Coal Thickness Research Articles

Evaluation of Coal Body Structures and Their Distributions by Geophysical Logging Methods: Case Study in the Laochang Block, Eastern Yunnan, China

The coal body structure is one of the key parameters in the exploration and exploitation of coalbed methane (CBM). Accurate identification of the structure of a coal seam is important for CBM exploration and development. In the present study, we collected coal seam logging data and drilling data from six CBM wells in the Laochang block in eastern Yunnan, China. Four logging curves, including gamma-ray logging (GR), density logging (DEN), acoustic travel time logging (DT) and compensated neutron logging (CNL), were selected. Principal component analysis was used to establish a geophysical identification approach for the coal body structures in the Laochang block. The geophysical identification results for the coal body structures were in good agreement with the actual coring data. We found that if the coal body structures score (F value) was less than 12, the coal seam belongs to Type I coal; otherwise, it is Type II coal. With increasing coal seam thickness, the proportion of Type I coal showed a decreasing power function trend. When coal seam thickness exceeded 1 m, it was dominated by Type II coal. The coal seams near well YW-04 had the highest degree of overall damage, followed by wells YW-02, YW-03 and YW-01. The coal seam in well YW-04 was affected by the superimposed regional and local tectonic tensile stress fields; the coal seam in well YW-02 was subject to the regional tensile stress field and the goaf, and the coal seams in wells YW-01 and YW-03 were subject to the joint effects of the regional tensile and local compressive tectonic stress fields. The distributions of the coal body structures in the Laochang block were the result of the combined effects of the coal thickness, goaf, tectonic type and tectonic stress field. These results may provide guidelines for the exploration and development of CBM in the Laochang block in eastern Yunnan. Furthermore, the scientific and quantitative evaluation method developed in this research can be applied to the identification of coal body structures in other CBM blocks worldwide.

The Study of Key Stratum Location and Characteristics on the Mining of Extremely Thick Coal Seam under Goaf

The basis of traditional ground pressure and strata control techniques is the key strata theory, wherein the position of the key stratum can easily be determined for coal seams with regular thickness and without goaf. However, in the case of mining ultrathick coal seams underneath goaf, the traditional methods used for the calculation of key stratum position need to be improved in order to account for the additional coal seam thickness and the presence of an upper goaf. This study analyzed the failure height and collapse characteristics of overlying strata during excavation for determining the structure of the failed overlying strata. The results indicate that the intercalation and overlying strata gradually evolve into a large “arch structure” and a small “arch structure” during longwall mining, respectively. A mechanical model of the bearing characteristics of the interlayer key strata structure was established according to the structure of the intercalation rock layer, which is a hinged block structure. The results of the model indicate that the maximum principal stress occurs when the key strata portion of the arch structure bears the overlying load. Consequently, the movement and position of the interlayer key strata can be evaluated throughout the mining process of the ultrathick coal seams underneath goaf. This method was used to determine the position of interlayer key stratum of overlying strata in Xiegou coal mine. And the results agree with that of the engineering practice. The results are significant to determine the key strata position during ultrathick coal seam underneath goaf longwall mining.

Sequence stratigraphy, paleogeography, and coal accumulation in a lowland alluvial plain, coastal plain, and shallow-marine setting: Upper Carboniferous–Permian of the Anyang–Hebi coalfield, Henan Province, North China

