Borehole Pressure Cells Research Articles

Moderate Cover Bleeder Entry and Standing Support Performance in a Longwall Mine: a Case Study.

Bleeder entries are critically important to longwall mining for the moving of supplies, personnel, and the dilution of mine air contaminants. By design, these entries must stay open for many years for ventilation. Standing supports in moderate cover bleeder entries were observed, numerically modeled, and instrumented by researchers at the National Institute for Occupational Safety and Health (NIOSH). The measurements of the installed borehole pressure cells (BPCs), standing support load cells and convergence meters, and roof extensometers are presented in this paper in addition to the numerical modeling results and visual observations made by the NIOSH researchers in the bleeder entries. The results include the effects of multiple panels being extracted in close proximity to the instrumented site as well as over one and a half years of aging. As expected, standing supports closer to the longwall gob showed the greatest load and convergence. The roof sag appeared generally independent of the proximity to the longwall gob. The BPC readings were driven by both the proximity to the gob and the depth into the pillar. The results of this study demonstrated that the entry roof can respond independently of the pillar and standing support loading. In addition, the rear abutment stress experienced by this bleeder entry design was minimal. The closer the mine development, pillar, or supports are to the gob, the greater the applied load due to rear abutment stress.

A coal rib monitoring study in a room-and-pillar retreat mine

The National Institute for Occupational Safety and Health (NIOSH) conducted a comprehensive monitoring program in a room-and-pillar mine located in Southern Virginia. The deformation and the stress change in an instrumented pillar were monitored during the progress of pillar retreat mining at two sites of different geological conditions and depths of cover. The main objectives of the monitoring program were to better understand the stress transfer and load shedding on coal pillars and to quantify the rib deformation due to pillar retreat mining; and to examine the effect of rib geology and overburden depth on coal rib performance. The instrumentation at both sites included pull-out tests to measure the anchorage capacity of rib bolts, load cells mounted on rib bolts to monitor the induced loads in the bolts, borehole pressure cells (BPCs) installed at various depths in the study pillar to measure the change in vertical pressure within the pillar, and roof and rib extensometers installed to quantify the vertical displacement of the roof and the horizontal displacement of the rib that would occur during the retreat mining process. The outcome from the monitoring program provides insight into coal pillar rib support optimization at various depths and geological conditions. Also, this study contributes to the NIOSH rib support database in U.S coal mines and provides essential data for rib support design.

An in situ monitoring campaign of a hard rock pillar at great depth within a Canadian mine

A recent research campaign at a Canadian nickel-copper mine involved instrumenting a hard rock sill drift pillar with an array of multi-point rod extensometers, distributed optical fibre strain sensors, and borehole pressure cells (BHPCs). The instrumentation spanned across a 15.24 m lengthwise segment of the relatively massive granitic pillar situated at a depth of 2.44 km within the mine. Between May 2016 and March 2017, the pillar's displacement and pressure response were measured and correlated with mining activities on the same level as the pillar, including: (1) mine-by of the pillar, (2) footwall drift development, and (3) ore body stoping operations. Regarding displacements of the pillar, the extensometers provided high temporal resolution (logged hourly) and the optical fibre strain sensors provide high spatial resolution (measured every 0.65 mm along the length of each sensor). The combination of sensing techniques allowed centimetre-scale rock mass bulking near the pillar sidewalls to be distinguished from microstrain-scale fracturing towards the core of the pillar. Additionally, the influence and extent of a mine-scale schistose shear zone transecting the pillar was identified. By converting measured rock mass displacement to velocity, a process was demonstrated which allowed mining activities inducing displacements to be categorised by time-duration and cumulative displacement. In over half of the analysed mining activities, displacements were determined to prolong for over an hour, predominately resulting in submillimetre cumulative displacements, but in some cases multi-centimetre cumulative displacements were observed. This time-dependent behaviour was more pronounced within the vicinity of the plumb shear zone. Displacement measurements were also used to assess selected support member load and elongation mobilisation per mining activity. It was found that a combined static load and elongation capacity of reinforcing members was essential to maintaining excavation stability, while permitting gradual shedding of stress through controlled pillar sidewall displacements.

