Objective Under the influence of backfill slurry, water inrushes in mining face, and water accumulation in goaves, the water content in cemented gangue backfill varies with the immersion time, thus altering the bearing performance of the backfill. Investigating the strength and damage characteristics of cemented gangue backfill during saturated immersion is crucial for maintaining the stability of the backfill and ensuring the safety of coal mines.

Methods Using uniaxial compression tests and scanning electron microscopy (SEM) of cemented gangue backfill during saturated immersion, this study analyzed the law of changes in the backfill strength under varying immersion times. Based on the statistical distribution of microelement strength, this study established a piecewise damage constitutive model for the cemented gangue backfill in the pre-peak stage. Using SEM, this study revealed the mechanism underlying the strength weakening of the cemented gangue backfill during saturated immersion.

Results and Conclusions  The results indicate that during saturated immersion, the cemented gangue backfill under uniaxial compression exhibited significant four-stage stress-strain curves. In the compaction and closure stage of pores and fractures, the maximum strain/stress and the strain at the peak stress point were positively correlated with the immersion time, while the peak stress and modulus of elasticit y were negatively correlated with the immersion time. With an increase in the immersion time, the primary failure morphologies of the cemented gangue backfill transitioned from shear failure to tension-shear conjugate failure, finally shifting to longitudinal tensile failure, with the failures concentrated in the central part of the cemented gangue backfill. This study established a piecewise damage constitutive model for the cemented gangue backfill in the compaction and closure stage of pores and fractures that considered the maximum stress and strain, modulus of elasticity, peak stress, and strain at the peak stress point. This model was verified using test data, indicating that the model-derived theoretical curves were roughly consistent with the test curves. Subjected to the surface tension and lubrication of water, as well as mineral dissolution, the cemented gangue backfill during saturated immersion manifested a three-stage strength weakening mechanism. As the cemented gangue backfill was saturated with water, its strength continued to decrease at a decreasing rate. The results of this study will provide certain data for coal mining based on cemented gangue backfill in environments with water-rich goaves.