Abstract:
In their ARMA 2015 paper “Numerical Modeling of Roof Support Plans at 4-Way Coal Mine Intersections” authors presented an approach to quantify reinforcement effect of a roof support plan over a designated area based on critical failure strains in tension, compression and shear in the immediate roof, pillars and immediate floor strata [1]. The above led authors to evaluate roof support plans at two coal mines in Illinois for an idealized four-way intersection with one cut corner [2]. In this paper authors use the above approach to analyze the effects of two variables on intersection stability: 1) Effect of error in estimating GSI on assessing stability of intersection, and 2) Effect of mining cut sequence in relation to the orientation of pre-mining maximum compressive stress. First, sensitivity of critical strain zones is analyzed due to an error in assessed GSI values used for estimating the Hoek-Brown failure properties for different lithologies. Then, the effect of two cut sequences is analyzed on the stability of the intersection. The developed 3-D numerical model was calibrated using in-mine measurements of convergence and instrumented bolt loads. Differential pre-mining horizontal stress field result in high shearing strains at pillar corners. The immediate roof strata in the N-S entry was subjected to high compressive strains of about 4 mm/m while low confining stress in the ribs resulted in high tensile strains. Contours of yielded zones in coal ribs extended 0.8 m (2.5 ft.) in the E-W direction that increased to 1.2 m (4 ft.) at the base of the pillar with failure occurring in the shear mode. Pillar corners and 0.4 m (1.25 ft.) of the ribs along the N-S entry fail in shear and tension. Some pillar settlement with shear failure occurs in the weak floor around all corners.
Introduction
Room-and-pillar and longwall mining systems develop underground excavations whose stability must be considered over their service lives. The primary concern is around intersections since 70-80% of roof falls in US coal mines typically occur there. An analysis of 2004 to 2008 roof falls in Illinois mines also showed that over 80% of these falls were concentrated at intersections [3].
Rock bolts are extensively used as primary and secondary support elements in coal mines around the globe. Typical cost of roof control in Illinois mines constitutes about 7-20% of the production cost depending upon site specific geo-mining conditions. In spite of considerable previous research on the topic, there is a need to improve the scientific basis for the design of roof control plans. Most of the currently practiced bolting layouts have been developed based on field experience rather than on a scientific basis. Therefore, it would be a contribution to develop an approach that can identify critical areas of instability around an intersection and assess the efficiency of a roof control plan. The critical zones can then be reinforced with appropriate support elements to develop sound roof control plans that will control the rock mass deformations around intersections.
