareous and siliceous sands under wet conditions, and between dry and wet calcareous sand. Results indicate that soil arching evolves through four stages: initiation, formation, expansion and adjustment, and residual state. Compared to siliceous sand, calcareous sand shows stronger arching and lower surface settlement, attributed to higher interparticle friction and stronger particle interlocking. Moisture enhances arch stability, while dry calcareous sand exhibits ongoing loosening and greater settlement. Well-graded sands and lower void ratios promote arch formation and reduce relaxed earth pressure, whereas a smaller burial depth ratio limits arch development. To quantify these behaviors, three normalized indices are proposed to evaluate unloading efficiency, surface deformation and arch mobilization, enabling unified comparisons. These findings provide a systematic understanding of deformation mechanisms and offer practical insights for geotechnical design in calcareous sand strata.