To evaluate the influence of uniformly tensioned roof bolts on roof stability, mechanically anchored roof bolts were installed using the conventional bolter and a specially developed thrust and torque controlled retrofitable box mounted on a bolting machine. Thirteen thousand bolts were installed over 19 crosscuts in a 4 heading development in an operating coal mine. Geological, bolt and mine parameters were recorded at bolt installation, and bolt tension histories, roof sag and other changes evident in the rooms, crosscuts and intersections were recorded over a 2 month period. In all, 24 variables and 11 responses were quantified at 67 bolt clusters.These data were analyzed statistically to determine the significant variables influencing roof stability. As the mine development was safe and stable during the monitoring period, roof sag after two months was used as an index to quantify "stability." The variables of significance were shown to be coefficient of variation of bolt tension, bolt anchorage capacity, roof lithology, roof shape, standard deviation of bolt tension, and dip of roof lithologies. It was concluded that of the variables with which the mine operator has control, bolt tension uniformity was the most significant parameter at this mine. Other variables, such as intersection type and time between mining and bolting, were not significant, whereas, some other potentially significant variables such as horizontal stress or roof joint frequency did not vary sufficiently for their influence on stability to be evaluated.
Roof bolts form a major component of underground coal mine roof control programs with the most commonly installed roof bolt being the mechically anchored tensioned roof bolt. The mechanically anchored roof bolt is anchored essentially at a point by an expansion shell that is activated and the bolt tensioned by rotating and torqueing the bolt head. This bolt system is generally the quickest to install with the lowest material cost but is not suitable for all geologies. Combinations of mechanical anchors and chemical grouts or mechanical full contact bolts (such as the Swellex or Split Sets) are used in roof lithologies where the simple point anchor mechanical bolt is unsuitable. This paper summarizes the results of a three year effort to evaluate the impact of improved techniques for the installation of mechanical anchored and torque-tensioned bolts on roof stability for roof lithologies typical of western United States coal mines. This research was conducted for the U.S. Bureau of Mines (USBM) in a program entitled, "Field Tests of Uniformly Tensioned Roof Bolts," with Mr. Robert Thompson as the Technical Project Officer. The USBM has supported several research and development projects to evaluate roof bolts reinforcement mechanisms and thus to increase bolt effectiveness for ground support. Programs related to the present study were initiated in 1975 with a project to develop accurate torque and thrust control for roof bolters. Subsequent programs evaluated in the laboratory and through field testing, the effect of hardened washers on the installed bolt tension (Rosso, 1977), and the effect of torque and thrust control with and without hardened washers on installed bolt tension and bolt tension variability. These programs resulted in a retrofitable control box to monitor and control torque output of the existing bolting machines (Brest van Kempen and Sweet, 1978). Further studies in 1979 and 1980, revealed a significant improvement in bolt tension uniformity (as measured by the ratio of bolt tension to standard deviation of bolt tension at installation) by improving the retrofitable control box for torque- thrust cont
Hydraulic fracturing was applied in horizontal drill-holes in the Salado salt formation near Carlsbad, New Mexico. Tests were performed approximately 650 m below surface to validate the design of a Waste Isolation Pilot Plant (WlPP) for the disposal of radioactive waste from defense activities of the United States. Hydraulic fracturing was performed primarily to determine whether the virgin in situ stress state at the WlPP site is isotropic and whether the magnitude of the virgin in situ stresses corresponds to the weight of the overburden. Beyond these limited objectives, however, measurements are now being reviewed to evaluate the usefulness of hydraulic fracturing in salt formations in general. Such measurements are desirable to determine stresses induced by mining and to monitor time-dependent stress changes around underground excavations in salt masses. Hydraulic fracturing was chosen because it is the only technique that can be readily applied in virgin ground far from existing mine openings. The method also appeared to yield directly the magnitude of the least compressive principal stress, regardless of the mechanical properties of the rock mass. A knowledge of the least principal compression was deemed sufficient to establish the isotropic character of the virgin in situ stress field under two conditions: (1) the Salado formation was mechanically isotropic, i.e., the fracture toughness did not vary normal and parallel to bedding so as to permit preferred fracture paths, and (2) pressure measurements were combined with determinations of the orientations of hydraulic fractures and, more generally, with observations of the characteristic hydraulic fracture patterns. The latter was possible by locating the drillholes for hydraulic fracturing along the axes of drifts that were mined several months later.
The success of general stress measurements by hydraulic fracturing of salt subjected to anisotropic stress conditions was and remains less certain. While the smallest in situ compressive stresses should still be equal instantaneous shut-in pressures, there may not exist any unique relationships between the greatest and intermediate in situ principal stresses on one hand and the primary breakdown and fracture reopening pressures on other. This problem would be a consequence of stress laxation due to salt creep around any drillhole in which hydraulic fracturing tests are conducted. Moreover, laboratory measurements suggest that creep and stress laxation could be so rapid that it might not be possible interpret the measurements for anisotropic stress conditions even if hydraulic fracturing were performed as soon after drilling as possible. The primary objectives of the stress measurements the WlPP were met by combining hydraulic fracturing tests with finite element analyses of the effects of salt creep, observations of hydraulically induced fracture patterns, and laboratory hydraulic fracturing tests on oriented core. In situ hydraulic fracturing tests in long drillholes were preceded by several trial tests to establish the best test methods and to demonstrate that salt could be fractured. The resolution of anisotropic stresses and special problems, e.g., in the identification of instantaneous shut-in pressures, are currently being addressed in hydraulic fracturing tests in a mine pillar, in tests lowing different delays between drilling and fracturing, and in tests with different frac fluids.
