Semester

Spring

Date of Graduation

1999

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mining Engineering

Committee Chair

Syd S. Peng.

Abstract

High horizontal stress plays an important role in ground control in longwall mining. It can significantly affect the stability of the panel entry system in both the development and mining periods. Many roof problems have been contributed to high horizontal stress, especially when the immediate roof is weak, or thinly bedded.;In this research, the stress distributions in the entry roof in longwall mining have been studied when the high horizontal stress occurs. Using a three-dimensional finite element method, the stresses in the entry roof have been analyzed for the different cases. It is found that the stress angle is the most important factor that affects the roof stability when the magnitude of horizontal stress is fixed. Generally, the longwall entries are in the worst stress conditions when the stress angle is about 60° ∼ 75°.;At the T-junctions of a longwall panel, the entries are subjected to large front abutment pressure. Under horizontal stress, the roof stresses at the T-junctions increase with the stress angle. In a single panel, the stress in the headgate entries is larger than that in the tailgate entries when the maximum horizontal stress is from the headgate side. In a multiple-panel system, the stress in the headgate is larger when the horizontal stress is from the gob side than that from the solid coal side. But, the stress in the tailgate is always larger than that in the headgate.;Roof failures often occur in the laminated roof, where the roof consists of some rock layers. In this situation, the slip between the coal seam and the roof/floor may occur. In addition, the roof separations can appear. In this study, the roof separations and the slip have been simulated. Because of the slip between the coal sewn and the roof/floor, the stresses in the entry roof relieve to some degree. If the coefficient of friction in the interfaces is small, the stresses in the roof relieve significantly. The stresses in the roof increase with the coefficient of friction and the stress ratio of the horizontal to the vertical stress. When the roof separations occur, the stresses in the roof increase. Usually, the lowest roof layer is subjected to the largest loading. On the upper surface of this layer, tensile stress occurs along the vertical direction, which will worsen the roof condition.

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