Shallow Seam Research Articles

Application of magnetotelluric method to the detection of overburden failure height in shallow seam mining

This paper studies the reliability of magnetotelluric (MT) method detection for overburden failure (OF) height in shallow seam mining. According to the geological conditions of the 52,505 working face in the Daliuta mine, five observation lines have been designed to detect the apparent resistivity (AR) of the overburden before and after mining. Analyzed the characteristic points of the face and the AR of the overburden on the main section and studied the characteristics of AR for various overburden lithologies. The AR of the overlying strata corresponding to different failure conditions is also different. Further obtained the height of the caved zone (CZ) on the working face was 25.6 m, and the height of the water flowing fracture zone (WFFZ) was 82.5 m. Finally adopted the numerical simulation and an empirical formula for calculation to validate the MT detection results and proved that it was reliable to use the MT method to detect OF height, thus providing a convenient, fast, economical, and reliable method for detecting OF heights when mining in shallow seams.

Study of a roof water inrush prediction model in shallow seam mining based on an analytic hierarchy process using a grey relational analysis method

Mining-induced water inrush is a sudden and destructive underground disaster caused by a mining disturbance. This disaster occurs frequently in the northern region of Shaanxi province in China due to overburden fractures in shallow seam mining, which pose a great threat to residents’ safety. It is therefore essential to construct an accurate prediction model. This study first applies selection hierarchy analysis to the main controlling factors of roof water inrush to study their weights using an analytic hierarchy process (AHP) including five factors: surface water catchment features, wateriness of the aquifer, water-resistant characteristics of aquiclude, combined influence of overburden, and mining disturbance characteristics. The grey relational analysis (GRA) method is used to calculate the correlation degree of each water inrush. The AHP-GRA method presents a comprehensive evaluative model combining the advantages of both approaches to analyze mining safety. Qualitative and quantitative indicators of the roof water inrush prediction model in shallow seam mining are established. Secondly, risk prediction of roof water inrush points and comprehensive water inrush is determined using engineering examples from the Hanjiawan coal mine. Results indicate that during safety mining, water inflow data are consistent with our prediction, thereby substantiating the model’s accuracy and providing a new method for predicting roof water inrush in shallow seam mining.

Drilling data of deep coal seams of the Cooper Basin: analysis and lessons learned

Deep (>4920 ft; >1500 m) coal seams of the Cooper Basin accommodate large amounts of natural gas; however, permeability of this unconventional resource is low and reservoir stimulation in prospective coal intervals is essential to achieve commercial production. This paper aims to analyse drilling data of deep coal seams of the Cooper Basin in South Australia. Drilling data obtained from mud logs are utilised to construct a drillability index (DI), in which rate of penetration is normalised by drilling factors, making DI more sensitive to coal rock strength. Analysis of DI and gas show information provides a preliminary screening tool for studying prospective deep coal seams, before performing in-depth reservoir characterisation and production tests. The decline in DI with depth is attributed to a compaction effect that makes deeper coal seams more difficult to drill through compared with shallow seams. The existence of a fracture network can reduce coal rock strength and consequently DI may increase. The increase in DI may be indirectly related to fluid flow characteristics of the coal seam helping in identifying prospective coal intervals. The DI is also affected by other factors and, hence, should be used in combination with reservoir information to yield conclusive indications. Gas show information and DI results were utilised to indicate the effectiveness of dewatering operation and hydraulic fracture confinement in the wells drilled in the Klebb area located in the Weena Trough.

Hydraulic support crushed mechanism for the shallow seam mining face under the roadway pillars of room mining goaf

While the fully-mechanized longwall mining technology was employed in a shallow seam under a room mining goaf and overlained by thin bedrock and thick loose sands, the roadway pillars in the abandoned room mining goaf were in a stress-concentrated state, which may cause abnormal roof weighting, violent ground pressure behaviours, even roof fall and hydraulic support crushed (HSC) accidents. In this case, longwall mining safety and efficiency were seriously challenged. Based on the HSC accidents occurred during the longwall mining of 3-1-2 seam, which locates under the intersection zone of roadway pillars in the room mining goaf of 3-1-1 seam, this paper employed ground rock mechanics to analyse the overlying strata structure movement rules and presented the main influence factors and determination methods for the hydraulic support working resistance. The FLAC3D software was used to simulate the overlying strata stress and plastic zone distribution characteristics. Field observation was implemented to contrastively analyse the hydraulic support working resistance distribution rules under the roadway pillars in strike direction, normal room mining goaf, roadway pillars in dip direction and intersection zone of roadway pillars. The results indicate that the key strata break along with rotations and reactions of the coal pillars deliver a larger concentrated load to the hydraulic support under intersection zone of roadway pillars than other conditions. The “overburden strata-key strata-roadway pillars-immediate roof” integrated load has exceeded the yield load that leads to HSC accidents. Findings in HSC mechanism provide a reasonable basis for shallow seam mining, and have important significance for the implementation of safe and efficient mining.

