## 6 Conclusions  
1. According to conditions of surface buildings and structures above the backfill panel in Tangshan Coal Mine, the control criterion of backfill mining for surface buildings was proposed; the compressive deformation, tensile deformation, and surface subsidence should not exceed \(-2.0\) mm/m, \(1.5\) mm/m, and \(500\) mm, respectively. Coal gauge was collected from Tangshan Coal Mine and formed into test specimens in the laboratory. The creep compression characteristics of the gangue backfill materials were measured, the deformation characteristics during creep compression were explored, and the corresponding constitutive equation of creep compression was established. 2. Based on the constitutive equation of creep compression of gangue backfill materials, a method for the simulation of the materials was proposed. In addition, the numerical model for surface deformation induced by backfill mining was established to simulate and assess surface subsidence above the backfill panel after different creep durations of backfill materials. As the creep duration prolonged, the maximum surface subsidence  
Figure 16: Buildings above the backfill mining area: (**a**) brickwork-concrete-structured shop fronts, (**b**) brickwork masonry houses. Figure 15: Measuring lines for surface subsidence: (**a**) south-north measuring line, (**b**) measuring line along the direction of advance of the backfill panel. increased (albeit at a decreasing rate). When the creep duration exceeded 270 d, the rate of surface subsidence decreased gradually and then tended to stabilize. * Through simulation, it is found the maximum values of surface subsidence, horizontal movement, inclination, curvature, and horizontal deformation above the backfill panel were separately 171.82 mm, 56.38 mm, 0.38 mm/m, 0.002 mm/m\({}^{2}\), and 0.29 mm/m. The maximum surface subsidence and horizontal deformation both met the fortification criterion for buildings on the ground surface above a mine. * By arranging two measuring lines above the backfill mining area, the cumulative surface subsidence along the two measuring lines was plotted; after the surface subsidence stabilized, the maximum surface subsidence reached 163.4 mm, satisfying the fortification criterion for surface buildings above the backfill panel. This indicates backfill mining basically does not influence the buildings on the ground surface above a mine. The measured values are consistent with the results of numerical simulation. * From the research results, it is clear the creep compression deformation of gangue backfill materials has a determining impact on the surface deformation. The particle size distribution can be adjusted and optimized to improve the creep compression deformation of gangue backfill materials. Especially for the particle size distribution of 0-30 mm, it has relatively small compressive deformation and can be the first choice for mine backfill. The reason is the existence of the small particles increases the coordination numbers of the large particles and decreases the amounts of the broken large particles, which effectively improves the deformability of gangue backfill materials. * Backfill mining of gangue is an effective method to control the surface subsidence and reduce the gangue waste accumulation. Because the mined-out space can be backfilled by gangue backfill materials, after that, the deformation induced by the creep compression of gangue backfill materials is gradually transferred to the surface. The results can provide reference to the surface deformation analysis under the support of gangue backfill materials in practical engineering for protecting the stability of buildings and structures. Conceptualization, M.L. and J.Z. ; methodology, M.L. ; validation, L.D. and Y.P. ; investigation, D.M. ; resources, Y.P. ; data curation, L.D. ; writing--original draft preparation, M.L. ; writing--review and editing, M.L., D.M., and P.H. ; supervision, J.Z. and P.H. All authors have read and agreed to the published version of the manuscript.