A case study of the stability of a non-typical bleeder entry system at a U.S. longwall mine

Longwall abutment loads are influenced by several factors, including depth of cover, pillar sizes, panel dimensions, geological setting, mining height, proximity to gob, intersection type, and size of the gob. How does proximity to the gob affect pillar loading and entry condition? Does the gob influence depend on whether the abutment load is a forward, side, or rear loading? Do non-typical bleeder entry systems follow the traditional front and side abutment loading and extent concepts? If not, will an improved understanding of the combined abutment extent warrant a change in pillar design or standing support in bleeder entries? This paper details observations made in the non-typical bleeder entries of a moderate depth longwall panel—specifically, data collected from borehole pressure cells and roof extensometers, observations of the conditions of the entries, and numerical modeling of the bleeder entries during longwall extraction. The primary focus was on the extent and magnitude of the abutment loading experienced due to the extraction of the longwall panels. Due to the layout of the longwall panels and bleeder entries, the borehole pressure cells (BPCs) and roof extensometers did not show much change due to the advancing of the first longwall. However, they did show a noticeable increase due to the second longwall advancement, with a maximum of about 4 MPa of pressure increase and 5 mm of roof deformation. The observations of the conditions showed little to no change from before the first longwall panel extraction began to when the second longwall panel had been advanced more than 915 m. Localized pillar spalling was observed on the corners of the pillars closest to the longwall gob as well as an increase in water in the entries. In addition to the observations and instrumentation, numerical modeling was performed to validate modeling procedures against the monitoring results and evaluate the bleeder design. ITASCA Consulting Group’s FLAC3D numerical modeling software was used to evaluate the bleeder entries. The results of the models indicated only a minor increase in load during the extraction of the longwall panels. These models showed a much greater increase in stress due to the development of the gateroad and bleeder entries--about 80% development and 20% longwall extraction. The FLAC3D model showed very good correlation between modeled and expected gateroad loading during panel extraction. The front and side abutment extent modeled was very similar to observations from this and previous panels.

Ground control monitoring of retreat room–and–pillar mine in Central Appalachia

In order to study pillar and overburden response to retreat mining, a ground control program was conducted at a Central Appalachian Mine. The program consisted of several monitoring methods including a seismic monitoring system, borehole pressure cells in the pillars, and time-lapse photogrammetry of the pillar ribs. Two parallel geophone arrays were installed, one on each side of the panel with the sensors mounted 3m into the roof. A total of fourteen geophones recorded more than 5000 events during the panel retreat. A MIDAS datalogger was used to record pressure from borehole pressure cells (BPCs) located in two adjacent pillars that were not mined during retreat. A series of photographs were taken of the pillars that had the BPCs as the face approached so that deformation of the entire rib could be monitored using photogrammetry. Results showed that pillar stability and cave development were as expected. The BPCs showed an increase in loading when the face was 115m inby and a clear onset of the forward abutment at 30m. The photogrammetry results displayed pillar deformation corresponding to the increased loading. The microseismic monitoring results showed the overburden caving inby the face, again as expected. The significance of these results lies in two points, (1) we can quantify the safe manner in which this mine is conducting retreating operations, and (2) we can use volumetric technologies (photogrammetry and microseismic) to monitor entire volumes of the mine in addition to the traditional point-location geotechnical measurements (BPCs).

Complete geomechanical characterization of rock mass with hydraulic borehole pressure cells : Lu, P H Key Questions in Rock Mechanics: Proc 29th US Symposium, Minneapolis, 13–15 June 1988P725–728. Publ Rotterdam: A A Balkema, 1988

Complete geomechanical characterization of rock mass with hydraulic borehole pressure cells : Lu, P H Key Questions in Rock Mechanics: Proc 29th US Symposium, Minneapolis, 13–15 June 1988P725–728. Publ Rotterdam: A A Balkema, 1988

886069 In situ determination of deformation modem and Poisson's ratio of a rock mass with hydraulic borehole pressure cells : Lu, P H Proc 28th US Symposium on Rock Mechanics, Tucson, 29 June–1 July 1987P273–281. Publ Rotterdam: A A Balkema, 1987

886069 In situ determination of deformation modem and Poisson's ratio of a rock mass with hydraulic borehole pressure cells : Lu, P H Proc 28th US Symposium on Rock Mechanics, Tucson, 29 June–1 July 1987P273–281. Publ Rotterdam: A A Balkema, 1987

New method of rock stress measurement with hydraulic borehole pressure cells : Lu, P H Proc International Symposium on Rock Stress and Rock Stress Measurement, Stockholm, 1–3 Sept 1986P237–245. Publ Lulea: Centek, 1986

New method of rock stress measurement with hydraulic borehole pressure cells : Lu, P H Proc International Symposium on Rock Stress and Rock Stress Measurement, Stockholm, 1–3 Sept 1986P237–245. Publ Lulea: Centek, 1986

