Shallow Coal Seam Mining Research Articles

Study on Influence Laws of Strata Behaviors for Shallow Coal Seam Mining beneath Gully Terrain

The study on influence laws of strata behaviors is the basic guarantee of safety mining for shallow coal seam beneath gully terrain. Taking 3302 mining face of Zhujiamao Coal Mine as the engineering background, the laws of strata behaviors for shallow coal seam mining beneath gully terrain are studied by field detection, theoretical analysis, and numerical simulation. The strata pressure observation and the hydraulic support working resistance show that the dynamic strata behaviors appear violently during mining beneath the gully bottom. The theoretical analysis shows that the rotation and breaking of key stratum beneath gully bottom under nonuniform load is the fundamental cause of strong dynamic strata behaviors. The numerical simulation of overburden movement and fissure development characteristics shows that the strata behaviors beneath the gully bottom are stronger than the strata behaviors beneath other areas. Additionally, according to the laws of dynamic strata behaviors, the safety measures for mining beneath gully bottom are put forward.

Variation in shallow sandy loam porosity under the influence of shallow coal seam mining in north-west China

To study the influence of coal mining on the porosity of shallow sandy loam under conditions of shallow seam mining in thick, loose layers in north-west China, a typical surface sandy loam stratum in Shaanxi Province was taken as the study area, and experiments were performed to test the variation of soil porosity at different depths of 0–10 m in strata before, during and after mining therein. The experimental results demonstrate that the overall average porosities in the disc-shaped edge area, the disc-shaped edge area to the disc-shaped basin bottom area and the disc-shaped subsidence centre area of shallow sandy loam in mining increased by (23.51, 18.07 and 22.61%) respectively compared with that before mining. Mining meant that the soil porosity in the period of stable subsidence after mining changed significantly in the disc-shaped edge area and the disc-shaped edge to the disc-shaped basin bottom area. The disc-shaped edge area shows a trend of slowly rising porosity with the increase of depth, and the disc-shaped edge area to the disc-shaped basin bottom area shows a trend of gradually increasing first and then gradually becoming stable. Although the porosity in the central area of disc-shaped subsidence increased before mining, its trend was similar to that before mining. Although the change in soil porosity in the period of post-mining settlement stability is greater than that before mining, it is best fitted by a quintic polynomial. In general, the rate of change of soil porosity in the study area shows similar trends with depth. It showed a U-shaped variation that first decreased, stabilised for a distance and then gradually increased. This study provides theoretical support for surface soil remediation and ecological environment restoration in coal mining areas.

Sliding instability characteristics and re-stabilization mechanism of key stratum in thin-topsoil SCS mining: a computer-aided case study from the Niushan Coal Mine, China

Roof step subsidence and support crushing accidents caused by the sliding instability of fractured single key stratum often occur in shallow coal seam (SCS). For this reason, the exploration of fracture displacement laws and control mechanism of key stratum in thin-topsoil SCS is of great significance for both roof control and safety production. From this perspective, in this study, numerical simulation and theoretical analysis were conducted to establish a mechanical model of the key stratum structure after sliding instability in thin-topsoil SCS. Moreover, its instability characteristics and re-stabilization control mechanism were analyzed. The results revealed that the fractured key stratum block could reach re-stabilization without any support crushing after sliding instability in thin-topsoil SCS depending on the significant unloading effect of the topsoil. The mechanical model of the key stratum structure after sliding instability was also analyzed to obtain the criterion for the re-stabilization of the sliding instable rock block. The computational analysis corroborated that increasing the support intensity could promote the re-stabilization of fractured key stratum block after sliding instability, and the required support working resistance could be obtained for preventing support crushing incidents.

