Mining-induced Fractured Zone Research Articles

Evolution of mining-induced fractured zone height above a mined panel in longwall coal mining

Evolution of mining-induced fractured zone height above a mined panel in longwall coal mining

Investigations of the height of fractured zones in overburden induced by undersea mining

Water inrush accidents are devastating to underground mining operations and cause damage. In particular, in undersea mining, mining safety is endangered as the mining-induced fractured zone extends upwards towards the vast amount of seawater. This paper describes a case study of the development of a fractured zone induced by undersea mining of a tilted ore body in the Xinli district of the Sanshandao Gold Mine in Shandong Province, China. A theoretical prediction model based on overburden movement during mining is proposed to analyse the developed height of the fractured zone induced by undersea mining of the tilted ore body. The Universal Discrete Element Code (UDEC) software program is adopted to establish a numerical model to investigate the movement and fracturing of the overlying strata during mining of the gold deposit from the – 280-m level to the – 200-m level in the Xinli district. For the study area in the – 200-m level, in which the gold deposit is currently being mined and is the closest to the seafloor, the theoretical (32.7 m) and numerical (36.23 m) results for the height of the fractured zone are in agreement with the field observations (32.1 m) from a panoramic borehole televiewer. These results indicate that the fractured zone in the Xinli mining area has not reached the seafloor and that the mine remains safe.

Dynamic monitoring of the mining-induced fractured zone in overburden strata, based on geo-electrical characteristics

Studies of the development height of mining-induced fractured zone in overburden strata are of great significance for safety and green mining of coal resources. Using geo-electrical response, a dynamic monitoring method for the fractured zone is investigated using the direct current resistivity method. Firstly, a numerical model of the mining-induced overburden is established and forward modeling is achieved from which the development of the caved zone and fractured zone is interpreted by inversion. The physical model then is designed, based on similarity theory and physical experiments, that displays visually dynamic deformation of the mining-induced overburden. After acquiring electrical data for the model, the inversion is done, based on which the development height of the fractured zone could be dynamically predicted. Finally, in order to prevent water inrush disasters, in situ investigations are carried out in one mine and height of the water-conducting fractured zone is determined. The researches show that the apparent resistivity inversion results, as the interpretation of caved zone and fractured zone, are approximately identical and the direct current resistivity method is proved to be an effective tool for dynamic monitoring of the mining-induced fractured zone.

Three-dimensional mineral grade distribution modelling and longwall mining of an underground bauxite seam

A multi-step interpolation approach was proposed to model the three-dimensional grade distribution and to demarcate orebody in a thin and anfractuous bauxite seam. A pre-mining demonstration procedure was established to virtually perform fully mechanized longwall mining in the modelled bauxite seam. Meanwhile, a comprehensive evaluation function expressed by the sum of dilution and loss rates of bauxite was utilized to determine the optimal cutting heights (1.71–2.69 m) and a suitable diameter (1.5 m) for the cutting drums mounted on a double-drum shearer to achieve no-waste and high-recovery mining of the demarcated 106,026 m3 bauxite orebody. In addition, a numerical model of strata containing geological defects (a karst cave and fault), discontinuities (joints and stratifications) and a mountain was established to simulate the discontinuous motion and overburden fracturing disturbed by the longwall mining in the modelled bauxite seam. The results indicated that the overburden presented reliable stability and impermeability, in which no large-scale caving in goaf, no water influx, no cave collapse, no fault slip, and no mountain slope landslide occurred. This was due to arched roofs above the Karst cave and the mining-induced fractured zone, the subcritical condition of the longwall panel in the dip section, and support from the protective pillar between two adjacent panels.

Influence of Spatial Relationships between Key Strata on the Height of Mining-Induced Fracture Zone: A Case Study of Thick Coal Seam Mining

The behavior of the overburden of working face 20104 at the Wangjialing coal mine was investigated using borehole imaging. The measured height of the conductive fracture zone (CFZ) in the overburden, 148 m, is significantly different from the height that is predicted by an empirical formula. The spatial relationships between key strata (KS) required for their fracturing and their influence on the CFZ’s height were analyzed. The results demonstrate that the spatial relationships between adjacent KS are a major factor behind the abnormal increase in the height of CFZ relative to the coal seam. The height of linkage (HoL) between KS was introduced and an equation for calculating this height was proposed. The study found that the fracturing of a KS could induce fracturing of the adjacent KS above it if their height difference was smaller than the HoL between them. Otherwise, the fractures resulting from the lower KS would terminate at the bottom of the higher KS. When the location of a high KS satisfies certain requirement, the spatial linkage between adjacent KS will allow for the conductive fractures arising in a lower KS to propagate through the high KS as well as the strata controlled by it, thus increasing the height of CFZ in overburden.

