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By Frank E. Chase

Sacrificial entries, roof slotting, and other tactics have been used to combat high horizontal stresses during roadway development in U.S. coal mines. In Australia, the "pillar extraction on the advance" mining method has been successfully employed as a horizontal stress control technique. The concept of advancing and relieving mine workings through pillar extraction developed from observations that some mines experience difficult ground conditions when advancing and retreating the first panel in a new reserve. However, conditions dramatically improve in subsequent panels. Apparently, the creation of a gob area stress relieves the adjacent panel workings. After much debate and close scrutiny, the first U.S. roof control plan to advance and relieve was approved in 1998, for the Sargent Hollow Mine in Wise County, VA. The major concern was that this mining practice places section personnel inby of the gob. The tentative approval permitted barrier pillar slabbing and removal of the adjacent development pillar during panel development. The intent of the plan was to provide a "stress shadow" for the advancing faces and outby workings to reduce the excessive number of roof falls. Sargent Hollow successfully extracted their first panel by pillaring on advance and retreat mining using posts. Mobile roof supports were employed to extract pillars while advancing the second panel. This paper summarizes Sargent Hollow Mine's experiences with the advance and relieve mining method.

Jan 1, 1999

By Kenneth L. Johnson

The effects of underground longwall coal mining on flow in perennial streams within the Pittsburgh seam basin were evaluated. Four streams above four different longwall mines were studied. Overburden depths at the study sites ranged from 500 to 600 feet. At all four locations, premining streamfow measurements were made to establish baseline flow conditions. Measurements were then repeated at each location after the longwall face passed beneath the stream. At one location, in which the longwall panel axis was roughly parallel to the stream, the streamflow was monitored as the longwall face advanced. Measurements at this location showed that temporary flow reduction occurred as the face advanced underneath; however, after the tensile portion of the traveling subsidence profile passed, this effect was lessened. At two of the sites in which the stream was roughly perpendicular to the panel axis, postmining streamfow increased across the panel and decreased over the development headings, such that the net lateral change in streamflow across the fully subsided panel and associated development headings was small compared to the measured flow.

Jan 1, 1997

By Yi Luo

Subsurface strata movements and deformations associated with underground mining activities could cause problems to subsurface structures and water bodies. By incorporating the methods for surface subsidence prediction with the subsidence parameters derived from collected subsurface subsidence data, methods have been proposed for the prediction of final and dynamic subsurface movements and deformations induced by underground longwall mining. Two new' types of strata deformations in the field of subsidence research, vertical and total strains that could he useful in assessing subsidence influences to the subsurface structures and water bodies, have peen introduced.

Jan 1, 2000

By Neil Styler

This paper presents an analysis of measurements of inter-burden de- formations above six longwall faces. An attempt is made to demonstrate some correlation between the movements at the various sites, and to examine their importance with respect to predicting caving height, disruption of overlying seams, and disruption of aquifers. This analysis demonstrates some significant differences between predicted surface subsidence, and inter-burden deformation. In addition it is shown that the caving height above a longwall face is equal to 8 to 12 times the extraction height, with a zone of fractured rock ex- tending to approximately 50 times the extraction height above the seam.

Jan 1, 1984

By Zhou Ying

Based on the mining condition and roof lithology of the 2-3 coal seam at Gengcun Mine, the simulated materials modeling method was used to study the movement process and characteristics of overburden strata movement and to analyze the movement laws of overburden strata during top coal caving mining. Based on the modeling results, variation of the peak abutment pressure and its effect on the top coal's crushing and caving in the mining process was discussed.

