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|Two examples of en echelon mining-induced fractures seen in hard¬rock 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 comers 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, frac¬tures 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.|
Additional chapters/articles from the SME-ICGCM book Proceedings 21st International Conference on Ground Control in Mining
|Pre-Driven Experimental Longwall Recovery Room Under Weak Ro||Longwall Mining-Induced Abutment Loads and Their Impacts on||Influence of Structural Stress Concentration and Structural||The NIOSH Shield Hydraulics Inspection and Evaluation of Leg||Study on Top-Coal Loss and the Optimum Drawing Interval of L||Stress Measurements for Safety Decisions in Longwall Coal||Re-Use of Rectangular Bolted Roadways in a Cover Depth > 100||Numerical Modeling of the Gob Loading Mechanism in Longwall||Deep Cover Pillar Extraction in the U.S. Coalfields||Evaluation of Pillar Recovery in Southern West Virginia||A Case History Investigation of Two Coal Bumps in the Southe||A Linear Coal Pillar Strength Formula for South African Coal||Anchorage Pull Testing for Fully Grouted Roof Bolts||Comparison of Some Aspects of Bolting Mechanisms Between Ful||Eclipse System Improves Resin Anchored Rebar Bolting||Design Considerations for Tensioned Bolts||Field Testing of the Fully Grouted Thrust Tensioned Bolts||Improvement in Pre-Tensioning of Strand Bolts in Australian||The Introduction of Roof Bolting to U.S. Underground Coal Mi||Support of Coal Mines in the United Kingdom||The Use of NDT Methods to Determine the Condition of Rockbol||Rockbolted Support of Retreat Longwall Gateroads at 1000m De||Roof Screening: Best Practices and Roof Bolting Machines||Numerical and Physical Modeling as Planning Tools for Rockbo||Stone Mine Design in Highly Fractured Rock||The Importance of Underground Stone Mine Roof Geology||Utilization of Ground-Penetrating Radar to Determine Roof Co||An Examination of the Loyalhanna Limestone's Structural||Highwall Stability in an Open Pit Stone Operation||Overview of Safety Considerations with Highwall Mining Opera||Highwall Monitoring to Combat Rockfall Accidents at Opencast||Seepage and Reinforcement Behavior of Grouting Into Slaking-||Floor Heave in Shallow Room-and-Pillar Mining||Analysis of a Stability Problem in an Underground Coal Mine||Comparison of Acoustic Emission and Stress Measurement Resul||Acoustic Scanner Analysis of Borehole Breakout to Define the||Estimating Rock Strengths Using Drilling Parameters During R||New developments with the coal mine roof rating||Application of geotechnical and geophysical parameters to im||Development of a Risk Rating System for Use in Underground C||Empirical and analytical design of large openings at a propo||Shear Mechanism for Mining-Induced Fractures Applied to Rock||Evaluating Techniques for Monitoring Rock Falls and Slope St||Developments in Sealant Support Systems for Ground Control||Stability Control of Clusters of Deep Openings Around Shaft||The Use of Pneumatic Stowing in Germany Considering Subsiden||A 3-D Semi-Analytical Method for Subsidence Prediction and S||Theory and Technology of Mining Subsidence Control by Grouti||Surface Subsidence Due to the Combined Effects of Undergroun|