Mechanisms behind strong strata behaviour in high longwall mining face-ends under shallow covers

Abstract Safe and efficient mining of shallow coal seams relies on the understanding and effective control of strata behaviour. Field measurements, theoretical analysis and numerical simulations are presented in this study to investigate the mechanism behind abnormal strata behaviour, such as roof collapse and severe roadway deformation, that occurs in high longwall face-ends under shallow cover. We observed that coal pillars with two sides being mined out become unstable when the cover depth exceeds a certain value. The instability of the coal pillar can alter the fracture line of the overlying strata, triggering a reversed rotation of the ‘curved triangle blocks’ that form after the breakage of the overlying main roof. The revolving blocks apply stress on the roof strata directly above the longwall face-end, resulting in roof collapse. The collapse of both the coal pillars and the roof also leads to the advancement and increase of the overlying abutment pressure, which further causes severe roadway deformation in front of the working face. The strong strata behaviour that occurs in high longwall face-ends with shallow cover is presented in this study and countermeasures are proposed, such as widening or strengthening the coal pillar, or implementing destress blasting. The countermeasures we proposed and the results of our analyses may facilitate the safe mining of shallow coal seams with similar problems in the future, and may improve the safety and efficient working of coal mines.

Linkage-induced mechanism and control technology of pressure bump and surface geological damage in shallow coal seam mining of gully area

Coal mining will cause pressure bump in mining face and surface landslide, subsidence, and another geological damage in shallow thick coal seam mining of gully region. To analyze the linkage-induced mechanism and mutual influence relation between underground pressure bump and surface geological damage. This paper proposes a novel approach to study the stress field, displacement field, and damage field distribution of overlying strata in coal seam mining under valley terrain. The influence of valley terrain on in situ rock stress field decreases with the increase of the buried depth of coal and rock. The coal and rock strata of different depth under gully terrain are divided into significantly affected area, moderately affected area, and non-influence area. Exploitation activities under a gully area are divided into uphill mining stage, slope top mining stage, and downhill mining stage. The pressure bump and surface geological damage are easily linkage induced at the uphill mining stage. Based on the control of underground and surface damage of shallow thick coal seam mining in gully region, the slicing mining and filling mining of thick coal seam are put forward.

Effects of mining speed on the developmental features of mining-induced ground fissures

Shallow coal seam exploitation not only causes serious surface subsidence but also induces extensive ground fissures, which severely threaten the safety of underground mining and the surface eco-environment. We use the Daliuta coal mine of the Shendong mining district in China as a case study to investigate and characterize the influence of underground coal mining on the development of ground fissures. A new comprehensive impact parameter K of geology and mining for ground fissure development is introduced to explain the effects of mining speed on the developmental features of ground fissures. The results show that ground fissures often develop in the surface tensile zone ahead of the working face advancing position and show an inverted C-shape on the surface, which is similar to periodic fracturing of the basic roof. The ground fissure angle increases linearly with mining speed and logarithmically with K, while the developmental cycle decreases linearly with mining speed. We propose a technical measure to control ground fissure disasters by adjusting mining speed, which provides a theoretical reference for eco-environmental governance in shallow coal seam mining districts.

Experimental Investigation on Crack Development Characteristics in Shallow Coal Seam Mining in China

The development of cracks in mining is the scientific basis for the safety and environmental exploitation of shallow multiple-seam. According to the “thickness of coal seam, interactive distance, and buried depth,” four mining coal mines are selected in Shen Fu-Dong Sheng coalfield (SFDSC). To research the mining conditions of shallow coal seam under different base-load ratio mining conditions and different working faces by the physics simulation and in-sit measurement, the key roof caves are sketched by different colors. This study shows that the typical shallow coal seams in the thin overlying bedrock and thick loose sand layer (LSL) as well as the development of the setup entry cracks (SEC) is dominated by LSL arch damage. The surface cracks are almost directly above the setup entry. The flat seam mining and the SEC development is dominated by parabolic type. The surface cracks are located inside the setup entry. With the mining height increased typically in a shallow coal seam, the rate of crack development and the extent of damaged area increased significantly. The SEC and boundary cracks are fixed. The dynamic periodic cracks (DPC) show the ability of the strata to self-repair. During the multiple-seam mining, the above three kinds of cracks have the phenomenon of activation and development. Through the reasonable coal pillar distance arrangement, the development of boundary cracks can be effectively controlled and the relatively uniform surface settlement and crack closure can be achieved. The purpose of reducing damage mining can also be achieved. Furthermore, it provides scientific support for the green mining in the shallow coal seam.