Jan 1, 2001

By Murali Gadde

Among different types of ground control problems associated with underground coal mining, those related to in situ stresses are the most common ones affecting the safety and economy of a mining operation. As a result, this has been the focus of many ground control research works done in the last three decades. This paper summarizes a recent work done on in situ stress related ground control issues of underground coal mine development workings. The approach used in this study was three¬-dimensional finite element modeling by an implicit code, ABAQUS/Standard. In the models, roof stability evaluations were made using Hoek-Brown rock mass failure criterion. Influence of the in situ maximum horizontal stress angle on the stability of both entry and intersection was examined to help determine the best development layout orientation. Also, the effect of in situ stress ratios on the stability of development openings was studied. To make the work more general, both 'low' and 'high' in situ stress fields were considered In general, the modeling results indicated that entries oriented in the direction of maximum horizontal stress were in the best condition while those oriented at 90° had the least stability. For intersections, the maximum and the minimum stability were seen at 0°/90° and 45°, respectively. However, under some combinations of input conditions, the best or the worst conditions were noticed at same other orientations as well. Based on these findings, layout design charts were prepared for both entries and intersections. It was also found that the change in ground conditions with change in layout orientation was more significant for 'low' in situ stress fields than for the higher ones. Change in the average safety factor with change in the ratio of in situ maximum horizontal to vertical stress resembled a lognormal distribution curve. However, the effect of the ratio of in situ maximum to minimum horizontal stress lacked such a clear trend for different input combinations considered.

Jan 1, 2004

By S. M. Matthews

The magnitude and orientation of the in situ stressfield has been determined as a key factor in controlling the stability of openings for both coal mine development and extraction. Monitoring of roadway and pillar behaviour in conjunction with numerical analysis has identified specific stress control measures that have been successfully applied to improve productivity in underground coal mining operations. The influence of the major horizontal stressfield on the stability of openings has been identified at a wide range of sites including mines in Australia, Britain, the USA, New Zealand and Japan. Stress mapping, stress measurement and stress monitoring techniques have been applied to define the in situ stress regime and, together with the correlation of measurements of rock deformation and rock reinforcement behaviour, has enabled appropriate stress control measures to be developed. Stress control measures for development headings have required the selection of roadway orientation, roadway sequencing, reinforcement density and pillar size. The monitoring of behaviour at a range of trial sites has confirmed significant improvements in development stability. Stress control measures for longwall extraction have required rational selection of longwall orientation and extraction direction. Where a mine layout design incorporating the optimum longwall layout design has not been possible, the use of sacrificial roadways has been successfully applied to protect key roadways from stress concentration effects. Comprehensive in situ monitoring of behaviour has been carried out to confirm the mechanisms involved.

Jan 1, 1992

By X. L. Yao

This paper analyses the effects of mining extraction geometry (width/depth ratio) and dip of seam on subsidence parameters based on a large Database, containing 120 longwall mining cases all within the UK coalfields. The subsidence parameters evaluated in this paper include the following: the maximum subsidence value with different goaf-treatments, the position of maximum subsidence, the position of half-subsidence, subsidence value over the rib-side, positions of maximum tensile strains, positions of inflection points and the magnitude of maximum tensile strains on both rise and dip sides. Additionally, the maximum compressive strain and two limit angles are also studied in this paper. Based on a comprehensive Database analysis, several equations are derived relating these parameters to the width/depth ratio and the dip of seam. The results should assist engineers to design new structures, or take measures to protect existing structures from strain effects. The findings of the research assist in extension of the SEH to inclined seams with greater certainty.

Jan 1, 1990

By C. Babcock

This Bureau of Mines report considers some equations for mine pillar design from 1833 to the present. Most of the equations are of essentially the same kind, using pillar width to height relationships. The effects of end constraint and test volume size, that were implied in some of these early equations have received special emphasis in recent years. Experimental testing in the laboratory throughout the period given has been on prisms or cubes of coal or rock. Most of the present concepts have been known since about 1912. Wilson has recently emphasized the effect of end constraint in his confined core concept, but a number of other earlier relationships have indicated an awareness of the effects of constraint as well.