Investigation on mining-induced fractured zone height developed in different layers above Jurassic coal seam in western China

The mining-induced fractured zone height (MIFZH) is of significant importance for water hazard prevention and regional eco-environmental conservation in the Jurassic coal field of western China. The paper discussed MIFZH developed in bedrock and Neogene laterite from two aspects of field measurement and theoretical analysis respectively. In theoretical analysis of MIFZH developed in bedrock, based on plate and shell theory, each stratum in bedrock above the coalface stress-decreasing zone was simplified as four clamped rectangular plates, and the value of the ultimate deflection of the thin plate and the height of the free space in the lower part of the stratum were compared to judge MIFZH. When MIFZH was developed in Neogene laterite, MIFZH was calculated by Pu’s theory and rock mass limit equilibrium theory in theoretical analysis; in on-site measurement, micro resistivity scanning imaging logging technology (MRSILT), overcoming the shortage of fluid leakage technology, was adopted to detect MIFZH, where its measured result proved the feasibility of theoretical analysis. The research results have important significance to water conservation mining and safety mining of the Jurassic coal seam in western China.

Evaluation of a coal seam roof water inrush: case study in the Wangjialing coal mine, China

The Wangjialing Mine in southern Shanxi Province is seriously threatened by roof water and is infamous for a water inrush disaster that happened there in 2010. A root-cause-analysis was conducted. Three key issues, the height of the mining-induced fractured zone in formations overlying the coal seam, the water yield of aquifers overlying the coal seam, and working face water inflows before and after pretreatment of roof water-bearing aquifers, were studied based on the “three maps–two predictions” approach. According to the multi-source information composite principle, abundance zoning maps of the roof aquifer were made using the overlapping function of geographic information system (GIS) for five controlling factors: aquifer thickness, total core recovery, drilling fluid quantity, permeability, and thickness of brittle and plastic strata. Borehole-specific data from in-situ pumping tests were used to verify the water abundance results. For areas that did not meet the verification requirements, the weights of the controlling factors were calibrated by reestablishing the analytic hierarchy process judgment matrix. The total height of the fractured zone within the #2 coal seam roof was calculated using an empirical formula. An established roof crack safety zoning map was used to evaluate that aspect. A three-dimensional numerical simulation of the groundwater flow system was established based on the site conceptual model of the roof aquifer and was used to predict the working face inflows. The results indicated that the 20,518 working face of the 205 panel had the greatest abundance of water.

In situ investigations into mining-induced overburden failures in close multiple-seam longwall mining: A case study

Preventing water seepage and inrush into mines where close multiple-seam longwall mining is practiced is a challenging issue in the coal-rich Ordos region, China. To better protect surface (or ground) water and safely extract coal from seams beneath an aquifer, it is necessary to determine the height of the mining-induced fractured zone in the overburden strata. In situ investigations were carried out in panels 20107 (seam No. 2-2upper) and 20307 (seam No. 2-2middle) in the Gaojialiang colliery, Shendong Coalfield, China. Longwall mining-induced strata movement and overburden failure were monitored in boreholes using digital panoramic imaging and a deep hole multi-position extensometer. Our results indicate that after mining of the 20107 working face, the overburden of the failure zone can be divided into seven rock groups. The first group lies above the immediate roof (12.9 m above the top of the coal seam), and falls into the gob after the mining. The strata of the second group to the fifth group form the fractured zone (12.9-102.04 m above the coal seam) and the continuous deformation zone extends from the fifth group to the ground surface. After mining Panel 20307, a gap forms between the fifth rock group and the continuous deformation zone, widening rapidly. Then, the lower portion of the continuous deformation zone cracks and collapses into the fractured zone, extending the height of the failure zone to 87.1 m. Based on field data, a statistical formula for predicting the maximum height of overburden failure induced by close multiple seam mining is presented.

Height of the mining-induced fractured zone above a coal face

The development height of a gas conducting fracture zone (GFZ) in the gob overlying strata is crucial to the gas drainage and safe production of a coal mine. In order to address the issues of excessive gas concentration and uncertain GFZ height in No. 7435 Face overlying strata of Kongzhuang Coal Mine, China, the caving characteristics of overlying strata were explored using both physical experiments on similar materials and numerical simulations of Particle Flow Code (PFC) software and verified each other. The relationship of cracks development to porosity changing characteristics was introduced to quantitatively determine the height of the local GFZ. The quantified GFZ heights were compared with those measured using the in-situ drilling flow method. The results showed that 1) PFC software could accurately simulate the overlying strata caving behaviors, thus saving manpower, materials and financial resources needed for related physical experiments, and 2) the temporospatial distribution characteristics of porosity could be used to forecast GFZ height, and are of significant importance for determination of GFZ. Overall, the conclusions are of engineering significance for accurate arrangement of boreholes for gas drainage and reduction of mine gas disasters.