Investigation of movement and damage of integral overburden during shallow coal seam mining

Most prediction methods used to calculate mining-induced subsidence are empirical, which cannot explain the mechanism of fundamental rock strata movement. In this paper, based on key stratum theory and the mechanical analysis of overburden, a new analytical solution, the Analogous Hyperbola Subsidence Model (AHSM), was proposed to describe the movement and damage of inner burden. Mining-induced rock strata movement was verified using the Distinct Element Method (UDEC), to assess the performance of the theoretical model. Three subsidence parameters (Smax/T, Sgoaf/Smax, D/H), generally used to describe surface subsidence trough, agree with statistical field measurements of the South Coalfield in Australia. The surface subsidence and the movement boundary in the overburden are very close to the theoretical predictions. Additionally, the primary key stratum damages the least in the overlying strata: its broken length is the smallest. The width of subsidence trough and maximum surface subsidence increase as the primary key stratum drops.

Development law of air leakage fractures in shallow coal seams: a case study in the Shendong Coalfield of China

Continuous air leakage from ground mining-induced cracks is a significant cause of coal spontaneous combustion, low oxygen at the working face and other disasters in shallow coal seams. This paper studies mining-induced cracks and air leakage caused by the repeated mining of shallow coal seams at the Bulianta coal mine, Shendong Coalfield, China. A similar simulation experiment was carried out in the laboratory, and then the ground mining-induced cracks were observed and the crack air leakage was detected. The results showed that the surface air flowed into the composite goaf before the fractures redeveloped to the surface during the process of lower coal seam mining. Due to the different development processes of vertical fractures in various regions, the width of the vertical fractures in the boundary area of the overlying goaf increases significantly after the repeated mining of coal seams, while the vertical fractures in the central area of the overlying goaf experience almost no change. The distribution of the surface air leakage cracks is in the shape of “回”. Graben-type cracks and half-graben-type cracks are surface air leakage cracks, which are the focus of surface air leakage problems, especially the half-graben-type cracks. The results are important in engineering for reducing air leakage from surface mining-induced cracks.

Goaf water storage and utilization in arid regions of northwest China: A case study of Shennan coal mine district

In arid and semiarid regions of northwest China, there are abundant shallow coal reserves throughout the region, yet during the mining process, the shallow water resources available are typically depleted due to the formation of fractures in the shallow subsurface. Water-preserved mining is the only option for coal mining in arid regions of northwest China. Based on the geological and hydrogeological conditions of the study area, this paper investigates the basic conditions and mechanisms of goaf water storage. A time series model for goaf water storage is proposed, along with a method for estimating the quantity of goaf water storage. Finally, hydrochemical characteristics of mine water and water quality characteristics after purification in the study area were analyzed. As indicated by the analysis results, after simple purification, the goaf water can meet industrial, ecological and agricultural water consumption standards. In the end, combined with current monitoring results of goaf water pressure and the status of goaf water utilization in the study area, this paper describes the quantity and the effect of utilizing goaf water storage. Goaf water storage can effectively alleviate the scarcity of local production and domestic water, and provides the most effective approach of large-scale water-preserved mining of shallow coal seams in the arid regions of northwest China.

Fracture Characteristics of Overlying Bedrock and Clay Aquiclude Subjected to Shallow Coal Seam Mining

Some of the coal deposits in the northwest region of China are at relatively shallow depths, covered by a thin layer of bedrock and a thick layer of wind-blown sand. We studied the mechanics and permeability of a clay aquiclude by X-ray diffraction and triaxial loading tests and the fracturing of the overlying bedrock and aquiclude in physical simulation tests. The results indicate that if the bedrock thickness is 90 m or more, the height, distribution pattern, and damage to the water-conducting fractured and caving zones (“belts”) are normal. If the bedrock thickness is 30 m or less, the “three belts make one” phenomenon occurs, and caving extends into the soil layer; the overlying rock is completely perforated by vertical fractures and the collapse spreads to the surface. When the bedrock thickness is between 30 and 90 m, the height of the water-conducting fractured zone is inhibited by the weathered rock and cohesive soil layers, and the height of the “two belts” is significantly decreased. This can be used to prevent and control coal mine water hazard.

