Creep Behaviors of High Fly Ash Content Cemented Backfill and Its Permeability Enhancement Effect of Overlying Strata

Abstract

The evolution laws of overlying strata permeability under the creep effect of backfill constitutes a critical environmental protection and safety challenge for deep mining. To address the aforementioned challenge, this study develops fly ash–cement backfill materials with a fly ash content exceeding 70.0% from the perspective of improving the utilization rate of coal-based solid waste resources, conducts conventional triaxial graded creep tests on the backfill, and investigates the creep characteristics of backfill mass. Based on numerical simulations of fluid–solid coupling in heterogeneous rocks, a global permeability calculation model for overlying strata in deep backfill mining is established, and the evolution law of overlying rock permeability is elucidated. The main conclusions are summarized as follows: (1) When the stress level reaches 0.9 times the peak deviatoric stress, the backfill exhibits three complete creep stages. The average total creep duration is approximately 660 min, and creep deformation accounts for roughly 20% of the total deformation upon failure. (2) According to the series connection characteristics of rheological mechanical elements, a creep constitutive model for backfill under equal confining pressure loading is established. Nonlinear fitting and verification are performed on the model parameters, with the coefficient of determination (R²) no less than 0.97 and 0.80, respectively. (3) A numerical calculation method for fluid–solid coupling of heterogeneous rocks is proposed. On this basis, the horizontal and vertical zoning characteristics of overlying rock permeability evolution subjected to backfill creep are revealed. The results provide theoretical support for safe deep coal mining and groundwater resource protection, as well as fundamental experimental data for further investigations on the creep behaviors of coal-based solid waste backfill materials.