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Mining Publication: Shear Mechanism for Mining-Induced Fractures Applied to Rock Mechanics of Coal Mines
Keywords: Geology Geophysics Rock mechanics Roof falls
This page is archived for historical purposes and is no longer being maintained or updated.
Original creation date: August 2002
Authors: BG White
Conference Paper - August 2002
Download PDF Document (5.50 MB)
NIOSHTIC2 Number: 20025564
Peng SS, Mark C, Wahab A, and Heasley KA, eds. Proceedings of the 21st International Conference on Ground Control in Mining (Morgantown, WV, Aug. 6-8, 2002), West Virginia University, Morgantown, WV, 2002 Aug; :328-334
Two examples of en echelon mining-induced fractures seen in hardrock mines provided a basis for inferring that fracture zones and bedding plane separations immediately surrounding mine openings are promoted by oblique shear into the openings. It is hypothesized that initial fractures or separations form at the corners of openings as a result of high stress and physical constraint on the rock's ability to deform elastically toward the opening. These conditions result in a locally preferred direction of shearing. The shearing, in turn, generates tensile stress that initiates a progression of systematically offset fractures approximately parallel to the direction of greatest compressive stress. The fractures or bedding separations create tabular rock layers that amplify shearing displacement through bending and dilation. Such shearing effectively reduces and redistributes the compressive stress, but significant dilation is an inevitable consequence. The combination of dilation and shearing and the progressive development of fracture zones have important implications with respect to ground support. The concept of mining-induced fractures forming as a result of shear is illustrated by two examples from coal mines. First, fractures seen at longwall faces probably result from shear associated with subsidence. The fracture zone that develops approximates or possibly defines the draw angle of subsidence. As the face advances, fractures extend downward along the lower edge of the fracture zone, while upper extensions of the fractures are pressed closed. Fracture zones in entry roofs provide a second practical example. Here, mining-induced fractures typically follow bedding planes. The shear zone model suggests that the first bedding separations develop near the edges of the roof and successive separations progress upward and toward the center. However, if the direction of greatest stress is inclined with respect to the roof, a fracture or bedding separation zone may propagate from one side only and also extend higher. Because coal ahead of the face provides some support against lateral shear deformation, bedding separation is inhibited near the face. Rock bolts installed close to the face ultimately become more strained and bent than bolts installed a few meters from the face, and bolts installed through the more remote part of a separation zone may ultimately experience the greatest tensile and bending strains. This model is supported by field data documenting progressive bolt failures that rapidly propagated downward across the roof during face advance.
Image of publication Shear Mechanism for Mining-Induced Fractures Applied to Rock Mechanics of Coal MinesConference Paper - August 2002
Download PDF Document (5.50 MB)
NIOSHTIC2 Number: 20025564
Peng SS, Mark C, Wahab A, and Heasley KA, eds. Proceedings of the 21st International Conference on Ground Control in Mining (Morgantown, WV, Aug. 6-8, 2002), West Virginia University, Morgantown, WV, 2002 Aug; :328-334
See Also
Elastic and Shear Moduli of Coal Measure Rocks Derived from Basic Well Logs Using Fractal Statistics and Radial Basis Functions
Geologic Factors Causing Roof Instability and Methane Emission Problems: The Lower Kittanning Coalbed, Cambria County, Pa
Methods of Determining the Orientations of Bedrock Fracture Systems in Southwestern Pennsylvania and Northern West Virginia
Pillar Mechanics of Coal Mine Bursts: A Control Strategy
Report on Early Strength Performance of Modern Day Weak Rock Mass Shotcrete Mixes
Reservoir Rock Properties of Coal Measure Strata of the Lower Monongahela Group, Greene County (Southwestern Pennsylvania), from Methane Control and Production Perspectives
Roof Support
Shale Failure Mechanics and Intervention Measures in Underground Coal Mines: Results from 50 Years of Ground Control Safety Research
Spatial Trends in Rock Strength - Can They Be Determined From Coreholes?
Stability of Underground Openings Adjacent to the Sink Hole at the NIOSH Lake Lynn Research Laboratory
Page last reviewed: June 8, 2016
Page last updated: September 21, 2012
Content source: National Institute for Occupational Safety and Health, Mining Program
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