Stability Evaluation of Multi-pillar and Roof System Based on Instability Theory

Pillars are often used to support the roof in underground mining. The multi-pillar and roof system is regarded as a multi-pillar and rock beam model. For a system composed of n pillars, the interaction force between the roof and pillars is obtained by a semi-analytical and semi-numerical method. Pillar failure may be progressive or sudden, depending on the equilibrium stability of the system in the post-peak stage. The initial conditions of multi-pillar failure and influence of one pillar progressive failure or unstable failure on adjacent pillars are analyzed based on the instability theory. If one pillar fails gradually, the stress transfer between pillars is also progressive. Once the first pillar fails suddenly, part of the stress is transferred to adjacent pillars, which may lead to further unstable failure of adjacent pillars and cascading failure of multiple pillars. The factors of pillar unstable failure mainly include geometric and mechanical parameters of the system. The mechanical parameters cannot be changed; however, entry (or stope in metal mine) geometrical parameters can be adjusted to reduce the possibility of unstable failure. The variations of pillar stability with entry widths are analyzed according to the factor of safety (FoS) and roof-to-pillar stiffness ratio r(k). If the pillars are arranged in a properly concentrated manner, FoS increases, r(k) decreases and the tendency of pillar unstable failure increases. Conversely, if the pillars are scattered close to the barrier pillars, the possibility of pillar unstable failure is reduced but the overall strength of all pillars is also reduced. Therefore, for a multi-pillar and roof system, the entry widths should be properly adjusted from an overall system perspective to ensure that both FoS and r(k) values are sufficiently large to minimize the possibility of pillar unstable failure.

Automated summary

Pillars are commonly used to support the roof in underground mining. The failure of pillars can be gradual or sudden, depending on the stability of the system. The stress transfer between pillars can also affect the stability of adjacent pillars. The geometric and mechanical parameters of the system play a crucial role in pillar failure. Adjusting the entry widths can help reduce the possibility of pillar unstable failure.