Jan 1, 1981

By D. W. H. Su

Following a roof fall in the B12 headgate of a CONSOL Pennsylvania Coal Company (CCPC) underground coal mine in February 1998, CONSOL R&D and Exploration initiated an underground roof geology reconnaissance program along the B13 and B14 developments to define potential roof instability areas and to recommend appropriate supplemental headgate support. This reconnaissance program revealed the presence of a thick, massive Pittsburgh sandstone channel overlying the B13 and B14 panels. Surface and underground core holes confirmed the close proximity and the very massive, nonlaminated nature of the sandstone. To mitigate anticipated adverse conditions at the B 13 and B 14 longwall faces due to delayed caving of the massive sandstone behind the shields, a task force consisting of Operations, Exploration, and R&D personnel was formed in May 1998. Several operational adjustments were made by mine management, including repairing damaged lemniscate links, maintaining immediate forward shield support, and implementing measures recommended by the task force intended to boost the shield set load density at the face. After reviewing available geotcchnical information, the task force also recommended fracing of the massive sandstone over the B13 and B14 panels to enhance caving characteristics and further reduce the bending-induced stresses in the sandstone near the face. Horizontal fracs were created at a horizon about 25 feet above the Pittsburgh seam main bench over the two panels in June and November 1998, respectively. To evaluate the effect of sandstone fracing and operational adjustments on the bending-induced stresses in the sandstone, three finite element models were constructed and analyzed. Results from the finite element analyses indicate that one horizontal frac, 25 feet above the Pittsburgh seam, would reduce the bending-induced stresses in the sandstone by approximately 50 percent, maintaining them at a level below the ultimate strength of the sandstone. Operational adjustments to increase the support set load density and support stiffness could reduce the bending-induced stresses in the sandstone by a maximum of an additional 10 percent. Shield pressure monitoring at the B14 midface confirmed the presence of a lower roof pressure zone inby and outby the 1314 frac holes. This lower face pressure significantly contributed to improving the B13 and B14 longwall productivity and safety. The total production downtime due to bad top at the B 12 face within the interval of channel influence was 30,600 minutes compared to about 3,500 minutes each for the B13 and B14 faces. The average longwall advance per machine shift for the B13 and B14 faces within the channel was about 70 percent higher than that for the B 12 face.

Jan 1, 2001

By M. K. Larson

The U.S. Bureau of Mines and Cyprus Shoshone Coal Corp. conducted a study of deformation mechanisms around a longwall gate-road system in an underground coal mine. Of particular interest was roof deformation over the headgate entry. The strata above and below the coal seam are very weak, carbonaceous mudstones that have planes of weakness oriented along the bedding. Visual observations of previous gate roads over several years suggested that deformation was time dependent and resulted in creep along joints or crack growth. Instruments were installed at a test site to measure deformation around the entry, strew changes in the pillar, and loads transferred to support systems. Multipoint extensometers, instrumented bolts, instrumented trusses, biaxial stressmeters, borehole pressure cells, and single-point extensometers were installed. Monitoring continued until the longwall face passed the test site by 38.7 m (127 ft). Results show that fracture zones arched over the entry from both ribs, most bolts and trusses were loaded past their yield point, and the largest change in secondary principal stress was almost horizontal in the pillar. Displacement measurements in the roof showed classic primary creep.