Characteristics of Strata Behavior during Thick Seam Mining by Fully-Mechanized Top Coal Caving in a Loess-Covered Gullied Region

This study systematically investigates the overburden movement and strata behavior at a fully-mechanized top coal caving (FMTCC) face in a thick coal seam in a loess-covered gullied region by field measurement and theoretical analysis. A comparative analysis was performed to examine how the attitude of surface gullies and the structure of overburden affect face support resistance. The characteristics and evolution mechanism of strata behavior during FMTCC under gully slopes in a loess-covered gullied region were discovered through a mechanical analysis of the roof structure and a comparison to the mining of a typical shallow seam under gully slopes. The study found that as the FMTCC face advanced, the strata behavior initiated in the middle part of the face and then continued to both ends, and its intensity was higher in the middle part than in the upper and lower parts. During mining under gullies, the increases in support resistance were generally less than 5% (except for the maximum increases of 7–9%), indicating that the occurrence of gullies on the surface had little influence on the strata behavior at the FMTCC face. Due to the increased burial depth and the number of key strata, the roof at the FMTCC face under gullies formed a stable load-bearing structure called a voussoir beam, which was the key difference from the strata behavior characteristics in the shallow mine under gullies.

Evolution mechanism of water-flowing zones and control technology for longwall mining in shallow coal seams beneath gully topography

The height and evolution mechanism of fractured water-flowing zones are of great importance for prevention of water inrush disasters and protection of immediate environments, particularly in ecological vulnerable areas. In this paper, comparative approaches including empirical formula calculation, physical simulation, numerical simulation and PROTEM transient electromagnetic analysis are employed to identify the development of the fracture zones caused by water-flowing so as to ensure mining safety and protection of groundwater in shallow coal seams beneath gully topography. An empirical formula was used to calculate the height of the water-flowing fracture zone, and an experimental study utilizing a model based on a similar material was adopted to analyze the movement and fracture development of the overlying strata. Results provide a reasonable basis from which a rational mining height may be determined. The PROTEM transient electromagnetic apparatus was employed to detect the water-flowing structure conductivity of the overlying strata induced by mining beneath gully topography in the field. The water-flowing fracture of the overlying strata can be estimated according to comprehensive analysis of the apparent resistivity changes of the field, gully surface crack development and aquifer water level. Accordingly, the control technologies in the field were implemented and discussed regarding preventing gully water from rushing into underground mining spaces through water-flowing fracture zones. Results indicate that safe and efficient mining of shallow seams beneath gully topography can be achieved by prevention of gully accidents and protecting the surface water resources of the ecological environment.

Effects of gully terrain on stress field distribution and ground pressure behavior in shallow seam mining

This study proposes a novel approach to study stress field distribution and overlying ground pressure behavior in shallow seam mining in gully terrain. This approach combines numerical simulations and field tests based on the conditions of gully terrain in the Chuancao Gedan Mine. The effects of gully terrain on the in situ stress field of coal beds can be identified by the ratio of self-weight stress to vertical stress (η) at the location corresponding to the maximum vertical stress. Based on the function η=f(h), the effect of gully terrain on the stress field of overlying strata of the entire field can be characterized as a significantly affected area, moderately affected area, or non-affected area. Working face 6106 in the Chuancao Gedan Mine had a coal bed depth <80m and was located in what was identified as a significantly affected area. Hence, mining may cause sliding of the gully slope and increased loading (including significant dynamic loading) on the roof strata. Field tests suggest that significant dynamic pressures were observed at the body and foot of the gully slope, and that dynamic loadings were observed upslope of the working face expansion, provided that the expanding direction of the working face is parallel to the gully.

Imaging the Underground Coal Gasification Zone with Microgravity Surveys

The paper describes results of microgravity measurements made on the surface over an underground geo reactor where experimental coal gasification was performed in a shallow seam of coal. The aim of the research was to determine whether, and to what extent, the microgravity method can be used to detect and image a coal gasification zone, especially caverns where the coal was burnt out. In theory, the effects of coal gasification process create caverns and cracks, e.g., zones of altered bulk density. Before the measurements, theoretical density models of completely and partially gasified coal were analysed. Results of the calculations of gravity field response showed that in both cases on the surface over the gasification zone there should be local gravimetric anomalies. Over the geo reactor, two series of gravimetric measurements prior to and after gasification were conducted. Comparison of the results of two measurement series revealed the presence of gravimetric anomalies that could be related to the cavern formation process. Data from these measurements were used to verify theoretical models. After the experiment, a small cavern was detected at the depth of the coal seam by the test borehole drilled in one of the anomalous areas.

