Shallow Coal Mining Research Articles

Backfill mining alternatives and strategies for mitigating shallow coal mining hazards in the western mining area of China

This study addresses the major geo-environmental hazards caused by shallow coal mining in China's western eco-environment frangible area. These hazards are related to the high overburden pressure, surface subsidence, soil and water losses, and land desertification, with consequent vegetation and wildlife losses. To mitigate these hazards, three alternative backfill mining methods are proposed, for three typical shallow coal mining conditions, using aeolian sand-based backfilling materials, which are readily available in this area. The main influencing factor is the backfill material compaction ratio. Its effect on aquiclude deformation and water-conducting fracture evolution are assessed by numerical and physical simulation methods. The potential application of the proposed backfill coal mining alternatives is evaluated and discussed in detail. The results obtained are considered to be valuable for developing a strategy for the coordinated exploitation of coal resources and environmental protection in China's western frangible eco-environment area.

Evolution Characteristics of Composite Pressure-Arch in Thin Bedrock of Overlying Strata During Shallow Coal Mining

Under the shallow coal mining condition and based on the cracking and movement rules of the roof in different mining stages, the mechanical models of symmetrical pressure-arch, stepped pressure-arch, rotating-squeezed pressure-arch in the mining field were established. Then, the instability criterion of each pressure-arch structure was derived. Through the similarity material experiment and numerical simulation, the evolution characteristics of the composite pressure-arches in the near- and far-field were revealed. Results show that the stepped pressure-arch in broken blocks of the basic roof was formed when the horizontal principal stress was greater than the in-situ stress. The broken blocks’ sliding could induce roof cutting off and strong weighting load. The structural characteristics of the overlying strata were determined by the thickness of the bedrock. The upper broken rocks formed a symmetrical pressure-arch in the near- and a far-field pressure-arch formed in the mining field. The median periodic broken rocks formed the rotating-squeezed pressure-arch, bearing the load of the loose layers and protecting the mining panel. After all bedrocks breaking, the latter arch foot of the far-field pressure-arch was transferred to the compacted caving zone. This study can provide a theoretical reference for the similar mining engineering.

A Numerical Study to Assist Assessment of the Stability of Shallow Coal Mine Goafs

Coal mine goafs are distributed widely across many parts of the world and their stability is a major cause for concern, particularly when designing new infrastructure. To reduce risk, the coal mine goafs are often stabilized using cement grout and such operations can be very expensive and difficult to verify. This paper uses the finite element method to examine the relative influence of a number of key factors controlling the stability and surface deformations of shallow, horizontal coal mine goafs overlain by sedimentary rock. Representative ranges in the stiffness and strength characteristics of coal and rock are examined and each material is assumed to satisfy the Hoek–Brown failure criterion. The analyses show that, for a typical maximum coal extraction rate of 40%, the critical goaf span varies linearly with the depth of the coal seam (for the maximum depth investigated of 45 m) and increases with the competency of the overlying rock. A relationship combining the rock quality and the ratio of the critical goaf span to the depth of the coal seam is proposed to enable assessment of abandoned coal mines with marginal stability. This relationship is shown to be consistent with observations made in two coal mining case histories.

Factors that trigger the development of mining-induced ground fissures, and standards to treat them in shallow coal mining areas

Factors that trigger the development of mining-induced ground fissures, and standards to treat them in shallow coal mining areas

Mechanical Performances of Pressure Arch in Thick Bedrock during Shallow Coal Mining

Due to the periodicity of mine pressure, it can easily lead to a wide-range damage of the overburden rock under shallow coal mining conditions, and the falling of the overburden rock is usually a serious threat to the mining safety. Based on the monitoring data of mine pressure in a typical shallow coal mine in northern China, the mechanical model of the surrounding rock in the mining field was built and the identification indicators of the pressure arch after coal mining was defined. Then, the arching mechanism, the stress distribution characteristics in different positions, and the bearing capacity of the pressure arch were analysed, and the evolution characteristics of the pressure arch under shallow horizontal coal mining were verified by numerical simulation and physical experiment. Results show that there is a core-bearing zone in the pressure arch, and the width of the core zone determines the bearing capacity of the pressure arch. The evolution of the pressure arch affects the formation and development of the caving arch of the loose zone in the mining field. The conclusions obtained in the study are of important theoretical value to direct the similar engineering practice.