To study the stratal stacking patterns and controls on coal accumulation in a lowland alluvial plain, coastal plain, and shallow-marine setting, the depositional facies, sequence stratigraphy, and paleogeography of the Carboniferous–Permian strata in the Anyang-Hebi (Anhe) coalfield, Henan Province, northern China were analyzed based on borehole cores. Ten facies were grouped into four facies associations, which represent alluvial plain, delta plain, delta front and prodelta, and offshore marine depositional environments. Ten sequences (S1 to S10, in ascending order) were defined on the basis of vertical facies stacking patterns. Sequences S1–S3, of Moscovian to early Asselian age, are composed mainly of offshore marine carbonate and mudstone, prodelta and delta front mudstone and sandstone, and delta plain sandstone, mudstone and coal. Sequences S4–S9, late Asselian to Wuchiapingian in age, consist of delta plain sandstone, mudstone and coal. Sequence S10, corresponding to the Changhsingian stage, is composed mainly of alluvial plain sandstone and mudstone. From S1 to S10, the Anhe coalfield experienced an environmental evolution from offshore marine, prodelta and delta front through delta plain to alluvial plain environments, an evolution from a low-accommodation to a high-accommodation setting, and a paleoclimatic change from humid to semi-arid conditions. Coal accumulation was controlled by a combination of relative sea-level fluctuation, paleoclimate and paleogeography. The cyclothemic nature of S2, including presence of parasequences, and the significant sea-level fall interpreted at the base of S4 may be related to Gondwanan ice dynamics in the late Carboniferous–early Permian. The thick coals are concentrated in the transgressive systems tracts of S2 and S4. The results of this study provide new insights into the environmental evolution of Carboniferous–Permian lowland alluvial plain, coastal plain and shallow-marine successions of the northern subtropical paleolatitudes and the controls on coal deposits in North China.

Comprehensive index evaluation of the spontaneous combustion capability of different ranks of coal

To investigate the effect of coal metamorphism on the probability of coal spontaneous combustion (CSC), five types of coal sample were subjected to a temperature-programmed testing system. The gas indicators and limiting parameters of CSC were analysed and the characteristics of different ranks of coal were observed during low-temperature oxidation. The CO production rate of each coal sample was positively correlated with temperature and the CO production rate of low-ranked coal was greater than that of high-ranked coal. The CO/O2 and CO productivity trends were the same. Analysis of the change in the alkene ratio revealed that the oxidation reaction became more intense if the coal temperature exceeded the dry cracking temperature. The oxygen consumption rate of each coal sample was positively correlated with temperature, low-ranked coal samples exhibiting higher oxygen consumption rates. Graham’s fire coefficient can be used as an auxiliary gas indicator to judge CSC tendency. Analysis of the limiting parameters of CSC revealed that the low limiting oxygen volume fraction and the minimal thickness of residual coal of high-rank samples were higher, whereas maximal air leakage intensity was lower. This paper further demonstrated differences in CSC tendency depending on the coal rank. These findings clarified certain factors affecting CSC, which may aid in preventing CSC in the future.

Innovation and future of mining rock mechanics

The 121 mining method of longwall mining first proposed in England has been widely used around the world. This method requires excavation of two mining roadways and reservation of one coal pillar to mine one working face. Due to considerable excavation of roadway, the mining roadway is generally destroyed during coal mining. The stress concentration in the coal pillar can cause large deformation of surrounding rocks, rockbursts and other disasters, and subsequently a large volume of coal pillar resources will be wasted. To improve the coal recovery rate and reduce excavation of the mining roadway, the 111 mining method of longwall mining was proposed in the former Soviet Union based on the 121 mining method. The 111 mining method requires excavation of one mining roadway and setting one filling body to replace the coal pillar while maintaining another mining roadway to mine one working face. However, because the stress transfer structure of roadway and working face roof has not changed, the problem of stress concentration in the surrounding rocks of roadway has not been well solved. To solve the above problems, the conventional concept utilizing high-strength support to resist the mining pressure for the 121 and 111 mining methods should be updated. The idea is to utilize mining pressure and expansion characteristics of the collapsed rock mass in the goaf to automatically form roadways, avoiding roadway excavation and waste of coal pillar. Based on the basic principles of mining rock mechanics, the “equilibrium mining” theory and the “short cantilever beam” mechanical model are proposed. Key technologies, such as roof directional presplitting technology, negative Poisson's ratio (NPR) high-prestress constant-resistance support technology, and gangue blocking support technology, are developed following the “equilibrium mining” theory. Accordingly, the 110 and N00 mining methods of an automatically formed roadway (AFR) by roof cutting and pressure releasing without pillars are proposed. The mining methods have been applied to a large number of coal mines with different overburdens, coal seam thicknesses, roof types and gases in China, realizing the integrated mode of coal mining and roadway retaining. On this basis, in view of the complex geological conditions and intelligent mining demand of coal mines, an intelligent and unmanned development direction of the “equilibrium mining” method is prospected.