Analysis of longwall goaf gas drainage trials with surface directional boreholes

Surface directional boreholes (SDB) were recently trialled at two Australian coal mines as a new application to longwall goaf gas drainage. This method has become increasingly attractive for use at coal mines where surface access to drill large numbers of vertical goaf wells are limited or restricted. To demonstrate the performance and effectiveness of the SDB and help improve future design and implementation, results of the field trials at the two Australia coal mines are analysed and presented in this paper. The SDBs, drilled with medium-radius-drilling techniques, had different configuration parameters, including number of branches, diameter, and relative locations above the mining seam and from the tailgate. No borehole stability issues were encountered during the gas drainage operation. The drainage methane flow rate varied in all SDBs; however, if applied properly, they can gain a consistent high flow rate of methane at about 1000L/s. It was observed that the drainage effectiveness and efficiency were significantly influenced by local geological settings, borehole horizontal and vertical placement, branching, and water filling. The key to having high and consistent gas flow in a SDB is to maintain a sufficient connection with the mining-induced fracture zone. In addition, drilling quality control for such boreholes is also crucial, to avoid any concave sections that might fill with groundwater and block the borehole. The trial results demonstrate that SDBs have the potential to become a common method of longwall goaf gas drainage.

An integrated method to calculate the spatial distribution of overburden strata failure in longwall mines by coupling GIS and FLAC3D

The spatial distribution of overburden strata failure is of significant importance to affect the safety of underground mining. Because the traditional methods cannot be applied in all coal mines due to geological conditions or mining structures, a method of coupling FLAC3D with GIS was presented to calculate the spatial distribution of overburden strata failure in longwall coal mines. After building the spatio-temporal database from the calculation results of FLAC3D, the height of the mining-induced fractured zone in the overburden strata can be calculated by using the given height function. The results of case study show that the height of the fractured zone reached the maximum value at the face advance equal to about the panel width. The outcome of the work presented will be helpful in practice to improve safety in the production.

Seismic characterization of the Grängesberg iron deposit and its mining-induced structures, central Sweden

We have conducted a reflection seismic investigation over the apatite-iron deposit at Grängesberg in central Sweden. At the time of closure in 1989, the mine was operated using the sublevel caving method down to approximately a 650-m depth. This mining technique caused subsidence and generated a network of faults that propagated from excavated zones at depth up to the surface. The Grängesberg deposit is the largest iron oxide mineralization in central Sweden and is planned to be mined again in the coming years. It is therefore imperative to have a better understanding of the ore geometry and the fault network. A reconnaissance survey consisting of two seismic lines with a total length of 3.5 km was carried out to address these issues. The profiles intersect the Grängesberg deposit and open pit, as well as the major mining-induced fracture zone present in this area. A drop-hammer source mounted on a hydraulic truck was used to generate seismic signals; cabled and wireless receivers were used for the data recording. Preprocessing of the data first required the cable- and wireless-recorded data sets to be merged before stacking all data available at each shot point. Source gathers exhibit reflections from the near surface, probably generated at lithological boundaries hosting the iron mineralization and other geologic structures. Deeper reflections were also observed. The metavolcanic assemblage hosting the mineralization and the anthropogenic fault network were depicted in the stacked sections, bringing in new elements to refine the geologic model of the area. This study also illustrated the ability of reflection seismic methods to delineate mining-induced structures in hard-rock environments. Low-velocity anomalies from the open pit and adjacent structures were depicted in tomographic sections along the two lines, which showed good agreement with known geologic features and the reflection seismic results.