Pressure-Arching Characteristics of Fractured Strata Structure during Shallow Horizontal Coal Mining

Pressure-Arching Characteristics of Fractured Strata Structure during Shallow Horizontal Coal Mining

Pressure-Arching Characteristics in Roof Blocks during Shallow Coal Mining

To reveal the performance of the stepped subsidence and the strong roof weighting during shallow coal mining, taking the fully mechanized mining face with large mining height in the Shendong mining area in China as the engineering background, theoretical analysis and numerical simulation were used to analyze the pressure-arching effect of the hanging roof blocks. Three typical pressure-arch models of the roof structure were proposed, such as the symmetrical pressure-arch of two key blocks, the step pressure-arch of multiple key blocks, and the rotative pressure-arch of multiple key blocks. Results indicate that the horizontal stress displays a nonlinear distribution at the abutments of the symmetrical pressure-arch, and there is a linear distribution of horizontal stress with a higher peak value at the midspan of the pressure-arch. The high horizontal stress at the arch abutment is necessary to form the rotative pressure-arch of multiple key blocks. The horizontal stress is relatively less at the arch abutment of the step pressure-arch structure. The main key block is easier to slide in this structure as the boundary horizontal stresses display the nonlinear distribution. The results are of instructive significance for roof weighting forecast and strata control during shallow horizontal mining for a thick coal seam.

Evolution Characteristics of Pressure-Arch and Elastic Energy during Shallow Horizontal Coal Mining

Evolution Characteristics of Pressure-Arch and Elastic Energy during Shallow Horizontal Coal Mining

Numerical Modeling of Pot-Hole Subsidence Due to Shallow Underground Coal Mining in Structurally Disturbed Ground

Stability analysis of underground mining is, generally, complex in nature and is difficult to carry out through analytical solutions more so in case of pot-hole subsidence prediction. Thus, application of numerical modeling technique for simulating and finding a solution is preferred. This paper reports the development of a methodology for simulating the pot-hole subsidence using FLAC3D. This study is restricted to geologically disturbed areas where presence of fault was dominating factor for occurrence of pot-hole subsidence. The results demonstrate that the variation in the excavation geometry and properties of immediate roof rocks play a vital role in the occurrence of pot-hole subsidence.

Experiment on Seepage Property and Sand Inrush Criterion for Granular Rock Mass

Water and sand inrush is one of the most serious threats in some shallow coal mines in China. In order to understand the process of sand inrush, experiments were performed to obtain the criterion for sand inrush. First, seepage tests were carried out to study the hydraulic properties of granular sandstone. The results indicate that seepage velocity has a linear relation with the porosity and particle-size distribution parameter. Then, sand inrush tests were conducted to investigate the critical conditions for sand inrush occurrence. It is determined that the sand inrush zone can be clearly distinguished based on the values of porosity and particle-size distribution parameter. Additionally, sand inrush tended to happen in the conditions of high porosity, high seepage velocity, and large particle-size distribution parameter. Further, general principles for preventing the water and sand inrush were proposed, such as reducing the porosity, improving the pore structure, and decreasing the seepage velocity. The proposed principles have been successfully used in situ to control the water and sand inrush.

Fractal characterization and methane adsorption features of coal particles taken from shallow and deep coalmine layers

Abstract Fractal characterization was conducted on coal samples collected from shallow and deep coal mine layers, to better understand the process required to exploit deep layer coal bed methane (CBM). Data from N 2 adsorption/desorption isotherms were used to assess the irregularities of coal pore surfaces ( Ds ) and the heterogeneities of pore structures ( Dp ) on the basis of modified Frenkel–Halsey–Hill (FHH) and Neimark equations. Fractal dimensions ( D ) calculated from the different analysis methods were compared. The results indicate that Ds values are sensitive to coal types that contain different fractal structural features. Shallow-deposited coal usually exhibits low Ds values, while coal collected from deep layers exhibits large Ds values. Carbon content and pore diameter are the main factors in determining the surface fractal dimensions. Deeper-deposited coal is characterized by rougher surfaces and more condensed pore networks. This is because these samples have higher carbon content and more micropores that formed under multiple conditions. Methane adsorption capacity is closely related to the Ds value, and a larger Ds value could indicate an increased capacity of methane adsorption.