Prevention and Control of Hydrogen Sulphide Accidents in Mining Extremely Thick Coal Seam: A Case Study in Wudong Coal Mine

Hydrogen sulphide is a toxic gas often present in coal seams and seriously threatens the lives and health of underground workers in coal mines. In this study, we theoretically modelled hydrogen sulphide generation in extremely thick underground coal mines with the +575 level #45 coal seam of Wudong Coal Mine as an example and obtained the on-site hydrogen sulphide emission pattern and spatial distribution features by combining field measurements and computational fluid dynamics simulation. The results showed that hydrogen sulphide mainly exists in the coal porous system in an adsorbed state. Because hydrogen sulphide has a molecular weight greater than the average molecular weight of air molecules, its concentration decreases with the increase of altitude to the bottom plate. When mining the upper stratified coal stratum, it diffuses widely in the working space; while when mining the lower coal stratum, it mainly concentrates at the bottom of the working face. Based on these analyses, on-site treatments were carried out using mixtures with different concentrations of sodium carbonate and sodium bicarbonate. In addition, different combinations of catalysts as well as type A and type B wetting agents were also tested. Eventually, a neutral KXL-I absorbent was developed, and the process of preinjecting absorbent and spraying absorbent was designed. The results showed that the newly developed KXL-I absorbent has high hydrogen sulphide absorption ability and is suitable for use as an absorbent in Wudong Coal Mine; preinjecting and spraying the absorbent can effectively prevent hydrogen sulphide disasters in the +575 level #45 coal seam in Wudong Coal Mine with the optimal final concentration of 0.9% and the absorption rate of 87% at the shearer of 66.6% at the support. Overall, our study provides valuable information for the prevention and control of hydrogen sulphide disasters in coal mines.

Research and application of coal and gas outburst early warning system based on gas geological features

The geological structure of coal mines has always been a dangerous object of attention in coal mine outburst prevention work. In order to realize coal mine safety information management and early warning of gas disasters, comprehensive use of gas geological theory, coal mine disaster warning theory, computer information technology and other analysis methods, considering the influence of geological structure, coal seam occurrence parameters, and gas parameters, an early warning indicator system for identifying the risk of coal and gas outbursts reflecting the geological characteristics of gas has been constructed. The coal and gas outburst risk identification and early warning system is constructed using the principle of multi-index step-by-step identification and extreme value determination, and it is applied on-site in the 3303 Measure Lane in the East Shaft Area of Sihe Mine. The research results show that the constructed early warning system can provide accurate early warning for the area (belt) affected by the geological structure by 10m, and can provide accurate early warning of coal and gas outbursts based on the outburst signs of gas geology such as the thickness of soft layers and changes in coal seam thickness. This technology provides effective support for coal mines to effectively prevent gas disasters and ensure coal mine production safety.

Discussion on Advanced Seepage Reduction Characteristics of Working Face under Seepage‐Damage Coupling

The water burst of roof on working face has been one of the significant geotechnical engineering problems that needs to be urgently resolved. The coupling effects of seepage and damage on the amount and intensity of water inrush from the roof are critically important. In this paper, the seepage‐damage coupling mathematical model of the aquifer in the working face is studied, and the seepage‐damage coupling mechanics model at different stages of the aquifer is established. Under the coupling of permeability and damage, the water‐soil characteristics of the aquifer in the 101163 working face of Mindong were numerically simulated by establishing the constitutive relation between vertical stress and permeability coefficient. The numerical results show that the stress concentration factor of the mining stress field gradually increases with the coal seam mining. The water‐flowing fractured zone of the overburden is close to the communication of the quaternary aquifer. When the coal seam is excavated 250–300 m. Three free surfaces appear in the groundwater pressure field, and a large falling funnel is formed to establish a deep flow S‐well well flow model. The research on the mining stress field and seepage field is carried out in combination with the Jakob formula. It is found that two sectors with reduced permeability of the fan surface are formed in front of the work. The variation law of the apocalyptic permeability infiltration under different mining distances, different coal seam thicknesses, different water pressures, and different roof management modes is studied systematically. The research indicates that the seepage flow under the condition of seepage infiltration of the lower aquifer should be between 50% and 100% of the traditional calculation method. The research results can help to deepen the understanding of the process of water inrush under the coupling of stress and seepage.

