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By Francis S. Kendorski

Underground stone mines tier construction aggregates and industrial minerals are being planned in increasing numbers throughout the United States. At the middle of the last century (1950s) underground stone mines were much more common, and characteristically had no thorough site characterization other than proving reserves and trial-and-error planning. Ground control employed was minimal. Consequently, there was a tendency to conduct minimal pre-mining studies. However, the deposits that lend themselves to little site characterization and minimal ground control are few today, with the best being mined out. Failures of a number of underground stone or similar mines from that era are more common than realized, Underground stone mines almost always use some combinations of square or rectangular pillars, sized to support the overburden load, and spaced to allow the use of larger, more-efficient equipment. Heights are controlled by the available stone quality, interbeds of waste, or technological or safety considerations. Prior to committing to a specific mine plan with subsequent significant capital investment, and prior to having available the wealth of knowledge and experience gained once mining underground, a safe and workable mine layout must he achieved. Early attention to the several different rock units involved (roof, mine level. floor) and their strength as a rock mass with the inherent discontinuities, will allow first approximations of acceptable mine geometries and sequencing.

Jan 1, 2000

By Kot F. Unrug

Since the introduction of longwall systems, the prediction of roof behavior and support requirements have become the major issue referring directly to safety as well as productivity. In more recent times, and especially in the USA where longwalls are exclusively equipped with costly hydraulic self- propelled support systems, a decision whether a longwall is applicable in a given condition, and when so, what kind of equipment should be selected, is of primary importance. There are numerous examples where lack of experience, and subsequently wrong selection of supports or winning machines, resulted in considerable losses. It should be emphasized that careful studies of prevailing geological and mining conditions, especially in a new area, are required to assure a successful operation. An evaluation of the conditions can be carried out in different ways utilizing such methods as: - Theoretical approach. applicable when geological conditions comply with the theoretical assumptions. - Empirical relationship which gives satisfactory results where the coefficients could be adequately determined. - Structural geological studies which must be undertaken in all cases, since the frequency of fractures and their orientation have a prevailing influence on roof behavior. - Scale model study on equivalent material for determination of failure conditions in the roof and mechanism of movement (flow) of broken rock matter. Coefficients for empirical formulas can be determined by these types of studies, as can boundary limits for mathematical modeling. - Mathematical modeling, e.g., finite element analysis. - Measurements and observations taken underground during the development period end utilization of experience for previous longwall panels in the area. Usually it is the most practical method where mining and geological conditions are comparable. An application of me or more of the above approaches remains to be a matter of judgement for particular conditions, however, the utilization of a practical experience, when available, is usually the safest way for planning a successful longwall operation.

Jan 1, 1982

By Lisa Burke

In underground coal mines, concrete block stoppings are widely used to control mine ventilation. Although stoppings are not intended as roof support, they are subjected to roof to floor convergence, and may fail as a result of this loading. Premature failure of stoppings can significantly affect mine ventilation, limiting the amount of fresh air reaching the working faces and increasing the risk for methane explosions. Softening materials are often used in stoppings to reduce the damaging effects of roof to floor convergence. However, to date, research has not focused on the behavior of stopping construction materials subjected to roof or floor loading, leaving the design process to trial and error. A combination of numerical simulations and large scale physical tests were employed to develop a scientific understanding of stopping performance. The first series of physical models were constructed using standard CMU (concrete masonry unit) blocks and tested in a load frame. The behavior of these walls was simulated with a distinct element model. A key finding was that very small variations in block size and irregularities in block shape significantly affect wall performance. These variations, which are inherent to the type of blocks typically used underground, create stress concentrations that initiate the failure of stopping walls. A second series of models employed concrete stoppings that incorporated woos planks, a shredded wood material, and foam planks like those commonly used underground. Numerical models were again able to match the physical test data. Analysis of the softening layers used in stopping construction indicated that the strength and stiffness of the material relative to that of the wall is crucial in determining the amount of roof to floor convergence the wall can withstand prior to failure. The product of this study is a numerical model that can be used to evaluate the performance of stopping materials and different wall geometries in a controlled environment. Testing was done in a laboratory setting with walls constructed on flat, parallel surfaces and subjected to uniform loading at a constant rate. Parametric studies with the model indicate that in this ideal environment it is possible to control the amount of vertical displacement the stopping can withstand by adjusting the number of softening layers built into the wall and the thickness of the layers. Additionally, under these conditions, the position of softening layers within the wall appears to have little effect on the amount of displacement walls can withstand prior to failure.