3D topographic stress perturbations and implications for ground control in underground coal mines

It is well known that the perturbed stress field beneath valleys can result in roof instabilities in shallow underground coal and stone mines. Quantitatively predicting the magnitude of these stress perturbations, particularly beneath complicated three-dimensional (3D) topography, has not become commonplace in mine planning, perhaps due to the complexity and time-consuming nature of the problem. Here we utilize 3D digital elevation models and the 3D boundary element method (BEM) approach to efficiently calculate the pre-mining topographically perturbed stress field in the vicinity of the Carroll Hollow coal mine in eastern Ohio. We find that regions of elevated compressive stress in the mine correspond to areas in which cutter roof failure is a common source of roof instability. Furthermore, both the magnitude and inclination of the principal stresses calculated from the 3D topographic BEM model are found to be consistent with observed failure distributions within the mine. We propose that the approach outlined in this study can be efficiently applied to the mine planning process in order to mitigate or avoid potentially hazardous mining conditions.

Expansive behavior of coal mine spoils treated with calcium-based stabilizers

Sulfate-induced swelling, resulting from the chemical treatment of sulfate rich soils, has been known to cause significant damage to pavements and other structures particularly in the south-western United States. This paper addresses the problem of sulfate-induced swelling in coal mine spoils, formed as a result of shallow strip coal mining, after treatment with calcium-based stabilizers. Samples of coal mine spoils were obtained from the Hawthorn coal mine area in Sullivan County, Indiana, and treated with the following calcium-based stabilizers: hydrated lime, quick lime and Portland cement, at dosages ranging from 1.5% to 9% by dry mass of the spoils. The swelling tests, conducted over a period of one month, were performed using the sand bath method in a custom setup housed in an environmental chamber. All specimens of the spoils treated with the stabilizers were found to exhibit swelling greater than that measured on the untreated spoils, with the greatest swelling observed following treatment with 3% hydrated lime. Swelling of the treated spoils was initially delayed and approached a constant value after approximately one month.X-ray diffraction and thermo-gravimetric analyses provide evidence of the presence of ettringite in all spoil–stabilizer specimens. While the formation of this mineral appears responsible for the observed expansion, the measured swelling strain could not be correlated to the amount of ettringite formed, and appears, instead, governed by the restraint provided by the stabilized matrix.Tests were also conducted to examine the effectiveness of the following two mitigation techniques: pre-compaction mellowing and double lime treatment. A pre-compaction mellowing period of 2–3days led to swelling strains comparable to those of the untreated spoils. No advantages could be identified in using double lime treatment.

In-situ stress measurements and stress distribution characteristics in underground coal mines in China

The theory, equipment and method of hydraulic fracturing stress measurement used in underground coal mine roadways in China are presented. A quality assurance system and a quality ranking scheme were developed for hydraulic fracturing stress measurements under specific conditions encountered in Chinese underground coal mines. During the last 8 years, a total of 204 hydraulic fracturing stress measurement campaigns have been carried out in 49 coal mines within 13 Chinese coal districts. The test sites which are located in sub-horizontal, inclined and steeply dipping coal measures range in depth from 69.2 m to 1 283 m. Each stress data record was assigned a quality between A and E according to a proposed data quality ranking scheme. A total of 97 stress data records of A and B quality were used to investigate the distribution characteristics of in-situ stresses within the coal districts. Generally, in-situ stresses increase with depth. In shallow coal mines, the rate of increase in horizontal stresses with depth is greater than the rate of increase of vertical stress. With increasing depth the rate of increase in horizontal stresses decreases. A considerable scatter in the in-situ stress test data may be due to distinct differences in both the strength and deformation moduli of strata located in varying geological environments and different coal districts. In addition, large folds and faults often result in a change in orientation of the maximum horizontal stress, and in distinct changes in stress values. Three types of stress field distribution have been noted with σ H > σ h > σ v found mainly in relatively shallow coal mines (< 400 m), the σ H > σ v > σ h type in moderately deep coal mines (400–600 m), and the σ v > σ H > σ h type predominantly in moderately deep to deep coal mines. The ratio of the maximum horizontal principal stresses to vertical stress is usually between 0.5 and 2.0 in the coal districts.

