Abstract:

Longwall recovery operations are generally accomplished by using either standard practices or alternative approaches using pre-driven recovery rooms supported by a variety of internal and external support systems. In this paper, a two-pronged approach is used for evaluation of ground response during longwall recovery operations. First, more elaborate multi-variable statistical analyses of 130 cases (reported by NIOSH) is completed, highlighting significant geologic and mining factors affecting the stability of pre-driven recovery rooms. Second, ground reaction to the longwall load is analyzed using boundary-element and finite-difference analyses, including three mining steps, different cave conditions, and a variety of primary, secondary and standing support systems with and without backfill.

While back fill support is shown to be effective in stabilizing ground conditions, standing support is shown to be inadequate to control convergence for the weakest geologic conditions. This prediction is in agreement with underground observations in other similar mines experiencing ground control problems with pre-driven longwall recovery rooms using rock reinforcement and standing support only.

Introduction

Longwall recovery operations are generally accomplished by using either standard practices or alternative approaches using pre-driven recovery rooms supported by a variety of internal and external support systems. Although a great majority of case studies reported by NIOSH and the industry have been successful, there have been some cases of significant convergence in front of the shields and other major roof weighting and support failures associated with main roof loading. To help improve stability and safety during the recovery operations, NIOSH assembled some 130 case studies and drew preliminary conclusions pertaining to correlations between mining, geologic conditions and support requirements (1). Due to limitations in site specific data and analyses techniques, uncertainties remain to-date with actual loading conditions and failure mechanism at individual sites where significant convergence was reported. Furthermore, it is important to understand the full load-deformation characteristics of support required to reduce the potential for excessive convergence within the recovery room, and shields becoming ironbound, creating very difficult conditions.

In this paper, a two-pronged approach is used for evaluation of ground response during longwall recovery operations. First, additional multi-variable statistical analyses of 130 NIOSH reported cases is completed, highlighting significant geologic and mining factors affecting the stability of pre-driven recovery rooms. Second, ground reaction to the longwall load is analyzed using boundary-element and finite-difference analyses, including three mining steps, different cave conditions, and a variety of support systems (primary, secondary, internal and standing) with and without backfill.

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