Overlying strata movement rules and safety mining technology for the shallow depth seam proximity beneath a room mining goaf

Aiming at the shallow depth seam proximity beneath a room mining goaf, due to that the shallow depth seam is exploited using the longwall mining and overlain by thin bedrock and thick loose sands, many accidents are likely to occur, including roof structure instability, roof step subsidence, damages of shield supports, and the face bumps triggered by the large area roof weighting, resulting in serious threats to the safety of underground miners and equipment. This paper analyses the overlying strata movement rules for the shallow seams using the physical simulation, the 3DEC numerical simulation and the field measurements. The results show that, in shallow seam mining, the overburden movement forms caved zone and fractured zone, the cracks develop continuously and reach the surface with the face advancing, and the development of surface cracks generally goes through four stages. With the application of loose blasting of residual pillars, reasonable mining height, and roof support and management, the safe, efficient and high recovery rate mining has been achieved in the shallow depth seam proximity beneath a room mining goaf.

Research Review and Advance on Shallow Buried Coal Seam Mining Pressure

In order to research the law of mine pressure and roof control in shallow buried coal seam, based on a comprehensive summary of mine pressure theory, technology application, engineering practice, summarized research results and status, analyzed research shortage, proposed research proposals. The results show that:1“short masonry beam” and “step beam” structure mechanical model and “key stratum in shallow seam theory” has provided theory basis for coal seam roof control shallow guidance. 2Rock and soil (bedrock and alluvium) coupling mechanism, loose layer dynamic structure theory," coal mining, water retention, ecology conservation, subsidence control" optimized portfolio on management theory and technical system of is the required research direction. 3Sichuan, Yunnan, Guizhou, representative of southwest mountainous shallow buried coal seam mining pressure research is conducive to development of mining pressure theory system.

Study on the Failure Height of Overlying Strata in Shallow Seam with Thick Bedrock

It was very important to obtain the failure height of overlying strata to ensure safety mining under water body and protect surface and underground water resources. Aimed at the failure of overlying strata of 6204 full-mechanized mining in Ordos, the paper proposed a new detection method of the failure height of overlying strata, and verified the accuracy and practicability of this detection method by comparing with the traditional borehole TV observations.

Study of Seismic Inversion Technology Forecasting Sandstone Waterborne of Coal Roof

Mechanism of water Inrush indicates that water resources is the essential factor for water inrush, roof and floor sandstone aquifer is the main source of water in the shallow seam mining process, take advantage of the physical property that sandstone is porous medium, using logging-seismic-geological comprehensive prediction method to determine the position of the sandstone porosity where is rich in water, the delineation of the rich water area, provide the evidence for the work of mine Water.

Application of a GPR technique for the monitoring of simulated underground coal gasification in a large‐scale model

ABSTRACTUnderground Coal Gasification (UCG) involves the recovery of energy from a coal deposit via the use of controlled combustion. A reactor equipped with apparatus enabling the analysis of temperature profiles and gas composition was constructed for the simulation of underground conditions, both with respect to the coal seam and the surrounding stratum characteristics. A three day experiment was performed in which large blocks of lignite were gasified, with an average gasification rate of 5 kg/h. The ground‐penetrating radar (GPR) reflection method was used to monitor the process and observe physical and structural changes taking place within the gasification zone; radar images of the different stages of the process were then recorded and analysed. The experiment demonstrated the potential for successful application of radar techniques in the observation and monitoring of the coal gasification process in a natural environment. Whereas the gasification of coal in shallow seams could be monitored by surface measurements, that of deep seams in operating coal mines or coal deposits could be monitored by measurements made from galleries or boreholes.

Key Protecting Strata Criterion of Water Conservation Mining and its Application

The entironment is deteriorating increasingly in north of Shaanxi Yushenfu mine with coal mining cosmically. The causation is the lapse of groundwater owing to mining. Water conservation mining has been the exigent request. Aiming at the need of water conversation mining in north of Shaanxi shallow seam mine, this article found key protecting strata criterion of water conservation mining according as the moving law of overburden rock and the mechanics model of fixed beam. The water conservation mine method in different overburden rock mine was put forward with the condition of no destroying water protecting strata. The feasibility of the key protecting strata criterion was validated using the choice of water conservation mine method in two different overburden rock frame mine in north of Shaanxi shallow seam mine. At the time, validating the computational result applying similitude simulation test and numerical simulation. The key protecting criterion offers a theoretic computational method for the choice of water protecting mining in different overburden rock frame mine.

