Floor failure analysis at a longwall mining face based on the multiple sliding block model

Abstract

This paper presents an analytical model of a floor failure at a longwall coal mining face based on the multiple sliding block model. During longwall mining, stresses and displacements of strata are constantly changing. High stress concentrations at the coal face can exceed the rock strength and initiate fractures in the strata that can, under unfavourable conditions, lead to large floor displacements and disruption of mining operations. Underground observations of the rock floor and the computational modelling of the longwall face indicate that two types of fracture dominate floor failure. Extensive lateral fracturing often develops along the numerous weak bedding planes that are typically present in the sedimentary strata while sub-vertical fractures form in response to changing stress abutments ahead of the longwall coal face. The fractures that initially develop ahead of the longwall face are subject to a “secondary” movement when exposed ahead of the longwall supports. In response to the stress relief as the coal is mined from above, strata move towards the opening and bending of the floor occurs. If fractures in the floor exist, the floor blocks will displace in response to the floor movement and interact at the fractured surfaces. This analysis attempts to explain how the stress distribution develops within the broken floor during an active movement of floor strata leading to high stress concentrations at the floor level. The analytical formulation in this paper is supplemented by numerical modelling, and results are presented to verify that the analytical solutions are in accordance with the numerical predictions.