Thin Bedrock Research Articles

Experimental Study on the Mining-Induced Water-Resistance Properties of Clay Aquicludes and Water Conservation Mining Practices

The Yushen mining area contains thin bedrock and a shallow buried coal seam, where JingLe group Hipparion clay and Lishi loess serve as a high-quality cement insulation cover. This study investigates the properties of the clay layer to determine the effect of the clay aquiclude on the mining water variation and fracture characteristics. Unloading hydraulic jack experiments were performed to test the physical and mechanical properties of the clay layer and the structure was analyzed in detail. The experimental results show that mining affects the soil cracks, leading to crack opening and subsequent bridging. The permeability coefficient of the soil layer initially increases with increased unloading and then decreases. A theoretical model is developed to determine the recovery mechanism of the clay layer water insulation based on the spatial movement of the clay. The results indicate the formation of a waterproof cover type of coal mud protection. Design methods are proposed to optimize the coal pillar size. Mining damage leads to the formation of a mud-covered bridge belt, which can be designed to appropriately reduce the protective layer thickness. The model is applied to the Hao Jialiang 2301 working face. The results provide important insight on the variation characteristics of the mining water insulation in clay layers and an important reference for accurately calculating the size parameters of waterproof protective coal pillars under mud-cap conditions to increase the upper mining limit of the working face.

Height of Water-Conducting Fractured Zone with Coal Mining in Loess Covered Area

Loess covered areas in China are short of water resources, but yet contain abundant coal resources. Height of the Water-Conducting Fractured Zone (WCFZ) is an important index for protecting water resources and the ecological environment. Using the geological conditions of the Zhangjiamao coal mine, Yushen mining area, China, a series of in-situ testing methods were used to observe the height of WCFZ in coal mining face. Then, a physical simulation was conducted to determine the developing process of WCFZ. Finally, the single factor and multi factor statistical analysis of the height of WCFZ was performed based upon the collected data. The results show that height of WCFZ is mainly controlled by mining height, followed by bedrock and loess overlying the coal seam, which includes thick bedrock-thick loess type (I), thick bedrock-thin loess type (II), thin bedrock-thin loess type (III), and thin bedrock- thick loess type (IV). This study provides support for the sustainable development of coal mining in the loess covered area.

Comprehensive analysis control effect of faults on the height of fractured water-conducting zone in longwall mining

In order to examine the height of a fractured water-conducting zone due to the presence of a fault, a typical working face in the Baodian Coal Mine which is located in the Yanzhou Coalfield of northeastern China is examined as a case study. Two subsurface conditions with and without a fault are considered. In this study, equations are developed to calculate the maximum span of the unsupported roof with and without the presence of a fault. Numerical models are used to examine the changes in stresses and the development of a plastic zone at different stages of mining. A comparison of the results based on theoretical analysis and numerical model shows that the height of the fractured water-conducting zone increases significantly with the presence of a fault. The height of the fractured water-conducting zone is about 1.50 times higher than the case without a fault. The results provide important insight into the prevention and control of water inrush in mines with thick and loose deposits and a thin bedrock.

Safe feasibility of retaining sand-proof coal rock pillars in full-mechanized caving mining of extra-thick coal seam

To solve the problem of increasing the mining upper limit by reasonably setting sand-proof coal rock pillars in an extra-thick coal seam under a thick loose stratum and thin bedrock of Yangcun Coal Mine in Yanzhou Mining Area, the geological and hydrogeological conditions of working face were systematically analyzed by taking 325 Working Face in No. 3 coal mining area of the mine as the background and combining with the mining experience of its adjacent working faces. The mining grade under the water body was determined to be grade II, consistent with the conditions for retaining sand-proof coal rock pillars on the roof of coal seam. On this basis, to avoid the limitation of a single method, this paper adopts the key stratum theory, empirical formula analogy, and numerical simulation; the heights of the maximum caved zone (CZ) and the water-conducting fractured zone (WCFZ) were determined as 32.8 m and 82.0 m, respectively. The thickness of protective stratum is 16.4 m; the height of sand-proof coal rock pillars in 325 Working Face is 49.2 m; the minimum thickness of bedrock pillar in 325 Working Face is 53.1 m. Thus, full-mechanized caving mining can be carried out according to the reservation of sand-proof coal rock pillars. The comprehensive application of various methods can effectively improve the reliability and accuracy of the calculation results. The research results provide the main reference for this working face and similar working faces in Yanzhou Mining Area in the future to increase the mining upper limit and set up safe coal rock pillars according to roof sand control.