Water outbursts in underground mining with steeply dipping coal seams: numerical simulations based on a mining case

The height of the water-conducting zone is of significant importance for the safety of underground mining under a water body. Fully backfilling a mine excavation with gangue provides the advantages of safety and efficiency during the extraction of coal under buildings, railways and water bodies. Based on a mine with steeply dipping coal seams, a systematic numerical study of the damage mode of the safety pillar and roof strata surrounding a stope with and without backfilling was performed. The fracture initiation, propagation and coalescence in the stressed strata, and the seepage fluid field evolution were numerically obtained during the advance of the working face, which provided the detailed evolution of the fracture zone and the interaction of the fractures surrounding the stope in the steeply inclined coal seam. When backfilling with gangue is used as part of the mining process, the degree and absolute values of roof strata deformation are clearly reduced. The safety of the pillar and roof strata are all characterised by a transition from uniaxial compression to biaxial compression, which evidently improves their stability. The damage around the safety pillar and the height of the mining-induced fracture zone in the roof strata are then effectively restrained. Because of the technical advantages of backfilling with gangue in steeply inclined coal seams, the backfilling technology is easy to apply and may result in good support of the overlying strata. The higher the stiffness of the backfill is, the lower the amount of subsidence and damage to the roof strata will be. Because the stress concentrations and deformation around the working stope are reduced, the backfilling is beneficial to the stability of the safety pillar in the current working stope and to worker safety in other stopes and adjacent coal seams. These findings may provide technical data for engineering designs and safety evaluations in similar coal mines.

Mining-induced movement properties and fissure time-space evolution law in overlying strata

Mining-induced fracture zone will be produced in the overlying strata after the coal was mined. In this article, the mining-induced deformation of overlying strata and the time-space evolution law of fissure were studied by the methods of physical simulation and field measurement. The results show that bed separation fissure and vertical fissure will appear in the overlying strata above mining face, which form the wedge-shaped fissure zone. The open degree of fissure depends on the size of uncoordinated deformation between neighbor layers, and the absolute strata sinking controls both the width of bed separation zone and the open degree of vertical breakage fissure. At last, the calculating formula was deducted based on theoretical analysis.

Mechanism and experiment of substituting high drainage roadway with directional long drilling group to extract pressure-relief gas

By establishing the numerical model in the vertical plane and the similar model in the horizontal plane of gas flow in goaf, the influence of high drainage roadway or drilling on the gas seepage field was analyzed, and the extraction mechanism was clarified. On this basis, the academic thought of directional long drilling group instead of high drainage roadway was put forward. And then using complex function theory, the permeation mechanical model of drilling group with circle distribution in the mining-induced fracture zone was established to explore the coupling relationship between the drilling quantity, extraction volume and the equivalent extraction rate of single drilling. Finally, combined with the concrete geological production conditions, the main parameters of directional long drilling group were determined. The distance between the drilling group center and the air-return roadway is 24 m, the height is 18 m, and the three drillings are in an approximate equilateral triangle distribution with a space of 8 m. The equivalent extraction square is 4.15 m2. It is shown that the effect of directional long drilling group is evident. The gas content in the upper corner is controlled below 0.95%, the content in the tail roadway is kept below the alarm value, and the content is over 50% in the drill, realizing the secure and effective extraction of coal and gas.

Numerical investigation of groundwater outbursts near faults in underground coal mines

Permeable geologic faults in the coal seam can cause intermittent production problems or unexpected amounts of groundwater outburst from the underlying aquifers. With the acknowledgment of the basic mechanism for groundwater outbursts, the groundwater outburst along the fault zones in coal mines are numerically investigated using RFPA, a numerical code based on FEM. The fracture initiation, propagation, and coalescence in the stressed strata and the seepage field evolution in the stress field are represented visually during the whole process of groundwater outburst. The numerically obtained damage evolution shows that the floor strata could be classified as three zones, i.e. mining induced fracture zone, intact zone and fault reactivation zone, in which the intact zone is the key part for resisting groundwater outburst and directly determines the effective thickness of water-resisting rock layer. With understanding of the evolution of stress field and seepage flow in floor strata, the groundwater outburst pathway is calibrated and the transformation of floor rock mass from water-resisting strata to outburst pathway is clearly illuminated. Moreover, it is shown that geometrical configuration, including inclination angle of faults and seam drop along faults, have an important influence on groundwater outburst. Finally, based on geological, hydrogeology survey and numerical results, the mechanism analysis of groundwater outburst in an engineering case is studied, which can provide significantly meaningful guides for the investigation on mechanism and prevention of groundwater outburst induced by faults in practice. ►This study provides supplementary information on the stress distribution and failure-induced stress re-distribution that cannot be observed directly in situ or in experiments, within the areas of floor rock mass with the influence of fault. ►This study gives an interpretation of the fracture initiation, propagation, and coalescence in the stressed strata and the seepage field evolution in the stress field during the whole process of groundwater outburst. The transformation of floor rock mass from water-resisting strata to outburst pathway is clearly illustrated with the fracture evolution. ►This study makes an assessment of safety regarding water-resisting floor rock mass containing a fault with different configuration, including inclination angle and seam drop. And concretely illustrate the influence of fault configuration on groundwater outburst by a case study.