The Bureau of Mines has conducted a study to assess the redistribution of mining induced stress associated with longwall panel extraction. The objective of this study was to assess the performance of various chain pillar configurations on gate road entry stability. To achieve this objective, field measurements were collected and analyzed to develop profiles of the stress redistribution occurring in the gate road chain pillars during longwall panel retreat mining. A better understanding of the redistribution of stress will allow for improved gate road design by more accurately defining the loads imposed on chain pillars.
The study was conducted at a mine in southwestern Pennsylvania which was experiencing severe ground control problems ahead of the face in the longwall tailgate entries. These problems had drastically cut production and blocked air courses for ventilation. Extensive maintenance was required to reestablish and maintain air flow for face ventilation. Similar but less extensive problems were experienced in the headgate.
The Bureau developed an instrumentation plan which involved a series of three longwall panels with four corresponding gate roads. Instruments were installed in each of the gate roads during development and monitored throughout the extraction of each panel. These instruments, vibrating wire stressmeters, were installed in adjacent pillars in each gate road. Using instrument readings taken prior to mining and subsequent readings taken with respect to longwall face advance, the stressmeters were used to determine the change in vertical stress. These data were then analyzed to gain a better understanding of the mechanics of vertical load redistribution in the gate road pillars. Stress profiles across the pillars show the stress changes as they occur in the center and outer edges of the pillar with respect to face advance.
The abutment-yield and yield-abutment-yield pillar configurations proved to be a more effective means of maintaining entry stability than traditionally sized rectangular pillars. Vibrating wire stressmeter data indicate that average pillar loads were lower and appeared to stabilize when an abutment-yield pillar arrangement was utilized. Data analyses also support the occurrence of severe roof and pillar deterioration which was visually observed in the tailgate entries during panel extraction.
A major constraint to successful longwall design is the lack of understanding of the induced stresses about the longwall panel and its boundaries. High stress concentrations on any part of the longwall system may lead to severe ground control problems which may result in hazardous conditions, lost production, decreased productivity, and higher mining costs. The inability to control stress redistribution associated with panel extraction impacts all areas of the longwall system including the accessibility and stability of gate roads, the stability of the face, the selection of longwall powered face supports, and the maintenance of tailgate entries for ventilation.
In many cases, the most severe longwall ground control problems are manifested in the tailgate entries. The inability of pillars to control the roof in the tailgate seriously impedes production by curtailing air courses required for face ventilation.