In this study, the spatial distribution of the mechanical strength of ultra-fine tailings cemented paste backfill (UCPB) in underground stopes was examined, and the micro-mechanism responsible for differences in spatial strength performance via changes in particle deposition was elucidated. To better understand this phenomenon, we constructed a similar backfilling stope model using the ultra-fine tailings of a gold mine. We manufactured specimens at different spatial locations and conducted a novel series of tests, including uniaxial compressive strength, shear strength, and conventional triaxial tests, to obtain the strength parameters in different spatial distributions. The strength parameters for the different UCPB locations in this model were revealed and examined along with the original pore and particle size distributions. Computer tomography (CT) and scanning electron microscopy (SEM) observations before and after the UCPB failed revealed that the particle deposition effect at different positions was the primary cause of the size effect. The strength distribution pattern demonstrated that the UCPB material has an apparent spatial effect. Consequently, considering the position relationship between the stope and filling mouth is crucial when designing the strength of adjacent mining rooms. This research may guide both backfill study and design.

Cemented tailings backfill (CTB) plays an important role in mine filling operations. In order to study the long-term stability of CTB under the dynamic disturbance of deep wells, ultrafine cemented tailings backfill was taken as the research object, and the true triaxial hydraulic fracturing antireflection-wetting dynamic experimental system of coal and rock was used to carry out a static true triaxial compression test, a true triaxial compression test under unidirectional disturbance, and a true triaxial compression test under bidirectional disturbance. At the same time, the acoustic emission monitoring and positioning tests of the CTB were carried out during the compression test. The evolution law of the mechanical parameters and deformation and failure characteristics of CTB under different confining pressures is analyzed, and the damage constitutive model of the filling body is established using stochastic statistical theory. The results show that the compressive strength of CTB increases with an increase in intermediate principal stress. According to the change process of the acoustic emission ringing count over time, the triaxial compression test can be divided into four stages: the initial active stage, initial calm stage, pre-peak active stage, and post-peak calm stage. When the intermediate principal stress is small, the specimen is dominated by shear failure. With an increase in the intermediate principal stress, the specimen changes from brittle failure to plastic failure. The deformation and failure strength of CTB are closely related to its loading and unloading methods. Under a certain stress intensity, compared with unidirectional unloading, bidirectional unloading produces a greater deformation of the rock mass, and the failure strength of the rock mass is higher. This study only considers the confining pressure within the compressive limit of the specimen. Future research can be directed at a wider range of stresses to improve the applicability and reliability of the research results.