Jan 1, 1995

By Shuren Wang, Wenbing Guo, Paul C. Hagan, Yanhai Zhao

"The overlying strata is often destroyed in large scale during shallow coal seam mining, and the sliding instability of the caved roof seriously threatens the safety of the mining field. The typical shallow coal mining in Shendong Mining Area in China was used as engineering background; the load distribution characteristics of the overlying strata were analyzed based on the monitoring data of the support resistance. Through theoretical analysis and numerical simulation, the formation and changing process of the pressure-arch in the overlying strata were studied, and the instability mechanism of the overlying strata was revealed by focusing on the accumulation and release law of the elastic energy in the pressure-arch. The results show that the continuous pressure-arch was formed when the horizontal stress exceeded the primary vertical stress of the mining field, and the elastic energy of the roof was released by the mining unloading effect. The elastic energy was accumulated in the pressure-arch, and the energy arrived the highest at the front arch foot. The accumulated energy at the arch foot was released by coal mining, and the shear zone could be formed. Therefore, the sliding of the caved zone along the shear zone induced strong roof weighting. The concentrated stress and the released energy during each mining increased with the panel advancing, and the height of the shear zone also increased. The conclusions obtained in the study are of important theoretical value to direct similar engineering practice.INTRODUCTIONThe instability of overlying strata during shallow coal mining, such as the large-scale roof falling and step-like ground subsidence, is a key problem that can restrict the safety mining in the mines (Ju and Xu, 2015). The self-bearing structure of the pressure-arch can form in the overlying strata after coal mining, and this structure can support the load of the upper strata and soil layer, so the weighting intensity of the panel is determined by the caved rock in the unloading zone under the inner boundary of the pressure-arch. A large amount of elastic energy is accumulated in the pressure-arch under the concentrated stress, and the released energy for mining is the internal cause of rock failure (Wang et al., 2017). It is an important problem to reveal the distribution characteristics of the stress and energy fields in the mine and to analyze the stability of the overlying strata during shallow coal mining based on the evolution characteristics of the pressure-arch."

Jan 1, 2018

By André Vervoort

In coal room and pillar, and pillar extraction panels, the sidewall conditions were investigated based on underground measurements and numerical simulations. Three-dimensional linear elastic models provided a useful and quick prediction of the amount of sidewall spalling. However, more complex constitutive models were required to simulate more accurately the extent of fracturing and its effect on the stress re-distribution. The input parameters of a Mohr-Coulomb model were determined by back-analysis of underground measurements, and the effect of the depth of the workings and the time dependent occcurrence of coal pillar fracturing were analyzed.

Jan 1, 1992

By David Gearhart, Peter Zhang, Gabriel Esterhuizen

"Researchers from the National Institute for Safety and Health (NIOSH) installed instruments in the track entry and in the pillars of a northern West Virginia longwall coal mine between two planned panels for the purpose of monitoring the behavior of the ground. The instruments included four cable bolt load cells to measure the change in gateroad loading, six borehole pressure cells to measure the loading in the pillars, three six-channel roof extensometers to measure roof sag, four convergence measurements on the cribs to measure the crib loading, and a multipoint borehole extensometer to measure the pillar expansion. The instrumentation was all connected through two multiplexers to a single Campbell Scientific permissible datalogger located about 350 feet outby the instruments. The goal of the project was to record all of the measurements as both longwall faces passed the site of the instrumentation and to use the data to calibrate numerical models.This report describes the recording of all of the measurements taken as the first panel approached and then passed the instrumentation site. This study provided valuable data related to the ground response and support performance that will be used to improve safety in longwall gateroads.INTRODUCTIONA longwall mine in northern West Virginia was selected as a second study site for an ongoing research project into gateroad layout and support design. The area was affected by a transition from limestone to sandstone in the roof. The entries were 16 feet wide, and the height was approximately 80 inches. The layout for the longwall panels was 1,170 feet wide by 4,000 feet long. The panel configuration was right-handed, and a three-entry gateroad was used between panels. The pillars next to the headgate were 137.5 feet by 115 feet center-to-center, and the pillars next to the tailgate were 275 feet by 100 feet center-to-center, with the crosscuts at 60 degrees. The width of the gateroad was a total of 215 feet."