Mining-induced stress measurement with hydraulic borehole pressure cells : In: Rock mechanics in productivity and protection (paper to the 25th symposium on rock mechanics, Evanston, Illinois, 25–27 June 1984) P204–211. Publ New York: AIME, 1984

Mining-induced stress measurement with hydraulic borehole pressure cells : In: Rock mechanics in productivity and protection (paper to the 25th symposium on rock mechanics, Evanston, Illinois, 25–27 June 1984) P204–211. Publ New York: AIME, 1984

Effect of Discontinuities on a Room-and-Pillar Coal Mine Plan: ABSTRACT

The highest safe extraction ratio is the goal of coal mine planning. This may be affected substantially by the interaction of regional in-situ stress with coal-measure discontinuities such as faults, interbedded weak and strong strata, rolls, and sandstone channels. The effect of discontinuities is to interrupt or concentrate the mechanics of stress transfer and cave development during coal extraction, often resulting in a costly change in mining plan. Heavy ground conditions were encountered when extracting pillars during retreat in a room-and-pillar panel at a mine in central Utah, resulting in the decision to bypass about 25% of the coal within the panel. Conditions which led to abandonment of the central portion of the End_Page 1172------------------------------ panel were observed in data obtained from instruments installed for subsidence monitoring purposes on the surface, in the overburden, and underground. Instrumentation included strain measurement at the surface and both absolute and relative stress measurement at mine level. Data from tape extensometer readings, overcoring, and borehole pressure cells support a rotated fault-block hypothesis. In the mine area, frequent nearly vertical faults are present and can be located on satellite imagery. Two major faults are present and can be located on satellite imagery. Two major faults striking obliquely across the panel appear to have isolated a block of rock that rotated toward the gob at the north end of the panel as mining progressed, creating adverse loading conditions at mine level. The combination of satellite imagery, ground mapping during mining, and selected instrumentation may hold some potential for improved mine development in areas experiencing similar ground control problems due to the interception of geologic discontinuities. End_of_Article - Last_Page 1173------------

A Rock Mechanics Case History of Elliot Lake

The Elliot Lake area is characterized by Proterozoic sediments containing uranium-bearing conglomerates separated by quartzite beds 10 to 100 ft (3.0 to 30.5 m) thick. The geological structure consists of a broad syncline with an east–west axis plunging about 5 °W, cut by northwest trending faults, and with steeply dipping east–west extension joints. All the mines use a stope-and-pillar method of extraction with narrow rib pillars about 250 ft (76 m) long on dip and sill pillars on strike.After the Elliot Lake Laboratory was established, detailed studies were undertaken to evaluate the methods that were available for the determination of the mechanical properties of the rock mass and its state of stress before mining. Practical studies were then made on the pillars, roofs, and abutments.Testing techniques were improved for the rock substance and the rock mass; however, much remains to be done to be able to characterize adequately the mechanical properties of the rock mass. A novel random sampling approach produced a suite of specimens many of which included fractures, with a mean uniaxial strength that was surprisingly little lower than the mean of only the solid specimens. The dispersion of values in such a suite was, of course, quite large. Of the other tests used, Brazilian tests were found to be useful for quality control of stress determinations using a strain recovery technique.The use of borehole pressure cells, seismic velocity, and borehole penetrometers as techniques for the determination of the mechanical properties of the rock mass remains questionable.The tectonic history of the region was resolved; it provides an explanation for the existence of horizontal stresses greater than vertical stresses and for the major principal stress to be oriented parallel to the axis of the syncline. It was also shown that the major principal stress axis is essentially parallel to the strike of extension joint surfaces, even when the strike deviates from the predominant 090° azimuth direction.After considerable experience with mining in these geologic conditions, which probably are more uniform than in most metal mines, the determination of stable spans of stopes and breadths of pillars can be done very well by judgment. However, for examining new layouts relatively simple theoretical analyses, particularly for the determination of stable pillar sizes, were found to provide a rational and useful basis for extrapolation.The stresses determined in pillars and abutment zones and the deformations of the roofs corresponded fairly well to values predicted by analytical and model techniques. The increased stress in the abutment zones extended into the solid for a relatively limited distance, which, in this relatively hard rock, seems to be related substantially to the span of the adjacent stope. All field measurements were subject to dispersion. The electrolytic analogue, which takes into account the three-dimensional aspects of the geometry of the tabular orebodies, showed that irregular mining boundaries have a distinct contribution to the variance of the pillar stresses. The finite element method was found to be flexible and useful in studying specific questions, particularly related to novel mining plans.