A research on the influence of geological factors to gas discharge from No.8 un-mined solid coal seam of Baode Coal Mine

Mining is gradually progressed toward the in-depth area of No.8 solid coal seam in No.3 panel of Baode Coal Mine. In order to secure safe mining in this area, a systematic analysis is conducted on the geological factors that influence gas occurrence. Based on the basic data actually measured at site, grey relational analysis (GRA) is adopted for predictive analysis of influencing factors (depth, coal seam thickness, metamorphic grade, sand to mud ratio of roof, sand to mud ratio of floor, geological structure and washout), followed by establishment of a grey relational model. Then, the relation degree among factors is calculated, thus identifying the main controlling factors of gas occurrence. The research result suggests: the main geological factors that influence gas occurrence in No.8 coal seam are geological structure and washout. A model equation is established for prediction of gas content using multiple regression method: y=3.2429+0.0047X1+0.0079X2-0.0180X3+0.0016X4-0.0215X5+0.4641X6+0.2001X7. This equation demonstrates high degree of fitting.

Study on Gas Occurrence Law in Xigou Coal Mine

The factors affecting the gas content occurrence and accumulation are quite complicated, in this paper, the gas occurrence law of Xigou Coal Mine is studied from the aspects of coal seam metamorphism, coal seam thickness, geological structure, surrounding rock lithology, burial depth and hydrogeological conditions, it provides guidance for safety production and gas development and utilization in later period.

Motion model and key parameters of overburden strata in longwall filling face

When the paste material is filled into the longwall working face of the coal mine, it firstly controls the movement of the overburden strata, thereby limiting the movement of the far field rock layer and reducing the subsidence. With this regard, the paste filling technology of the long-wall mining can address the limitation of “three down, one up” (mining under water body, building and rail, and on karst water body) technology in coal mining, thus improving the extraction rate of coal resource. To control the movement of the overburden strata near the working face, the prerequisite is to clarify the relationship among the parameters, including coal mining thickness, filling body thickness, filling step, filling body strength, etc. On this basis, this paper established an overall mechanical model of the filling body and surrounding rock, and determined the key technical parameters of the overburden movement, i.e., the filling rate, the strength of the filling body, and the filling step. Then, the influence of these parameters on the overburden movement and abutment pressure was analyzed through the numerical simulation method. The results show that at a higher filling rate, the overburden motion can be better controlled; the filling step only had a significant effect on the roof subsidence within the relevant step; as the early strength of the filling body was higher, the overburden subsidence was smaller and the abutment pressure in front of the face was better controlled. The research of overburden motion model and its key parameters have a good guiding significance for paste filling in the longwall face of coal mines.

Research on the Influence of Mining Height on the Movement Characteristics of Overlying Strata during Extremely Thick Coal Seam Fully Mechanized Sublevel Caving Mining

The study of the effects of mining height on overlying strata movement and underground pressure characteristics during extremely thick coal seam fully mechanized sublevel caving mining is very important for choosing the reasonable mining height and the support. Based on the geological setting and mining conditions at the Xiegou Coal Mine, the results of the physical simulation test and the numerical simulation technology will be used. Some conclusions can be drawn as follows: (1) With the mining height increase, the top coal gradually converted from tensile failure to shear damage, and the coal wall gradually transformed from shear failure to tensile damage. (2) When the mining height is 7.5 m, the full‐seam collapse distance, the immediate first weighting interval, and the main roof first weighting length are shorter than that when the mining height is 4m, and the periodic weighting length for the two mining heights is almost the same. (3) With mining height increase, the initial mining stage and the transition stage become shorter, and the production rates become better. (4) The law of the abutment pressure peak and the sphere of influence increase slightly, and the working resistance of support needed to be strengthened. (5) The subsidence quantity of the top coal in the control area increases along with the mining height in a quadratic polynomial way but decreases along with the initial supporting force in a negative logarithmic rule. (6) After assigning the subsidence, the regression relation between the initial supporting force and the mining height is a quadratic polynomial.