CO2 Storage in Shallow Underground and Surface Coal Mines: Challenges and Opportunities

Effective storage of CO2 requires a better understanding of coal and minerals such as clays to develop new sorbent materials and sequestration technologies.

Reconnaissance spatial analysis of the hydrogeology of closed underground coal mines

A method is demonstrated to employ combined mapping and field techniques to unravel complexities in the hydrogeology of flooded and unflooded underground coal mines of shallow (120 m, 400 ft) depth. For a study area in the Appalachian coal basin, underground mines, coal outcrop, coal structure, and mine-drainage discharge locations were mapped onto an integrated geographic information system platform. The Upper Freeport coal seam (Allegheny Group) in the study area is relatively shallow (120 m, 400 ft), with many closed free-draining mines and numerous discharges. The mapping of coal outcrop and structure was accomplished using (1) high-resolution aerial orthophotographs, (2) digital outcrop polylines, and (3) a coal-bottom structure surface created by geospatial interpolation of point data mostly abstracted from original mine maps. Mapping results integrated with field measurements of mine-water discharge allow inference of likely pool locations and potential interconnections between underground workings, which are not apparent from mine maps alone. The results are reconnaissance level in that the interpreted pool locations and depths are unconfirmed by drilling or water level measurement; therefore, any action based on such results should be preceded by fieldwork to confirm hypotheses offered by the reconnaissance analysis. Nonetheless, such a preliminary interpretation of hydrogeological conditions based on little or no subsurface (well) water level information may be highly useful for both interpreting subsurface conditions and for planning future research. The technique is based on a combination of mine mapping, digital analysis of geologic structure, and surface reconnaissance fieldwork. Results are directly applicable to areas of reclamation of shallow coal mining where mine closures have resulted in an uncontrolled discharge of mine drainage.

Case histories of dc resistivity measurements to map shallow coal mine workings

Two recent events have focused attention on the need for mapping underground mine workings: the failure of the Martin County Coal Corporation tailings impoundment near Inez, Kentucky on 11 October 2000 and the 24 July 2002 Quecreek Mine inundation that trapped nine miners for 77 hours in Somerset County, Pennsylvania. In both cases, unexpected conditions related to abandoned mines were the sources of the accidents. In the first case, the overburden between an abandoned mine and the base of a slurry impoundment was too thin and the slurry broke into the mine. The consequence was that the slurry broke out and flooded two separate watersheds with coal refuse. In the second case, miners accidentally excavated into the flooded workings of the abandoned Saxman Mine, which they thought was hundreds of feet away. Conditions such as these require that the location of underground workings be defined. This is not easy. Detailed mine maps may be unreliable or missing. Conventional exploration (drilling) can easily miss targets as small as a mine entry. Because of these difficulties, the National Resource Council appointed the Committee on Coal Waste Impoundments to evaluate remote sensing technologies to locate mine workings. In Pennsylvania, a Commission on Abandoned Mine Voids and Mine Safety, convened in September 2002 as a result of the Quecreek inundation, has also identified various geophysical technologies that could be applied to the mapping of mine workings. Most geophysical techniques have been around for many years and the physics used to detect underground mines hasn't changed. Basic interpretation strategies also haven't changed. What has changed is the ability of the geophysicist to gather data rapidly. Multielectrode systems have greatly improved the efficiency of data acquisition with the dc resistivity method—e.g., measurements can now be made automatically. Until recently, the dc resistivity method was limited by …