Jan 1, 2018

By Charles D. Albright

FMC built its first angle bolter in 1969. This was a single mast unit which could drill holes for conventional bolts or angle holes for installing roof trusses. From this drill we learned very quickie that the mast design was an important factor to the success or failure of any angle bolter. The first mast installed on this machine (EXHIBIT 1) was fixed in length with endless chain and hydraulic motor feed. It took less than a shift underground to determine that the fixed length was not going to work. The undulating top created conditions where the mast was too long to be positioned at a 450 angle. Several mast designs were tried and discarded before we felt we had the answer to our problems. (EXHIBIT 2} We developed a mast that would extend or retract without moving the position of the drill unit. This allowed the operator to "wedge" the mast between the floor and roof before starting his drill feed. This arrangement worked exceptionally well, but unfortunately the demand, as perceived by our Sales Department, for angle bolters did not materialize. This unit became an "orphan" and was used by many coal mines until it died a natural death. FMC did not decide to re-enter this market again until 1979. At this time many coal producers were contacted to determine what features should he incorporated into a dual mast angle bolter. From this information we were able to arrive at three main areas of concern that we should consider when designing a new machine. 1. Eliminate overturning moment on drill unit. 2. Provide some sort of steel support near roof line to aid in starting a 15° hole. 3. Dual mast units need a method of raising center of mast rotation to facilitate various mining heights and still maintain a 45° angle and proper distance from rib. I would now like to discuss each of these individually. EXHIBIT 3 indicates the overturning moment resulting from the feed cylinder thrust and the reaction of the drill hit against the roof. To eliminate this moment a mast was designed using two feed cylinders mounted in the plane of the drill center line, thus removing all overturning moments on the drill unit carriage. This mast has been used extensively in vertical drilling with no downtime attributed to wear on mast guides and drill unit supports. The cylinders are designed to provide uniform thrust and feed speed throughout the total stroke. For manufacturing and economic reasons, the final mast design on the first drill was not used for this new drill. A drill steel support mounted on the mast, and positioned by a hydraulic cylinder provides the needed guidance near the roof for starting a hole at a 45° angle. (EXHIBIT 4) This support does not surround the drill steel but simple cradles the drill steel for "collaring" the hole. Once the hole is started, the support can he lowered. The support plate is hinged to allow the drill mast to pass. The advantage of having vertical motion of the center of rotation on a dual mast drill is shown in EXHIBIT 5. Following the criteria recommended by Birmingham Bolt Company, the truss bolts should he at a 45° angle and one fifth of the width of the entry from the rib. Therefore, in an entry 20 ft. wide, the hole should be drilled 4 ft. from the rib and at the 45° angle previously mentioned. With the limitations inherent to dual boom drills, that is, the masts cannot pass each other nor can one mast pass the center line of the machine even if the other is positioned out of the way, it is impossible to accommodate much variation in seam height and maintain the recommended criteria. As shown in EXHIBIT 5, a center of rotation 17 inches above the floor will start a hole 4 ft. and 45°angle at a height of 67 inches. Any height above 67 inches

Jan 1, 1982

By Dan Payne

Roof falls and other ground control incidents make up a significant portion of fatal accidents in underground mines. BHP Billiton has undergone a review of all fatalities in their operations and divided these into 10 fundamental categories of which underground ground control is one. Each underground roof fall fatality was investigated and the root causes workshopped to develop a protocol for the control of fatalities due to falls of ground. This protocol sets out 22 requirements which each BHP Billiton underground mine site must comply with. This paper describes the development, requirements and application of the protocol within BHP Billiton.