Jan 1, 2004

By M. Y. Fisekci

This paper concentrates on subsidence measurements, applied over the thick and steep seam mining in the Rocky Mountains Region of Western Canada. The studies to date indicate that two new subsidence monitoring techniques appear to be the most suitable methods for these conditions. The computerized telemetry and aerial photogrammetry systems are being tested for the reliability of the systems during the winter months within the net- work of laser surveying over the workings of the new hydraulic mine.

Jan 1, 1981

By Paul C. Hagan, Jianhang Chen, Serkan Saydam

"Modified cable bolts are commonly used in underground mines due to their superior performance in preventing bed separation when compared with plain strands. To better test the axial performance of a wide range of cable bolts, a new Laboratory Short Encapsulation Pull Test (LSEPT) facility was developed. The facility simulates the interaction between cable bolts and surrounding rock mass, using artificial rock cylinders with a diameter of 300 mm in which the cable bolt is grouted. Furthermore, the joint where the load is applied is left unconstrained to allow shear slippage at the cable/grout or grout/ rock interface. Based on this apparatus, a series of pull tests were undertaken using the MW9 modified bulb cable bolt. Various parameters including embedment length, test material strength and borehole size were evaluated. It was found that within a limited range of 360 mm, there is a linear relationship between the maximum bearing capacity of the cable bolt and embedment length. Beyond 360 mm, the peak capacity continues to rise but with a much lower slope. When the MW9 cable bolt was grouted in a weak test material, failure always took place along the grout/ rock interface. Interestingly, increasing the borehole diameter from 42 mm to 52 mm in weak test material altered the failure mode from grout/rock interface to cable/grout interface and improved the performance in terms of both peak and residual capacity.INTRODUCTIONFully grouted cable bolts have been used in underground mining industry for many decades. Initially, plain cable bolts manufactured by winding seven flexible steel wires together were commonly used (Thorne and Muller, 1964). Laboratory and field tests lead to a better understanding of the transfer process between grouted cable bolts and the surrounding rock mass, which is beneficial to improve the reinforcing performance of cable bolts in engineering practice (Fuller and Cox, 1975; Cox and Fuller, 1977; Schmuck, 1979; Stillborg, 1984). Nevertheless, as in many cases the initial cable was sourced from discarded steel with a smooth surface along the strand, plain cable bolts did not always meet performance expectations (Windsor, 1992). Over time, it was realised that cable surface geometry plays an important role in determining anchorage performance. Consequently, modified forms of cable bolts were developed. For instance, twin-strands which were made by using spacers to combine two plain cables together improved anchorage capacity two-fold (Matthews, Tillmann and Worotnicki, 1983). However, this required a large borehole to be drilled. Epoxycoated strands had remarkable performance in practice but were difficult to install without damaging the epoxy layer (Dorsten, Hunt and Kent, 1984). In the 1990s, there was a rapid development in the cable bolt design. A typical example was birdcaged cable bolts. They were initially suggested by Hutchins et al. (1990) via unwrapping traditional seven-wire strands and generating a number of nodes together with antinodes along the strand. But it was difficult to make birdcaged cable bolts that could be coiled for transporting."

Jan 1, 2015

By K. Goshtasbi Goharizi

This paper discusses experimental results of physical scale models employed to study the stability of a pillar between rectangular roadways. A programme of laboratory investigations utilising a bi-axial test rig has been undertaken to examine the influence of various stressfields on the stability of pillars between mine roadways. The effect of pillar height/width ratio on the overall stability is studied. Observations of failure characteristics in and around pillars are discussed especially in relation to the various stressfields applied to the models.