Formation Mechanism and Reduction Technology of Mining-Induced Fissures in Shallow Thick Coal Seam Mining

Surface mining-induced fissures formed in shallow coal seam mining have serious impact on safety mining and water resources protection. This paper proposes a novel approach to study the formation mechanism and dynamic development of surface mining-induced fissures in shallow coal seam mining. This approach combines field tests, theoretical analysis, and numerical simulations based on the geological condition of shallow coal seam mining in Chuancao Gedan Coal Mine. Two typical surface mining-induced fissures, step-type fissures and collapse-type fissures, are generated in shallow coal seam mining. The fissures with large vertical throw or horizontal opening severely impact water resource protection and surface ecological environment. Surface mining-induced fissures are generated periodically and changed dynamically with the advancing of working face. The vertical throw and horizontal opening of surface fissures are changed dynamically with the movement of loading key strata. The movement forms of loading key strata determine the morphological development of surface fissures. Downward sliding movement of broken rocks causes step-type fissures, while downward rotation movement leads to collapse-type fissures. The degree of the downward sliding and rotation of broken rocks determines the vertical throw and horizontal opening of surface fissures. This paper proposes mining technologies to reduce damaging ground fissures in shallow coal seam mining and analyzes their control effects.

Groundwater system for the periods of pre- and post-longwall mining over thin overburden

The groundwater loss is one of the primary issues for environmental destruction. Underground longwall mining methods can cause rapid subsidence of the overburden strata. Especially in shallow coal seam mining. Since the subsidence through the overburden to ground surface, the groundwater may be affected and drawdown into the lower levels above the longwall panels. This paper analysed the potential impact of longwall mining on groundwater over thin overburden. Four water wells were drilled down to the proposed deformation zone above two longwall panels to record the different water levels’ variations throughout the mining period. A numerical groundwater flow model (GGU-SS-FLOW3D) was performed to predict the water levels in the periods of pre- and post-mining conditions. To set-up model, local mining conditions and regional geology characteristics were ascertained in this study. The measurement data from the four water wells were adopted to calibrate the flow3D model. The field measurement and GGU-SS-FLOW3D model can help to better understand the variation of the groundwater system related to the ground subsidence from longwall mining over thin overburden.

The Height of Water-Conducting Fractured Zones in Longwall Mining of Shallow Coal Seams

The height of the water-conducting fractured zone is of significant importance for both the prevention of water inrush induced disasters and the design of longwall mines. To ensure the mining safety of shallow coal seams under water-rich aquifers, an approach for forecasting the height of the water-conducting fractured zone is proposed by analyzing the damage zone distribution in FLAC3D model. First, 30 numerical models with different face lengths and mining thicknesses of no. 1101 coal face at Luxin Mine, China, a typical shallow coal face, were built using FLAC3D. Then, the heights of water-conducting fractured zones and the thicknesses of protective layers with different face lengths and mining thicknesses were obtained from numerical simulations and empirical formulae. Results show that the height of the water-conducting fractured zone increases with both the face length and mining thickness. And there is a critical value of face length, which is about 300 m in no. 11 coal seam at Luxin Mine. To mine no. 11 coal seam, a slicing mining method is suggested and the face length and mining thickness of the first slice are recommended as 200 and 3.0 m, respectively.

On-Site Radon Detection of Mining-induced Fractures from Overlying Strata to the Surface: A Case Study of the Baoshan Coal Mine in China

Large-scale longwall mining of shallow coal seams may cause mining-induced fractures that can project completely through to the surface. This could lead to a series of mine safety and environmental issues, further deteriorating the already fragile ecological environment in the Western mining areas in China. Therefore, an accurate and effective understanding of the spatiotemporal evolution law of mining-induced fractures in overlying strata and its relationship to upper aquifers is critical. In this paper, the application of the geophysical-chemical properties of radon in mining engineering is explored as a potential solution to the shortcomings of existing surveying methods. A radioactive measurement method is proposed for the detection of the development of mining-induced fractures from overlying strata to the surface in the Baoshan Coal Mine (BCM). The on-site test indicated that the first weighting step is approximately 60 m, the average periodic weighting step is approximately 20 m, and the influence coverage of the advanced abutment pressure is approximately 30 m. The presented method could be used as an indirect technical support to increase the safety of coal mining by acting as a simple, fast, and reliable method of detecting mining-induced fractures in overlying strata.