Spatiotemporal evolution of coal spontaneous combustion in longwall gobs: A case study from mining discontinuation to resumption

AbstractCoal mining is frequently halted for long periods of time due to geological faults, and the process of resuming mining is often fraught with danger. To address this issue, in this study, the previously developed COMBUSS‐3D software was optimized to simulate the process between mining discontinuation and resumption, based on the multi‐field coupling theory. The optimized software takes the temperature distribution in gobs when coal mining ends as the initial condition, and it considers the condition of spontaneous coal combustion after remining. The actual advance distance of the working face is introduced into the program compilation and solved by linear interpolation. These improvements ensure that the calculation results are more realistic. The results demonstrate that the high‐temperature zone is distributed on the air intake side of the gob, and its final location largely depends on the remining advance rate. The hazardous area of spontaneous ignition gradually moves to a deep suffocation zone of the gob when the mining of the coal wall resumes, the residual coal is no longer oxidized, and CO formation is gradually decreased, which is verified by the theoretical expectations and on‐site observations. The spontaneous combustion period can be effectively prolonged by reducing the thickness of crushed coal during the mining stage or by increasing the longwall advance rate after resuming mining. This study can provide significant guidance and suggestion on locating the hazardous area in gobs and suppressing the spontaneous coal ignition.

Impact of GPR antenna height in estimating coal layer thickness using spatial smoothing techniques

High-resolution algorithms to estimate the depth of near-surface interface using ground-penetrating radar (GPR) can provide a measurement of thin to thick top layer using synthetic data for surface-coupled antennas. In practice, the air-coupled antenna configuration in the target depth estimation needs to determine the air-gap distance between the antenna module and the surface. In this study, the authors present an improved approach of estimating the antenna height for measuring thin to thick top coal layer using the high-resolution algorithms. In GPR, backscattered echoes are generally correlated in nature. In order to apply high-resolution algorithms, the authors use two improved spatial smoothing techniques for fighting against the correlation between echoes. Here, in the existence of air–coal–shale–clay and air–coal–shale–coal interface, step frequency continuous wave GPR is used to measure the thickness of the coal layer. Simulation results on the antenna height estimation using synthetic data models, full-wave model and plane wave model are shown to assess the estimation algorithm's effectiveness.

Failure Mechanism and Control Technology of Thick and Soft Coal Fully Mechanized Caving Roadway under Double Gobs in Close Coal Seams

The surrounding rock of the roadway under double gobs in the lower coal seams is partially damaged by the mining of the upper coal seam and the stress superimposition of the stepped coal pillars. What is worse, the upper layer of the roof is collapse gangue in double gobs, which makes the anchor cable unable to anchor the reliable bearing layer, so the anchoring performance is weakened. The actual drawing forces of the anchor bolt and anchor cable are only approximately 50 kN and 80 kN, respectively. The roadway develops cracks and large deformations with increasing difficulty in achieving safe ventilation. In view of the above problems, taking the close coal seam mining in the Zhengwen Coal Mine as the engineering background, a theoretical calculation is used to obtain the loading of the step coal pillars and the slip line field distribution of the floor depth. The numerical simulation monitors the stress superimposition of stepped coal pillars and the distribution of elastoplastic areas to effectively evaluate the layout of mining roadways. The numerical simulation also analyzes the effective prestress field distribution of the broken roof and grouting roof anchor cable. A laboratory test was used to monitor the strength of the grouting test block of the broken coal body. Then, we proposed that grouting anchor cable be used to strengthen the weak surface of the roof and block the roof cracks. From on-site measurement, the roadway was seen to be arranged in the lateral stress stabilization area of the stepped coal pillars, the combined support technology of the grouting anchor cable (bolt) + U type steel + a single prop was adopted, the roadway deformation was small, the gas influx was reduced, and the drawing force of the anchor bolt and the anchor cable was increased to approximately 160 kN and 350 kN, respectively. The overall design and control technology of the roadway can meet the site safety and efficient production requirements.