Pillar failure in shallow coal mines—a recent case history

The mechanisms of subsidence over coal mines in which troughs develop above longwall workings or crown-hole collapse occurs over room-and-pillar workings are sufficiently widespread to be quite well understood. Other mechanisms whereby room-and-pillar systems fail are known from stratiform workings—in particular, from evaporite mines, where deterioration of the pillar supports is more rapid. Documented examples from coal workings that involve delayed room-and-pillar failure are, however, relatively rare.A case history of a row of terraced properties whose location coincided with a developing subsidence trough affords evidence of room-and-pillar failure. The terraced properties provided a reference, in the form of a level damp-proof course, from which the progress of the settlement could be monitored and interpreted. Mining records indicated that the centre of the trough was located over a line of 2.2 m high split pillars in room-and-pillar workings at 44 m depth. Borehole evidence showed that some of the pillars were either much smaller than shown or possibly absent altogether. The seat-earth floor was inundated with groundwater and the roof strata were weak mudstones, making the extended pillar system vulnerable to deterioration and failure. The mudstone roof had collapsed extensively over the voids and the resulting debris limited the residual void space available for subsidence. Consequently, some support was provided to the strata as subsidence developed and the process ceased after about 260 mm of settlement at the surface.A large volume of ground was lost into the subsidence trough at the surface. Subsidence at the level of the workings was not conclusively proven by monitoring or measurement, but other possible mechanisms within the superficial deposits that could account credibly for this loss of ground could not be identified. Failure of the room-and-pillar system was the only mechanism consistent with the available evidence and, in view of the condition of the mine workings at that location, is considered to be the cause of the surface subsidence.The absence of other case histories suggests that, although there may be circumstances in which such failure can occur, they are relatively infrequent and, unless there is a means of recording the subsidence and evaluating the mining position with reasonable confidence, room-and-pillar failure is likely to continue to be difficult to identify.

地表および岩盤沈下の基本モデル ウオールタイプ採炭に伴う岩盤の挙動（第１報）

This paper summarizes the theoretial and experimental comparison of physical characteristics of thick and hard rock strata which lies above wall type faces in The Republic of South Africa (Sigma Colliery) and Japan (Kushiro Colliery) and its effect to restrict surface subsidence. The difference of subsidence together with goaf solidification by the nature of rock strata is analyzed. When it is compared with the Dolerite case at an elastic modulus of 70 × 103 MPa, which is known that the 200 m face length is a minimum criteria for stable long wall operation, it is found that the face operation by the face length of more than 100 m with the hard and thick rock strata at an elastic modulus of 15 × 103 MPa will be an appropriate standard for Kushiro case. This is because of the occurrence of hanging phenomenon and its periodical heavy goaf flushing, when face standards, particularly face length is inappropriate. Thus, face design considerations are largely affected by the existence of thick and hard rock strata found in Dorelite (South Africa), Pocahontas Sand Stone (USA), Coalcliff Sand Stone (Australia) and Harutori Conglomerate/Sandy Shale (Japan).The author also indicate that the existence of very hard and thick rock strata above face can be pragmatically utilized to avoid sea water penetration in shallow coal mining district by way of applying comparatively shorter face length which will not allow sea bottom subsidence. As the results of analysis, design considerations for wall type mining, particularly face length is offered to suit various mining conditions.

Estimation of grout take for small-scale developments in areas of shallow coal mining: some examples from the East Pennine Coalfield: P. J. Lloyd, J. C. Cripps & F. G. Bell, in: Engineering geology of construction, ed M. Eddleston & others, (Geological Society of London; Engineering Geology Special Publication, 10), 1995, pp 135–141

Estimation of grout take for small-scale developments in areas of shallow coal mining: some examples from the East Pennine Coalfield: P. J. Lloyd, J. C. Cripps & F. G. Bell, in: Engineering geology of construction, ed M. Eddleston & others, (Geological Society of London; Engineering Geology Special Publication, 10), 1995, pp 135–141