Study on the Supporting Method of Crossheading in Shallow-Buried Coal Seams Working Face Using FLAC Simulation

Taking 1403 coal face of Fengjia coal mine in northern Shaanxi as background, put forward three supporting schemes of crossheading according to different buried depth and coal seam interval. Applying FLAC software, established relevant calculating model, analyzed the stress field and surrounding rock deformation caused by shallow seam mining before and after supporting. Combining the result from site monitoring, obtained the deformation law of crossheading surrounding rocks in the short distance seam: the vertical deformation is the main deformation under the A support scheme. The field tests showed that: the supporting schemes according to the section of crossheading is feasible and it ensured haulage gate roadway’s surrounding rocks stability , met the requirements of safety production in the coal mine, the supporting schemes of crossheading proposed in this paper has reference value to similar project.

Buckling and Post-Buckling Behavior for Roof Strata in Longwall Mining

In accordance with the occurrence behavior of roof strata and the fracture characteristics of key stratum in shallow seam longwall mining, this paper studied the post-buckling behaviors of key roof stratum in the process of mining by using initial post-buckling theory, which derived a critical load and a breaking span of the main roof during the first weighting, determined the final subsidence for broken key stratum, and presented an application with the example of Daliuta 1203 face. The results indicate that the rock blocks a in are state of non-equilibrium after main roof breaking, the equilibrium path of main roof is unstable from breaking to final subsidence; thick unconsolidated layers above roof have effect on post-buckling behaviors of key stratum; the stability for bifurcation point equilibrium configuration and post-buckling equilibrium path of roof strata could be revealed and an effective method for determining displacement field of imperfection structure could be provided by using initial post -buckling theory.

A Study on the Roof Fracture Mechanism of Large Cutting Height Workface in Shallow Thick Coal Seam

Based on the high mining height of large cutting height workface in shallow thick coal seam and the few falling waste rock in goaf, the key roof can not be supported effectively, the facture mechanical model of key roof was established. The theoretical calculation formula of key stratum fracture step and working resistance of support were obtained by using fracture mechanics. The results show that the fracture step of key roof relate to not only the mechanical character of key roof and the load of overlaying rock seam, but also the working resistance of support and horizontal pressure in key roof. The reasonable working resistance of support and the step of roof fracture were analyzed in 1-2coalmine 51104 face of a mine in Dongsheng area. The theoretical results are well agreeable with the field measured results.

Arch Math Model of Thick Sand Layer Mining Unload Failure in Shallow Seam

Under the basis of the “arch” failure of thick sand layer in preliminary period in shallow seam through field experiment, arch math model is set up. The conclusion establish the basis to study the coupling problem between key stratum (key block) and the sand layer.

Study on Movement Law of Overlying Strata during Shallow Thinner Seam Mining in Shendong Mining Area

Given the particularity and the first time for shallow thinner seam mining in Shendong mining area, the overlying strata movement law during shallow thinner coal seam mining has been studied in this paper based on the fully mechanized thinner coal face of NO.71301 in Shi Getai coal mine, resorting to observation on spot, numerical simulation and other methods. Through research, the important laws and features had been obtained, such as, initial pressure step, periodical pressure step, the obvious partition of periodical weighting, the unitary elasticity distribution of front abutment pressure, and the stress shell morphology over the mining rock, which provides important characteristic for roof control and foundation for the powered support selection.

Research on downward crack closing of clay aquiclude in shallow coal seam safety mining

The water resisting property of aquiclude is the key factor of water conservation and safety mining, and the mining induced cracks in aquiclude is major factor of water resisting property. The aquiclude is composed by loess layer and red clay layer in Yushuwan Coal Mine, and the water reaction property of clay and loess of aquiclude was tested by soil mechanics method. The permeability coefficient of the loess is 0.856 m/d and the clay is 0.434 m/d. The dilatability coefficient of the loess is 16.1% and the clay is 14.6%. Through physical solid-liquid simulation with whole stress-stain similarity, the distribution of “downward crack zone” and “upward crack zone” was found to be the major factor of aquiclude stability. The downward crack closing length is about 30% of the downward crack length. The expanding of clay and loess with water are principal factors of downward crack closing. At last, the mechanical model of downward crack closing was constructed, and the criterion of crack closing was put forward at all. This work will provides the theoretical base for aquiclude stability research and safety mining in shallow seam.