Hydrocarbon Generation Potential and Depositional Setting of Eocene Oil-Prone Coaly Source Rocks in the Xihu Sag, East China Sea Shelf Basin.

The Xihu Sag in the East China Sea Shelf Basin is a focus for hydrocarbon exploration and development. Hydrocarbons in the Xihu Sag are believed to have mainly originated from coals in the Paleogene Pinghu Formation (shortened as Pinghu coal). In this study, the hydrocarbon generation potential, origin of organic matter, and depositional setting of the Pinghu coal were analyzed by means of optical microscopic analysis, bulk organic geochemistry, and molecular geochemistry analysis. The results reveal that the maceral compositions of the Pinghu coal are characterized by a predominance of vitrinite (73.91–96.13%) with relatively high contents of liptinite (1.47–23.02%) and only minor amounts of inertinite (0–5.18%). Type II–III kerogen and high values of TOC (total organic carbon) (8.24–56.77%), EOM (extractable organic matter) (14 601–112 259 ppm), and HI (hydrogen index) (178.76–291.18 mg·HC/g·TOC) indicate that the Pinghu coal is both gas- and oil-prone and could not only generate but also expel hydrocarbons. The results of vitrinite reflectance (0.54–0.90%), Tmax (421–453 °C), and biomarker-related parameters, including CPI (carbon preference index) (1.10–1.61), OEP (odd-to-even predominance) (1.09–1.49), 22S/(22S + 22R) for C31 homohopane (0.59–0.62), and 22S/(22S + 22R) for C32 homohopane (0.57–0.60), suggest that these coaly source rocks have entered the hydrocarbon generation threshold, most of which have entered the expulsion threshold. Biomarker-related parameters of ∑n-C21-/∑n-C22+ (0.38–3.62), Pr/Ph (3.33–9.23), Pr/n-C17 (1.91–14.88), Ph/n-C18 (0.35–1.83), 22S/(22S+22R) of C31 homohopane (0.59–0.62), 22S/(22S + 22R) of C32 homohopane (0.57–0.60), 1,2,7-TMN/1,3,7-TMN (0.74–14.39), and 1,2,5-TMN/1,3,6-TMN (2.22–21.07) suggest that organic matter in the Pinghu coal was deposited under relatively oxic peatland conditions characterized by a predominance of terrestrial higher plant input, especially the resin-rich Pinaceae and Taxodiaceae. The absence of combustion-derived PAHs indicates that neither vegetation fire nor peat fire occurred very frequently during the formation of peat. Areas in the Xihu sag with considerable thicknesses of coal should be paid particular attention for future hydrocarbon exploration. From a global perspective, Cenozoic coaly source rocks, which are characterized by a relatively high content of aliphatic components, should be paid special attention for their oil-prone properties related to the advent of conifer plants.

QUALITATIVE CHARACTERISTICS OF THE COAL DEPOSIT KOTEZI IN THE BUGOJNO BASIN

<p>This paper presents study results of the qualitative characteristics of coal deposit Kotezi. Regional geological researches were conducted from 1983 to 1987, and detailed from 2014 to the end of 2018.</p><p>Tests were performed on the following coal quality parameters: average thickness of pure coal in coal layers, total moisture content, ash content, total sulfur content and lower heat value. Researches were conducted in the field, laboratory and cabinet. Subsequently, an analysis and interpretation of the obtained research results was carried out. Comparing the research geological results of all represented coal layers, deviations of the considered parameters were determined.</p><p>ls of the Bugojno basin belong to the younger coals, soft to medium hard, no luster (matt), brown to black colored, and have brown streak. Their fracture is plate-like and particleboard. They do not have a distinct lignite structure, except in the lower layers. With their general habitus, they resemble younger brown coals, and belong to humic coals which are relatively low in carbonation. In the vertical profile of coal layers, the highest quality components are from the upper part of the layer, while the slightly lower quality components are from the middle and lower parts of the layer.</p>

