Numerical analysis of the stability and minimum required strength of sill mats considering creep behavior of rock mass

The underhand cut-and-fill mining method is widely employed in underground mines, especially when the surrounding rock mass or orebody is of poor quality or subjected to high stresses. Such a method typically requires the construction of sill mats with cemented backfill to provide operators with safe artificial roofs. It is critical to correctly estimate the minimum required strength of sill mat to minimize binder consumption and ensure its stability upon base exposure. Over the years, only a few publications were devoted to determining the minimum required cohesion (cmin) of sill mat. None of them took into account rock wall closure associated with the creep of surrounding rock mass. The effect of rock wall closure associated with rock creep on the cmin of sill mat remains unknown. To fill this gap, a series of numerical simulations were performed. The influence of rock creep on the cmin of base-exposed sill mat is, for the first time, investigated. The numerical results indicate that Mitchell's models could be suitable for large spanned sill mats subjected to negligible wall closure. This is however seldom the case in practice, especially when mine depth is large. In general, the cmin of sill mat increases as mine depth increases. Neglecting rock creep would lead to a significant underestimation of the cmin of sill mat. When mine depth is large and rock mass exhibits severe creep, the cemented backfill having a ductile behavior (i.e., with low stiffness but high strength) should be considered to reduce binder consumption and avoid crushing failure. In all cases, promptly filling the mined-out stope below the sill mat can improve the stability and reduce the cmin of sill mat.