Jan 1, 2006

By Andrew Starkey

Ground anchorage systems in the form of rock bolts are used extensively throughout the world as supporting devices for civil engineering structures such as bridges and tunnels. It is widely recognized that non-destructive testing methods for ground anchorages need to be developed as a high priority [1 ], with only a small proportion of anchorages currently being monitored in service [2]. The condition monitoring of anchorages is a relatively new area of research, with the objective being a wholly automated or semi-automated condition monitoring system capable of repeatable and accurate diagnosis of faults and anchorage post tension levels. The GRANIT tm system developed by the University of Aberdeen is capable of measuring the integrity of ground anchorages and has won two major UK Awards - the National John Logic Baird Award for Innovation in 1997 and a Design Council Millennium Product Award in 1999. The normal procedure with the GRANIT tm system [3,4] is to train a neural network on dynamic response data that has been sampled from an anchorage over a range of post-tension levels. Further data is needed in order to test the neural network. The paper presents neural networks that have been trained on data taken from a number of anchorages on different sites, and the results of the diagnosis of these neural networks on new test data is presented. The results show that the cross-diagnosis of anchorages is possible, where a neural network trained on one anchorage is capable of successfully diagnosing the post-tension level for an anchorage of similar construction, and the high level of accuracy that can be attained with the GRANIT tm technique is shown. The paper also introduces a mathematical model of the GRANIT tm process.

Jan 1, 2001

By Bruce W. Jack

An extensive underground roof-monitoring program was conducted in order to determine the roof strata behavior under various conditions. In total 29 sites at 5 different collieries were monitored using the sonic probe extensometer. Results showed that, in drill and blast sections, there are often pre-existing openings in the roof prior to the installation of the support. An estimated 42 per cent of the total roof displacement monitored in drill and blast sections took place prior to the installation of support. A comparison between roadways and intersections indicated that, for a 40 per cent increase in the span, taken across the diagonal of art intersection, relative to the roadway width, the magnitude of the displacement in the roof increased by a factor of 3. Total displacements measured were relatively small, only 2 reaching double figures of 12 mm. The remainder were all less than 7 mm. The results also showed no evidence of a substantial increase in the height of the bed separated, potentially unstable roof strata, as is the case in the high horizontal stress-driven beam-buckling mechanism experienced in for instance some Australian coal nines. Therefore, it was concluded that, in South African collieries, the effects of horizontal stresses is generally relatively small compared to overseas collieries.

Jan 1, 2000

The purpose and current practice of cable trusses in underground coal mines is briefly outlined and details of various cable truss designs are presented. The main part of the paper describes the in-situ performance of cable trusses installed as secondary supports in gateroads and recovery roads of longwall coalfaces. The performance analysis is based on the results of in-situ monitoring carried out at a number of sites at two underground coal mines within Australia. Truss performance has been assessed as load developed in relation to roof convergence, face position and time. Additionally, the influence of strata conditions, installation pattern/density, roadway layout and geometry have been taken into account. An analytical model has been developed to predict truss loads. The principal variables are listed and the basis of the model is described. Based upon behaviour observed in the field the model has been progressively upgraded and validated against measured performance. Guidelines on truss performance and factors which influence it are presented and the implications of these to the use of cable trusses is discussed. Possible methods for improving truss performance in certain strata conditions are outlined and some comments are made on the value of cable trussing compared to cable bolting in these situations,

Jan 1, 1992

By George B. Quaye

Conventionally, roadways in South African collieries are 6 to 7 m wide. This dimension is chosen so as to allow maneuvrability of appropriate mine machinery and equipment, to meet production requirements and to ensure roof stability. Over the last two decades, a great deal of research effort has been expended to understand the behaviour of roadways in different strata within this width range. The results have improved the understanding of the strata behaviour to such an extent that 6 m wide roadways are developed with a high degree of confidence. With the advancement of technology and the quest to mine at a lower cost, some mines in South Africa have started considering the benefits of other types of mine machinery that will require much wider roadways to ensure optimum productivity. The roadways required will have to be about 10 - 14 m wide. The challenge confronting these coal mining companies is the potential instability that might be associated with such wide roadways. This dilemma is aggravated by the fact that, apart from theory, nothing is known about the expected behaviour of such wide roadways. This led to experimentation at a selected site in a South African coal mine where 10 m and 14 m wide roadways were developed and monitored. This paper highlights the experimental procedure adopted, discusses the results obtained, compares the prediction of elastic theory to reality and set the precedence towards understanding the behaviour of wider roadways in South Africa.

Jan 1, 2001