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Geology roof control and mine design.
Coal Age 2001 Dec; :29-31
Geology is an integral part of roof control, mine design, and production operations. Yet, the importance of geology, coal/rock as an engineering construction material and its properties and behavior within the planned mine areas, in overall mine production operations has not been fully appreciated. Of all of the engineering disciplines, mining engineers work with the most challenging construction materials. They must deal with rock materials as they exist in their natural states and design mine structures without well-known and defined properties. Further complicating the design process is the variability of the in-situ rock materials with rock types and rock properties varying widely from place-to-place. Predicting properties in advance of mining is difficult and oftentimes impossible. This is why some types of roof-control failures may suddenly occur in some areas when a single roof-control technique is consistently applied to an entire panel, section, or mine. Many roof-control techniques are primarily applicable to the geological conditions that were assumed in the development of those techniques and variations of the rock or rock properties may require modification or significant redesign of the support technology. A change in the geologic properties often results in significant variation of the rock behavior that is outside of the original design criteria limits assumed by the original authors in the development of the roof-control techniques. When this occurs, those techniques may not work, and in the worst case, the support system fails. Therefore, it is important to know as much as possible about geological conditions. There have been many studies performed and papers published in the past regarding how various geological anomalies or defects have caused entry stability problems (roof falls, rib rolls, and floor heave). The anomalies, such as clay veins/dikes, coal seam rolls, hillseams/mountain seams, joints/faults, kettle bottoms, and sandstone channels, are larger in size and oftentimes easily recognizable. A series of reports were developed to help identify these features and recommend preventive measures to reduce their impacts. However, other features involving minor or gradual and subtle changes in geology that are not easily identifiable or observable have been responsible for major roof-control failures. Thick sequences of thinly bedded sandstones, siltstones, and shales (most commonly called "stack rock") are often difficult to support given the tendency to delaminate following displacement after mining or horizontal stress concentrations. While the compressive strength of the individual rock units is high, the stability of the strata is very low. The laminations weaken the rock sequence to the point that even after roof bolting the rocks are not structurally sound enough to remain stable. This type of roof has been found to be responsible for massive high roof falls covering many pillar blocks. Recently a type of coal mine roof rock was observed to be extremely unstable and virtually impossible to support prior to complete failure. This rock interval appeared to be highly fractured in an orthogonal pattern with the fracture pattern being extremely closed-spaced (less than 1/16 inch). The rock interval failed immediately following mining and failed in a near-flowing motion as one would expect unconsolidated sand grains to displace. The close spacing of the fractures, the calcite coating on the plane surfaces, and the degree the fractures were interconnected resulted in a rock unit that essentially was unconsolidated. This rock unit appeared within the mine roof suddenly and required a significant effort for mining in the area. Partings in coal seams are important features for coal mining operations and they are frequently used for cutting horizon control. Partings also play a very important role in entry stability, especially in high coals and/or under deeper overburdens. A stronger parting normally enhances the stability of the pillars although it is more difficult to cut during the mining operation. On the other hand, a much weaker parting, depending on its location and thickness, could be the deciding factor in pillar stability in thick coals and/or under high cover, rather than the strength of the coal itself. All conventional pillar design formulas do not consider the fact that coal seams usually contain one or more partings at various elevations and the role of the partings in pillar and entry stability. Most partings found in coal seams are weaker than the host coal and normally contain varying amounts of clay minerals. The clay minerals are subject to weathering and deteriorate once in contact with the wet/dry cyclic nature of the ventilation air. These weathered partings tend to promote rib rolls and sloughages over time, particularly under the influence of abutment pressures. In high coals and/or under high overburdens, the deterioration could be so severe that it seriously threatens mining operations. Since mining engineers do not and cannot select the rock materials to build the mine structures and the in-situ rock materials are known to vary considerably from place-to-place, it is extremely important to be aware of the geological character of the roof and coal seam. Obviously the key is the timely identification and recognition of subtle or sudden changes in rock strata, the impact of the changes on structural stability within the mine, and the development of an approach for minimizing the impact if the conditions signify a shift from stable to unstable.
Rock-falls; Risk-factors; Risk-analysis; Mining-equipment; Mine-shafts; Geology; Miners; Mining-industry; Construction; Construction-materials
Page last reviewed: April 12, 2019
Content source: National Institute for Occupational Safety and Health Education and Information Division