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Author: Admin | 2025-04-27
From the ribline as shown in figure 12. Accurate calculation of the angle of draw has been the focus of longwall subsidence research over the last decade. Most studies on the subject conclude that the appropriate angle for eastern U.S. applications ranges between 15° and 25° for flat-lying coalbeds. The caving angle is not explicitly defined in rock mechanics literature, but observers have referred to it as a “negative angle of draw.” This implies that it projects out over the gob from the ribline as shown in figure 12.Studies of surface subsidence and caving behavior may provide some insight into a clear definition and value of the caving angle. King used this angle to estimate the side abutment load for longwall pillars. He assumed that this angle defined a failure plane where the weight of the overburden was evenly distributed between the pillars and the gob. King referred to the angle as a “shear angle,” and he estimated its value at 25°. Mark took a similar approach in developing a method for determining longwall pillar loads; he referred to the angle as the “abutment angle.” He estimated it at approximately 10° to 23° based on back-calculation from pillar stress measurements conducted in three coalbeds. Singh referred to the angle as the “angle of fracture” and used physical models to study its behavior and formation in different mediums. He noted the shear-tensile failure in its development and estimated its range from 5° to 15°. Peng implicitly called it a caving angle and based on observation estimated its range from 15° to 35° with an average of 25° applicable for most conditions. Different geological conditions may be the determining factor for this broad range of values. Obviously, caving mechanics warrants further study to better delineate appropriate caving angles based on stratigraphy.From purely a geometric perspective, the angle of draw and the caving angle can be used to determine the extent of the subsidence trough. Figure 12 illustrates a simplified example of this zone’s influence using an angle of draw of 25° and a caving angle of 25°. Strata bending progresses gradually, beginning at the angle of draw, and becomes more pronounced superjacent to the ribline. Upon crossing this boundary, fracturing and poor roof conditions due to tensile stress are most apparent. Proceeding farther, the inflection point denotes the end of the tension zone and the start of the compression zone. Beyond this,
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