Overcore measurements at the North Fork Valley (NFV) coal mines in western Colorado have shown that horizontal stresses are highly anisotropic. Measurements have been made in four mines at various depths. In many measurements, the maximum horizontal stress is three to four times higher than the minor horizontal stress. At the West Elk Mine, maximum and minimum horizontal stresses of 24 MPa and 6 MPa, respectively, have been measured at depths of 640 meters (m). Under highly anisotropic stress conditions, ground control problems associated with both high and low horizontal stresses can develop. While high horizontal stresses can produce cutter failures and floor heave, low horizontal stresses can allow block fallouts. This paper summarizes the horizontal stress measurements in NFV mines and, in particular, in the West Elk Mine where their role in roof fall and floor heave failures is discussed. This experience has led to an improved use of ground support and safer mining operations at depths of 640 to 700 m.
"Underground mine rib stability is an ongoing safety issue in all mining applications. Rib-related accidents present at least as many problems as roof-related ground control. Underground coal mine rib safety is continually emphasized as a primary safety concern at every mining operation. The application of rib support materials is increasing as more difficult mining conditions are experienced due to deeper cover, more variable depositional environments, thicker seam heights, and multi-seam mining effects. As the coal seams become thicker, the need for rib control increases. A variety of rib control methodologies has been documented and research continues on mine design methods and support techniques to reduce or eliminate rib-related accidents.This paper presents an alternative rib bolting technology using proven conventional rib support systems that allows mine personnel to more efficiently install rib support during development operations. This alternative rib bolting method allows for additional rib support with a single support installation.INTRODUCTIONRib control continues to garner the watchful eye of both concerned operators and government officials due to many factors: inherent safety within entries and crosscuts, actual rib stability history, safety improvement, cost control, improved mining methods, and maintaining production goals. As demonstrated through many years of actual practice, desired outcomes can be achieved by scrutiny, innovation, and combining improved practice methodologies.Rib surface control continues to be a necessary part of the production process. However, production and safety issues persist at the point of installation using current control apparatuses and methods to install rib controls. Among others, current issues of rib control are quantity of bolts required, overall time required for installation, ease of apparatus installation, flexibility of the system for appendage installation and operator exposure. A need exists to decrease the time miners spend installing rib control apparatuses, while increasing the safety of such installation processes. At present, installing rib control devices (primary rib bolts, plates, mesh, and, in most cases, secondary rib bolts and plates) is performed in a very congested area and is time-consuming process at the primary coal-winning area: the face. The MatLok system addresses these problems."
The Ohio Valley Coal Company (Ohio Valley) has operated the Powhatan No. 6 Mine in Belmont County, Ohio since 1972. Ohio University owns a 455-acre research farm over a portion of the Ohio Valley reserve containing 51 acres of old-growth forest known as Dysart Woods. In 1988, when Ohio Valley applied for a new mining area for its first longwall located several miles away from Dysart Woods, Ohio University claimed that mining at that distance would lower the regional water table and kill the old trees. In response, Ohio Valley began to study potential impacts from longwall mining on hydrology, soil moisture, and tree growth. Early studies examined the impacts of longwall mining on hydrology, soil moisture, and tree growth directly over mined areas compared to woodlots that were not undermined. A subsequent study examined tree-ring growth for a five-year period before and after longwall mining for several woodlots, some directly over panels and others at distances of up to 2000 ft away. No significant differences in growth rate associated with longwall mining were observed. New research was begun that compared longwall-mining impacts on groundwater hydrology, available soil moisture, and tree growth in two stands of mature trees located near Dysart Woods in order to confirm the earlier results. Using a test technique known as "Before-and-After-Controlled-Impact-Pairs," data was collected from one woodlot (test) that was undermined and from another (control) that was not undermined. Data collection began several years prior to longwall mining and has continued for several years after mining. The data for the test site shows that although the shallow and deep groundwater levels were affected by longwall mining, the soil moisture, vigor rating, or the growth rate of trees did not decline significantly compared to the control site where there were hydrologic impacts. The study shows that there are no known significant impacts to trees from longwall mining. The results of these studies were used by Ohio Valley to obtain State approval to longwall to within 300 ft of Dysart Woods
Implication of Highly Anisotropic Horizontal Stresses on Entry Stability at the West Elk Mine, Somerset, Colorado