Dynamic Evolution Law of Overburden Rock in Shallow‐Buried Super‐High Fully Mechanized Working Face and Determination of Support Strength

The objective of this study was to reveal the law of overburden movement and stress evolution during the mining of super‐high fully mechanized mining faces. Based on the 12401 fully mechanized mining face of Shangwan Coal Mine in Shendong, this study conducted research and analysis using the methods of similarity simulation experiment, numerical simulation, and field measurement. The results showed that the maximum and minimum principal stresses in the coal seam in front of the working face are concentrated with the advance of the working face. The degree of stress concentration increases with the increase in the advancing range, and the concentration degree of the maximum principal stress and the change gradient is greater than that of the minimum principal stress. But the range of the peak lead coal wall is lower than that of the minimum principal stress of the peak lead coal wall. The phenomenon of stress recovery exists in the goaf. With the increase in the advancing range of the working face, the degree of stress recovery gradually increases, and the degree of maximum principal stress recovery is higher than that of the minimum principal stress recovery. The large fractures observed near the working face are closely related to the underground pressure, relatively large fractures appear on the surface, and the fractures become narrower near the two pathways. Only caving and fissure zones exist in the thin bedrock overburden, and the bending subsidence zone changes with the bedrock thickness. The support strength of the hydraulic support should not be less than 1.47 MPa. This research on the overburden movement and stress evolution law of a super‐high fully mechanized mining face can provide theoretical guidance for the exploitation and utilization of extrathick coal seam resources. It has broad engineering prospects.

Failure Analysis of Thin Bedrock and Clay Roof in Underground Coal Mining: Case Study in Longdong Coal Mine

Abstract It was found that movement in the overburden after mining was complicated under the condition of thin bedrock and clay roof (TBCR). In order to explain the small–big periodic weighting phe...

Study on the Influence of Bedrock Thickness on Deformation and Failure of Overlying Soil Layer in Thin Bedrock Coal Seam Mining

The water-blocking action of the clay Cenozoic overburden layer is an important factor that affects safety of thin bedrock coal seam mining. The change of thin bedrock thickness has an important influence on the deformation and failure of overlying soil layer. In this work, similarity simulation experiment is used and two indexes of failure height and crack density in soil layer are selected to analyze the influence of thin bedrock thickness on the deformation and failure of overlying soil layer. The experiment results show that the failure height of soil layer increases with the decrease of the bedrock thickness. The change of failure height is small when the bedrock thickness is greater than 30 m (mining thickness is 6 m), while when the bedrock thickness is less than 30 m, the failure height of soil layer increases rapidly with the decrease of the bedrock thickness. The main effect of the decrease of the thin bedrock thickness on the deformation and failure of the overlying soil layer results in the increase of shear/tensile cracks. The shear/tension crack density is 0.04 strip/m when the bedrock thickness is 70 m, while the shear/tension crack density increases to 0.14 strip/m when the bedrock thickness reduced to 20 m.

Deformation and Failure Characteristics of Overburden Under Thin Bedrock and Thick Alluvium: A Case Study in Baodian Coal Mine

Underground mining will lead to deformation and failure of overburden and frequently associated with water inrush. In view of the special situation of No.8 mining area in Baodian Coal Mine (thin bedrock and thick alluvium). According to geological engineering characteristics and overlying lithified strata, different bedrock height models were established using physical and numerical simulation. The deformation and failure of overburden, mining-induced stress, and height of the caving zone are then studied using these models. The results show that mining-induced stress is different in overburden and clay, with a large variation near the rock and clay layers. Findings also illustrate that the clay layer has an inhibitory effect on mining-induced stress. The height of the caving zone increases with the height of the bedrock, and the height of the caving zone obtained by physical and numerical simulation are similar. Through the comprehensive comparison of the height of the caving zone, it can be inferred that the height of the caving zone does not reach the thinnest position of the bedrock and no large water inrush will occur. The results obtained in this study provide an important framework for the study of coal mining safety under similar geological conditions in other locations.