Research on Cracks Zone of Clay Aquiclude in Overburden

The mining induced cracks in aquiclude are major factor of water resisting property and mining safety under water. The aquiclude is composed by loess layer and red clay layer in Yushuwan coal mine. The water reaction property of clay and loess of aquiclude is tested by soil mechanics method. The permeability coefficient of loess will be 0.856 m/d and clay will be 0.434 m/d in Yushuwan mining condition. The dilatability coefficient of loess is 16.1% and the clay is 14.6%. Through physical solid-liquid simulation with whole stress-stain similarity for Yushuwan shallow coal seam mining, the distribution of “downward crack zone” and “upward crack zone” is found to be the major factors of aquiclude stability and the “downward crack” is the main channel of the subsurface ground water flowing down. Furthermore, the downward crack closing in wet clay is revealed and the 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. This work will provide the theoretical base for aquiclude stability research.

Characteristics and stability of slope movement response to underground mining of shallow coal seams away from gullies

Underground pressure is abnormal during mining of shallow coal seams under gullies. We studied gully slope movements, subject to underground mining, with physical simulation and theoretical analysis. The rules disclose that the slope rock slides horizontally in response to mining in the direction of gullies and rotates reversely with the appearance of a polygon block in mining away from gullies. We focused our attention on the case of mining away from a gully. We built a mechanical model in terms of a polygon block hinged structure and investigated the variation of horizontal thrust and shear force at the hinged point in relation to the rotation angle under different fragmentations. The Sliding-Rotation instability conditions of the polygon block hinged structure are presented based on the analyses of sliding instability and rotation instability. These results can serve as a theoretical guide for roof control during mining away from gullies in a coalfield defined by gullies.

Aquifer protection during longwall mining of shallow coal seams: A case study in the Shendong Coalfield of China

Large-scale underground mining of shallow coal seams can have significant impacts on ground surface and potentially leads to incidental damage to the overlying aquifer with subsequent environmental hazards. In order to deal with the apparent conflict between water protection and high production in underground mining, an aquifer protection mining technique was trialed in panel #51201 in the Shangwan colliery, the Shendong Coalfield, China. Overburden failure, water level of the unconsolidated aquifer, surface cracks, and ground subsidence were monitored in a series of boreholes using borescope, and also by observations in a nearby water well and by surface surveys. Monitoring results indicate that the aquifer protection mining technique can be successfully applied by modifying a few mining parameters such as mining height or advance rate under some suitable geological conditions. The critical zone in the stratigraphy appears to be the weathered bedrock located immediately below the aquifer. The weathered bedrock will remain impermeable after mining as long as its thickness is greater than the mining height. The development of the mining-induced overburden failure is affected by the key strata but ultimately controlled by the soft aquiclude such as the weathered zone. It was observed respectively that the height of the caved zone was observed to be 5 to 6 times the mining height, whilst the height of the fractured zone is about 10 to 11 times the mining height.

Field trials of aquifer protection in longwall mining of shallow coal seams in China

The large-scale mining of shallow coal seams has a significant impact on the overlying aquifers and surface ecological environment. To protect the aquifers and maximize the coal resource recovery, field trials were undertaken during the operation of the LW32201 in Bulianta coal mine, Shendong, China. With a severely weathered rock (SWR) layer and two key strata (KS) in the overlying strata, aquifer protection in longwall mining (APLM) relies mainly on the rapid advance. In some localized zones, special measures should be taken to achieve the APLM, including lowering mining height, backfill and slurry injection. To further understand the mechanism and applicable conditions of the APLM and validate the effectiveness of the APLM, variation of the water table in the aquifer was observed as the longwall face passed through the zone. This paper also discusses the mechanism and basic requirements of the APLM and the relationship between the fall of the water table and the surface subsidence. The results of the field trials indicated that APLM in shallow coal seams could be successful under suitable conditions.

Study on water resisting property of subsurface aquiclude in shallow coal seam mining

According to the characteristic of clay aquiclude of overburden, the proper simulation materials and proportions of mixture for simulating the plastic clay aquiclude layers were developed, and the plastic similarity conditions were setup. Thus, the similitude and simulation method in whole stress-strain was progressed. Furthermore, the simulation condition and material proportions in water reaction property and crack closing property were also put forward. Based on systematical tests, the development and distribution of mined cracks in roof and subsurface aquiclude was found and the stability of aquiclude was analyzed at all. At last, the key section and key index of aquclude stability was advanced. It is found that the movement of clay aquiclude follows the movement of the underlying bedrock layers. The basic caving mechanism of the overburden roof strata was also presented.