Faulkner, Gavin J. (Department of Mining and Minerals Engineering, Virginia Polytechnic Institute and State University) | Kesh, Someswat (Department of Mining and Minerals Engineering, Virginia Polytechnic Institute and State University)
Powered supports in longwall mines essentially operate under two conditions:
Observations of the land surface show that subsidence over active and abandoned underground mines is controlled to some extent by the stratigraphic and structural geology of the mines in addition to the methods of mining and the thickness of the seam mined. Exposures in highwalls of surface mines that intersect abandoned 6nderground mines in the Warrior coal basin of Alabama illustrate this, showing sandstone beds that act as beams bridging openings and interbedded shale beds crumbling. Stratigraphic changes observed in the district are extremely variable and calculations regarding subsidence used for one area will not apply to another area if there is a major change in rock types. Rocks exposed in road cuts and open pit mines demonstrate a marked difference in weathering characteristics. Some rocks, such as shale, weather very rapidly, by crumbling and becoming plastic, whereas other rocks, such as sandstone, resist weathering. The continuing rock weathering after mining in openings produced by the mining activity and in fractures caused by roof movement, apparently, is a major cause of the surface subsidence that may occur years after the mines are abandoned.
The purpose of this report is to emphasize the importance of stratigraphic and structural geology in evaluating the surface subsidence over underground mines. Subsidence problems are in two fields. One set of problems deals with surface collapse over abandoned mines, some of which were closed more than a half century ago. The second set of problems are surface movements over active mines. The stability of the surface over abandoned mines is a major concern in Alabama because municipal developments extend over abandoned mines in both the Warrior and the Cahaba coal basins, and occasionally there are subsidence problems. The surface over the abandoned Birmingham red-iron ore mines on the southeast slope of Red Mountain and below Shades Valley is now a potential problem because this area, formerly held as a mining re- serve, is open for development (fig. 1).
Surface stability is an international problem. European problems are that mining is conducted under densely populated areas (Brauner, 1973). In the United States problems are that some cities, like Birmingham, are expanding with housing and other construction over mines that have been abandoned years ago, and they are having subsidence problems (Matheson and Pearson, 1985). A problem facing active mines is that government regulations require subsidence plans be filed before permits to mine are granted (Chen and Peng, 1985). The stability investigation of the Warrior coal basin includes observations made throughout the entire district. The writer visited surface mines in each area, examining the highwalls, which provide excellent geologic cross-sections. The subsidence and distortion of strata were mapped and photographed in the highwalls of pits that had intersected abandoned underground mines. The reconnaissance included the Birmingham red-iron ore district. Field work included traverses in old and new housing developments that overlie abandoned mines, looking for evidence of subsidence in houses, streets and walks where cracks would be visible. Approximately 12 months were spent on the field investigation.
This paper presents an application case history of the planning methods used for developing, costing and scheduling an underground hydro- electric pumped storage facility in weak rock. This method covers the system configuration, excavation methods and costs, initial support systems and final lining systems. This study was performed by the Omaha District, Corps of Engineers for the proposed Gregory County Hydro- electric Pumped Storage Project, and presents a synopsis and the results of the studies performed.
The initial construction planning studies for the proposed Gregory County Hydroelectric Pumped Storage Project have recently been terminated; however, these studies were carried out in enough detail to assure that the proposed plan is constructable and has a positive cost-benefit ratio when compared to alternate sources of power. The proposed project consists of two stages. Each stage would have an installed capacity of approximately 1,180 M.W. It is envisioned that Stage I would be basically completed before Stage II is started. The proposed project would be located in south central South Dakota in Gregory County, on the west side of Lake Francis Case, approximately 56 km (35 miles) north of the Fort Randall Dam. The project configuration contains the following major facility components: - An embankment forebay providing 5.3*108M3 (40,000 acre-feet) of storage for State I with provisions for expansion to 10.6*108M3 (80,000 acre-feet) for Stage II, constructed on top of the relatively flat Missouri Plateau which is about 213 meters (700 feet) above Lake Francis Case. - An underground powerhouse containing three 93.3 MW reversible pump turbines (each stage) with surge chambers located on the tailrace tunnel. - An intake structure and one vertical shaft, 244 m (800 feet) long by 9.14 m (30 feet) diameter, and tunnel 275 m (800 feet) long by 9.14 m (30 feet) diameter, for each stage to provide water passage between the forebay storage area and powerhouse. - A tailrace tunnel, 2,487 m (8,160 feet) longby 10.2 m (33.5 feet) diameter, for each stage for water passage from the powerhouse to the discharge channel. - A discharge channel including trashrack structures to provide water passage from the tailrace tunnel to Lake Francis Case.
These components are shown on Figures 1 and 2. (available in full paper)
The rock formations at the project are indurated and compacted sedimentary marine deposits. The stratigraphic sequence in ascending order is the Carlile shale, Niobrara chalk and Pierre shale. The average unconfined compressive strengths are 6.45 MPa (935 psi), 6.89 MPa (1,000 psi), and 2.93 MPa (425 psi), respectively. Figure 2 shows a geologic cross-section at the site. The underground powerplant will be located in the Niobrara chalk formation which is approximately 52 m (170 feet) thick at the powerplant location. The chalk is overlain with about 214 m (700 feet) of Pierre shale. The top of the powerplant is situated so that 12 m (40 feet) of Niobrara chalk would be between the top of the powerhouse and the Pierre shale. The Pierre shale is weaker than the Niobrara chalk and it is desirable to contain the higher stresses from the opening within the Niobrara chalk.