Backfill mining is the most environmentally friendly mining method at present, which can effectively control the surface subsidence, improve the recovery rate, and has good social and economic benefits. The purpose of this study is to solve the environmental problems caused by solid waste, combined with the rich geographical advantages of aeolian sand in the Yushenfu mining area of China. The rheological properties of the aeolian sand-fly ash-based filling slurry with different fly ash content are studied by experiments, and the strength development law of the filling body under different age and fly ash content are studied from the macroscopic and microscopic points of view. The rheological experiments showed that the increase of the amount of fly ash has a significant effect on the thixotropy, plastic viscosity, and yield stress of the filling slurry. Additionally, rheological properties of aeolian sand-fly ash-based filling slurry conform to the Bingham model. With the increase of the amount of fly ash, the performance of the filling slurry has been significantly improved. Uniaxial test and scanning electron microscope observation showed that the influence of fly ash on the strength of the filling body was mainly reflected in the late stage of maintenance, but was not obvious in the middle stage. Fly ash particles mainly bear the role of “water reduction” and a physical filling effect, which makes the filling slurry thicker and the internal structure more closely spaced. The volcanic ash reaction of fly ash is lagging behind the hydration reaction of cement; the secondary product of the delayed reaction is filled in the pores of cement hydrates, which can greatly reduce the porosity of the backfill body and increase the later strength of the backfill body. It provides a guarantee for the safe replacement of coal pillars in the working face.

Filling mining is an important direction in green coal mining. In the filling site, a layered filling body can be formed due to technological problems. In this paper, we take Shanghe Coal Mine (Shaanxi Province, China) as the background. In order to explore the mechanical properties and failure modes of layered backfill, specimens with different layered proportions (1:1, 1:3, 3:1) were made and studied concerning the aspects of wave velocity, porosity, strength and failure modes. The experimental result demonstrates that with the increase of curing time and fly ash (FA) content, the porosity of ASPLB decreases, the wave velocity and the strength increases. In addition, the layered structure has a significant effect on the strength and failure mode of the specimen. Uniaxial compression experiments showed that after 28 days of curing with Ratio III, the strength of layered backfill (LB) was reduced by 14% and the strength of 3:1 LB was increased by 16.7% and 40% compared with 1:1 LB and 1:3 LB, respectively. A digital speckle experiment showed that the failure mode of ASPLB is a vertical fracture without penetration, and the fracture propagation of layered ASPLB is hindered by the stratification. Based on the above research, the scheme that meets the requirements of the Shanghe Coal Mine is determined, and its reliability is verified, providing guidance for scientific stratification and the filling of gob.