Research on mechanism and control of asymmetric deformation of gob side coal roadway with fully mechanized caving mining

Considering serious asymmetric deformations and failures of the gob side roadway in fully mechanized top coal caving face and taking coal pillar roadway as engineering background, we have discussed the surrounding stress condition and the deformation failure patterns of coal pillar roadway by the theoretical analysis, numerical calculation, field tests. The results show that the deformations and failures of gob side coal roadway with fully mechanized caving mining are characterized by “two-way” asymmetric failure patterns along vertical and horizontal directions. The reason for the asymmetric deformation was uneven stress conditions of surrounding rocks under the effect of strong mining. The asymmetric deformations and failures of coal pillar roadway surrounding rocks were closely related to caving thickness in former panel. The fracture position of main roof varied with top coal thickness, and limit caving thickness was 9.4 m. Based on the asymmetric deformation characteristics and mechanisms of roadway, this paper has proposed a control strategy which mainly adopts asymmetric high prestressed truss anchor cable. Field tests were carried out in the repair section of roadway. The results show that roadway asymmetric deformation was well controlled and long-term stability of roadway was realized, which provides a reference for similar asymmetric deformation roadway.

Periods of refracted P-waves in coal seams and their applications in coal thickness estimations

The direct and accurate estimations of coal thicknesses are prerequisites for intelligent mining practices. One of the most effective methods for detecting the distributions of coal thicknesses in coal mining panels is the in-seam seismic (ISS) method. In the present study, after examining the formation processes and propagation characteristics of refracted P-waves in ISS data, it was concluded that the refracted P-waves in coal seams are mainly formed by the multiple transmission and reflection of the P-waves between the coal and rock interfaces of roof and floor at critical angles. This results in the refracted P-waves having strong periodicity, and these periods are proportional to the coal thicknesses. This study adopted numerical simulation models with different coal thicknesses, and the aforementioned periodicity characteristics were examined. It was found that the coal seam thicknesses could be calculated using the periods of the refracted P-waves. However, in thin- or medium-thick coal seams, it was found that multiple transmitted P-waves overlapped and the periods could not be read directly. Therefore, in order to solve this problem, this study composed source wavelets with the main frequency of the source signals and then composite synthetic P-waves by convoluting the source wavelets with the sequences of various coal thicknesses. The suitable estimated coal thickness corresponded to the minimum value of the errors between the synthetic and actual refracted P-waves. An experiment was conducted in the No. 42224 panel of the Chaigou Coal Mine in order to validate the proposed method. The experimental results revealed that the estimated coal thicknesses from the refracted P-waves were consistent with the actual geologic conditions in the coal mine. Due to the fact that the refracted P-waves arrive earlier than other waves in seismic records, the refracted P-waves could be easily identified and processed. Overall, the proposed method was found to be a simple application process for accurate coal thickness estimations.

Investigation on the Ground Pressure Induced by Hard Roof Fracturing at Different Layers during Extra Thick Coal Seam Mining

The fracturing of hard roofs in different layers would result in complex ground pressure on the working face, such as supports collapsed and severe roadway deformation. However, the mechanism of the ground pressure induced by hard roof fracturing in different layers is still unclear. In the paper, a physical model of a 20 m extrathick coal seam mined with hard roofs existing was established based on the physical simulation similarity criterion. The overburden fracturing structure, abutment stress distribution, and failure characteristics of the coal body were monitored by a noncontact strain measurement system and resistance strain gauges, to reveal the mechanism of ground pressure induced by hard roof fracturing. Furthermore, on-site measurement was used to monitor and analyze the ground pressure affected by hard roofs in different levels. The results provide a theoretical basis for the control of ground pressure in extrathick coal seam mining with hard roofs.