Stability analysis of the composite support pillar in backfill-strip mining using particle flow simulation method

Backfill-strip mining is proposed as a sustainable mining method to address the shortage of backfill material and high filling costs at present. The overlying strata in backfill-strip mining are mainly supported by the combined support pillar (CSP) of the residual coal pillar and the filling body. The stability of the CSP in backfill-strip mining is important to control the surface subsidence and reduce surface environmental damage. The particle flow code (PFC) simulation method is used in this study to investigate the deformation characteristics, failure behaviours, and stress distribution of the CSP for assessing its stability. The different influencing factors of the stability of the CSP, including geological mining factors, backfilling mining techniques, and the sizes of the residual coal pillar and the working face, are discussed. The results show that the shape of the CSP looks similar to a saddle. The vertical stress of the coal pillars is larger than that of the filling body. The subsidence value of coal pillars is smaller than that of the filling body, but the horizontal movement value of the coal pillars is large. Among these influencing factors of the stability, the residual coal pillar width has the greatest influence on the CSP. The different widths of the residual coal pillar lead to changes of the support formation and the bearing stress weight of the CSP, which make a big difference in the stress distribution characteristics, the movement deformation characteristics, and the stability of the CSP. On this basis, the CSP is divided into four types according to the stability and the support characteristics of the CSP, and their deformation characteristics and stability are summarised. The research results are important for guiding the stability assessment of CSP in backfill-strip mining and preventing the subsidence disaster whilst promoting sustainable extraction of coal resources.