Key Takeaways & Executive Findings
- •• Rock creep significantly increases the minimum required cohesion (cmin) of sill mats, especially at large mine depths. • Neglecting rock creep can lead to unsafe underestimation of cmin, risking sill mat failure. • For deep mines with severe rock creep, ductile cemented backfill (low stiffness, adequate strength) is recommended to reduce binder use and prevent crushing. • Prompt filling of the stope below the sill mat improves stability and reduces cmin.
Abstract
The underhand cut-and-fill mining method is widely employed in underground mines, especially when the quality of surrounding rock mass or ore body is inferior 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. Accurate estimation of the minimum required strength of the sill mat is crucial 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 mats. None of them considered rock wall closure to be associated with the creep of surrounding rock mass. Moreover, the effect of rock wall closure associated with rock creep on the cmin of the sill mat remains unknown. Thus, a series of numerical simulations was performed to fill this gap. The influence of rock creep on the cmin of base-exposed sill mat was investigated for the first time. The numerical results indicate that Mitchell’s models could be suitable for sill mats subjected to negligible wall closure. However, this scenario is rare, especially when mine depth is large. In general, the cmin of sill mats increases as mine depth increases. Neglecting rock creep would significantly underestimate the cmin of sill mats. When mine depth is large and the rock mass exhibits severe creep, cemented backfill with ductile behavior (i.e., with low stiffness but enough strength) should be considered to reduce binder consumption and prevent crushing failure. In all cases, promptly filling the mined-out stope below the sill mat can improve its stability and reduce its cmin value.
1. Introduction
Backfill prepared with cement, tailings, or waste rocks is widely utilized in underground mining operations to improve ground stability, reduce ore dilution, and increase ore recovery [1–15]. Sill mats constructed with cemented backfill are used in underhand cut-and-fill mining methods to protect mining workers and machines from direct exposure to problematic ground conditions due to poor rock mass or high potential for rock bursts. Another application of sill mats constructed with cemented backfill is in the recovery of ore sill pillars. In all cases, determining the required strength of the sill mat is a critical concern to ensure safe and economical design because the cement content of the sill mat should be as high as possible from the point of view of safety but as low as possible in terms of backfill cost. Therefore, an optimization should be performed, involving an estimation of the minimum required cohesion (cmin) of the sill mat.
To date, the strength design of sill mats remains a great challenge because their interaction with the rock mass (i.e., hanging wall and footwall) has not yet been fully understood. Mitchell [16] was probably the first to investigate the failure mechanisms of base-exposed sill mats using centrifuge physical model tests. Four failure modes (i.e., sliding, flexion, rotation, and caving) were identified, and an analytical solution for each failure mechanism was proposed using limit equilibrium analysis to determine the minimum required strength of the sill mat. Subsequently, Stone [17] proposed an empirical chart solution with a factor of safety (FS) = 2 to evaluate the required strength of the sill mat by considering its flexural failure mode. The graphical solution of Stone [17] was later updated by Pakalnis et al. [18–19] after compiling some practical cases in terms of the strength design of the sill mat. Although these works provide fundamental insights into the mechanical behavior of sill mats, they suffer from the assumption that rock walls are stiff and immobile. In practice, wall closure is unavoidable during and after stope excavation below the sill mat. Thus, it must be considered in any stability analysis and required strength estimation for the sill mat, as confirmed by recent publications through numerical modeling [20–23]. The increased horizontal compressive stress in the sill mat due to rock wall closure can l
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Chuan Fan, Li Li, Guangsheng Liu, Xiaocong Yang, Weidong Song, Lijie Guo, Ruofan Wang (2025). Numerical analysis of the stability and minimum required strength of sill mats considering creep behavior of rock mass. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-3029-y
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Frequently Asked Questions
What is the main objective of this study?
The study aims to numerically investigate the influence of rock creep on the minimum required cohesion (cmin) of sill mats in underhand cut-and-fill mining, addressing a gap in existing design methods that neglect rock wall closure due to creep.
How does rock creep affect the required strength of sill mats?
Rock creep leads to increased wall closure, which increases the horizontal compressive stress in the sill mat, thereby increasing the minimum required cohesion (cmin). Neglecting rock creep can significantly underestimate cmin, leading to unsafe designs.
What numerical method was used in this study?
The study used FLAC3D, a three-dimensional finite difference code, to perform numerical simulations of sill mat behavior considering rock creep.
What are the practical implications for mine design?
For deep mines with severe rock creep, the study recommends using cemented backfill with ductile behavior (low stiffness but adequate strength) to reduce binder consumption and prevent crushing failure. Promptly filling the stope below the sill mat can also improve stability and reduce cmin.
How does this study improve upon existing design methods?
Existing methods like Mitchell's models assume rigid rock walls and are only suitable for negligible wall closure. This study incorporates rock creep, providing more accurate cmin estimates for realistic conditions, especially at large depths.
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