Jan 1, 1992

By Q. F. Wang

Spontaneous combustion hazard of loose coal in the top-coal caving region of entry affects the safety of the entry and top- caving longwall face seriously. In this paper, by combining on- site temperature tests with lab tests and numerical simulation, the characteristics of microcirculation close to the top-coal caving region are analyzed. A model for the temperature fields of the microcirculation in loose coal is presented. Based on this model, the temperature distribution and possible self-ignition region can be predicted. Further, the heat and mass transfers in microcirculation are studied. Finally, the mechanism of oxygenation during the spontaneous combustion of microcirculation in loose coal of the top-coal caving region of entries is put forward. The research results in this papa may provide ideas for the prevention and putting out of the fire caused by spontaneous combustion in the caving zone.

Jan 1, 2004

By Y. Luo

This paper presents a number of mathematical models for predicting the different phases of the complete surface subsidence process associated with underground longwall mining. A computer program developed based on the mathematical models is briefly introduced.

Jan 1, 1992

By Quanzhong Gu

Stability of a pre-exnacted recovery room plays a vital role when longwall face approaches and during a longwall face move. In an attempt to extend the application of pumpable cribs into recovery room support and to limit roof bolt installation when a longwall face approaches the recovery room, yieldable and cuttable pumpable cribs (YCPC) were employed in the recovery room at the northern West Virginia mine. The YCPC consists of two layers of materials with different strengths. This allows considerable deformation from the floor and/or roof by sacrificing the soft layer while maintaining relatively high load capacity under mining¬induced front abutment loads. The thickness and stiffness of the soft layer, installation and quality control are the key factors when using the Yieldable and Cuttable Pumpable Crib to stabilize longwall recovery room- Underground observation and monitoring results show that the Yieldable and Cuttable Pumpable Crib is a viable solution to insure stability of the recovery room and can save at least two days for a longwall face move at the mine.

Jan 1, 2004

By G. C. Xiao

A database, incorporating historical data from British mines, to study the factors affecting inflows from proximate aquifers into longwall workings, showed that a main casual factor was the immediate roof strata lithology. When this comprised principally stronger sandstone, there was a tendency for periodic inrushes associated with face weighting. This phenomenon is explained by the collection of water in bed sepa¬ration cavities, where free water connected to a high pressure head aquifer source, may be trapped.

Jan 1, 1990

By Roy H. Grau

The trend in underground room-and-pillar cod mining is to employ remote-control continuous mining machines to take extended cuts of 40 A or more. Roof fall safety hazards to the mine worker associated with 4ed cutting are not completely de6ned but are known to include removal of the miner operator from the protective canopy, non-compliance with approved cutting depth and CM operator positioning. This report summarizes ground control research performed by the National Institute for Occupational Safety and Health (?-JIOSH) researchers at the Pittsburgh Research Laboratory concerning extended cuts. A combination of statistical analysis and underground investigations have been conducted. Statistical analysis of extended cut use, along with roof fall accidents and fatalities data delineate the extent of the problem in U.S. coal mines. Additionally, an underground study is described where the long-term roof stability of extended cut areas are compared to standard cuts.

Jan 1, 1997

By Robert W. Bruhn

A 125 foot high steel lattice frame tower supporting a 500 kV EHV transmission line and located over the gateroads of a longwall mining operation was subjected to ground movements from the mining of adjacent longwall panels. Under an agreement with the owner of the powerline, the mine operator was required to minimize mining impacts on the transmission line. In view of this requirement, the mine operator prepared a forecast of the expected subsidence ground movements prior to the onset of mining. These forecasts, in conjunction with finite element analyses of the tower superstructure under the imposed ground movements and wind loadings. suggested that the structural capacity of the tower would probably be diminished little, if at all, by mining. Ground movements determined by survey during mining of the two longwall panels were found to be in close agreement with predictions. Strains measured in selected tower members during mining generally agreed with values calculated by the finite element analyses. The study confirmed a methodology to predict subsidence and to assess its effects on steel lattice transmission towers.

Jan 1, 1990

By H. N. Maleki

Extensive measurements and underground observations in three Western U. S. coal mines are integrated in this paper to determine in-situ pillar load-deformation characteristics for narrow (30 it wide, 80 ft long) pillars in two-entry gate road systems. In spite of similarities in the regional geology, coal pillar laboratory mechanical properties and gate pillar geometries, the pillar peak strength, post-failure behavior, and failure mechanism were shown to be significantly different. Pillar peak strength was shown to be dependent on depth at one site, approaching burst-prone stress levels of 4000 psi. At another site, the pillar peak strength was lower because of lower confinement; this was related to the higher frequency of cleats and the lower friction- al properties of the roof/floor and coal contact. Two failure mechanisms were identified - one in the pillar, and the other in the mine floor. The roof stability was good at all three sites because of the thick-bedded nature of the roof strata and limited total gate span in a two-entry system. Existing pillar design techniques were shown to be inadequate for design, requiring adjustments for depth of cover, cleat frequency, and roof /floor frictional properties.

Jan 1, 1988

By George J. Karabin

Since the inception of a qualitative pressure arch theory by Haak4 in 1911, much work has been done in an effort to apply this concept to underground mining. It has been used in both longwall and room and pillar mining to determine roadway and pillar stability (with varying degrees of success). This paper presents a summary of a case study where the pressure arch concept was extended in a totally qualitative manner to define the source of roof control problems in the mains of a Pennsylvania mine. It was concluded that, due to the method of mining employed, a pressure arch formed over a portion of the entries, and the high abutment stresses accompanying it resulted in extreme roof deterioration. The following discussion will include a general presentation of the arching mechanism, the physical response of the roof strata under mining-induced abutment stresses (both from the case study and in general), a computer simulation of ground stability under both "normal" and "abutment" stress conditions, and finally, potential means of alleviating such conditions.

Jan 1, 1982

By David Newman

Landslides and soil creep often occur with varying degrees of severity on steep slopes within Southern Appalachia. Ground movement may take place over years with subtle changes in topography and vegetation or it can occur rapidly in response to water inflow or disturbance at the toe of the landslide. Although the ground movement typically is restricted within the soil and colluvial layers, failure can extend into the weathered rock zone. Certain areas of Appalachia are described as "slide prone" because of historical precedent. To develop a profile of "slide prone" areas, the strength and physical properties of the soil and colluvium, topography, groundwater and surface water runoff, present and historical land use, and any surface disturbance are examined in detail. These attributes are useful in determining whether an area is potentially unstable before surface development including construction, logging, or mining. Recommendations are provided to avoid initiating ground movement concurrent with mining operations.

Jan 1, 1998

By Thomas L. Vandergrift

The type and severity of coal mine entry failures are affected by the strength and stiffness properties of the roof, floor, end coal. Knowledge of the mine-wide trends of these properties Is valuable in developing effective ground control plans. Within the context of a larger effort to Improve entry stability In high horizontal stress fields, the U.S. Department of the Interior, Bureau of Mines, has evaluated three portable, on-site test devices to determine their suitability for identifying mine-wide physical property trends. The devices evaluated were the borehole shear tester, the Schmidt harmer, and the point load tester. Despite the lack of standardized data Interpretation procedures, each device provided useful on-site physical property data. The Schmidt homer was preferred for trend identification because of Its speed and simplicity. Schmidt hammer data were collected over a large area of an underground coal mine to compare calculated roof strength trends with a known geologic feature.

Jan 1, 1990

By Hamid Maleki

Mining in the Book Cliffs Coal Field of Utah has historically been associated with seismicity because of high-stress environments and the presence of stiff, competent rocks. Recognizing the geotechnical risks in this coal field, mine engineers have dedicated considerable resources to the study of mine seismicity. In spite of these efforts, there are no engineering procedures for assessing mine seismicity for a new property and in developing mine designs. This paper presents the results of ongoing geotechnical monitoring and stress analyses at a western U.S. coal mine. The intent is to demonstrate the application of geotechnical methodologies in evaluating mine seismicity and stress-control technologies including evolutions in yield pillar designs for variable topographic conditions of the Book Cliffs Coal Field. We show that narrow yield pillars are quite effective in limiting strain energy accumulation along the gateroads and thus the potential for pillar seismicity. At the same time, the pillars provide sufficient resistance to control roof-floor convergence at the headgate, gradually unloading to residual strength inby the face position. Lagging, nonuniform caving of overburden rocks results in periodic stress concentrations at the tailgate comer. The sudden release of strain energy triggers sudden failure of critically loaded coal in this area of the high stress gradient. The three-entry gate pillar configuration promotes formation of long cantilevers and load transfer to the tailgate, contributing to formation of cutters and coal bumps. The source of seismicity is investigated while simulating slip along three sets of joints in two stiff stratigraphic units during multipanel extraction.

Jan 1, 2003

By Anton Sroka

The German hard coal production predominantly derives from underground multiseam mining activities. Beside common surface damages, ?inner mine damages? on underground infrastructures (i.e. main roads, cross cuts and shafts) due to mining excavations occur to an increasing extent. As these "inner mine damages" dramatically increased because of the development towards high performance longwall mining technologies, this matter raised to be a severe problem concerning mining economics. To protect durable and long lasting underground infrastructures, precautionary measures such as suitable mine layouts, adapted planning of mining operations and moderate face advances have to be considered. Some examples of "inner mine damages" recorded by means of mine surveying methods are given in this paper. An empirical-mathematical model based on the principles of mining subsidence engineering methods will be introduced. This model allows to minimize ?inner mine damages? and helps to support and optimize mining processes.

Jan 1, 1999

By Michael J. Friedel

The fundamental problems associated with ground control reflect the inability of mining personnel to "see" the development of anomalous conditions. Toward this end, the U. S. Bureau of Mines is investigating the application of seismic refraction tomography for imaging the process of pillar yielding, at the Foidel Creek coal mine, near Steamboat Springs, CO. The inherent three-dimensional problem of imaging coal pillar stress is reduced to a single plane by considering only the stress transmitted from the pillar to the adjacent floor rock. This approach exploits the fact that refracted seismic energy is essentially confined to the adjacent higher velocity sandstone strata. The tomographic analysis using ray-tracing based on wavefront migration resulted in a robust P-wave tomogram from which regions of high velocity indicate zones of high stress concentration, whereas low velocity zones indicate regions of stress relief. The interpreted region of high stress concentration occurs at the pillar end nearest to the working face of the adjacent longwall panel. This finding is corroborated by vertical pressure cell measurements recorded in the pillar. The stress diminishes toward the pillar perimeter with local minima correlating to the locations of observed pillar spalling. Time-sequence monitoring, as the longwall face advances, is recommended for evaluating changing stress conditions.

Jan 1, 1993

The U.S. Bureau of Mines (USBM) has developed the coal Mine Roof Rating (CMRR) to aid in ground control. This paper describes the application of the CMRR at four coal mines operated by a major U.S. coal company. The mines are widely separated geographically, and cover a wide variety of geologic conditions and mining methods. The study found that the CMRR was a useful predictor of roof performance. In mines where the CMRR varied, and other parameters held constant, roof falls were associated with low CMRR values. Important similarities in roof performance were found in mines with similar CMRR values, despite radically different geologic environments. The analyses also indicated that statistics can be used to evaluate ground control performance. For these mines, a "roof fall probability" can be estimated from the CMRR and the intersection span. A spreadsheet program has been developed to facilitate treatment of CMRR data, and it is briefly describe

Jan 1, 1994