Law of Movement of Discontinuous Deformation of Strata and Ground with a Thick Loess Layer and Thin Bedrock in Long Wall Mining

The surface discontinuous deformation caused by coal mining has great damage to the ecological environment and threatens the safety of human lives. Focusing on the problem of discontinuous deformation (ground fissures and collapsed pits) in mining areas with a thick loess and thin bedrock, this paper uses a coal panel in southern Shanxi in China as research background, and uses field investigation, theoretical analysis and the particle flow code 2D (PFC2D) numerical simulation method to study the movement of overburden and discontinuous ground deformation of mining areas with a thick loess layer and a thin bedrock. The results show that with the continual advance of the working face, the failure of the overlying rock, the changing of force chain shape and the development of cracks under this geological and mining condition have their unique rules. This study analyzes the law of movement of overburden in coal seam mining, explains why discontinuous deformation of the surface occurs in case of a thick loess layer and thin bedrock, and provides reference for the prediction of fracture development under the same geological conditions and the application of the PFC2D in coal seam mining in different geological conditions.

Height of water-conducting fractured zone in a coal seam overlain by thin bedrock and thick clay layer: a case study from the Sanyuan coal mine in North China

The accurate determination of the height of the water-conducting fractured zone (HWCFZ) in a coal seam covered by thin bedrock and thick clay layer is crucial for aquifer protection, mine safety, and eco-environmental conservation. Using the Sanyuan coal mine in North China as a case study, theoretical analysis, numerical simulation, and field measurements were used to analyze the regularity of the HWCFZ in this area. Per mechanical theory, a geometrical model for calculating rock bending and subsidence was established. Two arcs of equal curvature and lengths and opposite curvature direction were used to fit the curve section of intermediate rock layer after bending. The theoretical method for solving the HWCFZ was given by calculating the rock layer's tensile ratio. In addition, rock failure process analysis (RFPA) software was used to simulate the development of the water-conducting fractured zone (WCFZ) during the mining process, and the HWCFZ was measured by the drilling fluid loss measurement, both which verified the feasibility of the theoretical calculation. To determine the primary controlling factors affecting the HWCFZ, the seepage characteristics and plastic properties of the Quaternary clay layer were also analyzed using laboratory tests. Results show that the Quaternary clay with a depth greater than 130 m has a permeability coefficient of E−10 m/s, a liquidity index less than 0.25, and a plasticity index more than 17, indicating the clay layer is impermeable and of great importance to inhibiting the development of WCFZ. The research can provide significant theoretical insights for effectively preventing water hazards on mine roofs.

Deformation Rules for Surrounding Rock in Directional Weakening of End Roofs of Thin Bedrocks and Ultrathick Seams

With the deployment of China’s energy strategy in the western regions, complex geological mining conditions such as thin bedrock and ultrathick seams in western China have caused a series of problems such as serious deformation of the surrounding rock at the ends of the working face and the increase in the lead abutment pressure of the roadways; the research on end roof deformation in the resource exploitation in western China has become one of the great demands of the industry. Based on the failure characteristics of rock mass, relying on the actual mining geological conditions of a coal mine in Inner Mongolia, the failure characteristics of the overlying rock strata under the influence of mining were simulated and analyzed using similar material simulation experiment, which intuitively reproduced the failure and deformation processes of the immediate roof, main roof, and key strata and revealed the mechanical mechanism of the directional weakening of the end roof. It is of great significance for the stability control of the surrounding rock at the end of the fully mechanized caving face in the thin bedrocks and ultrathick seams, reducing the abutment pressure of gate roadway and controlling the spontaneous combustion of residual coal in the goaf.

Study on the Influence of Bedrock Thickness on Deformation and Failure of Overlying Soil Layer in Thin Bedrock Coal Seam Mining

Study on the Influence of Bedrock Thickness on Deformation and Failure of Overlying Soil Layer in Thin Bedrock Coal Seam Mining

Research on Overburden Movement Characteristics of Large Mining Height Working Face in Shallow Buried Thin Bedrock

The overburden movement of the large mining height working face of shallow buried thin bedrock (SBTB) is a complex engineering problem with “time-space-intension”, which is of great significance to realize efficient and safe mining in the northern Shaanxi mining area. Based on the research object of No. 22201 working face in Zhangjiamao Coal Mine, the roof structure characteristics of large mining height working face in SBTB are researched by field drilling measurement, laboratory test, physical and numerical simulation. The results show that: (1) Based on the measured data of the drillholes, it is concluded that under the mining conditions of SBTB with large mining height, the roof movement is ahead of the weighting of the working face, and the working resistance has a significant time effect. The advanced movement distance is about 20 m, which can be used as an early warning index of the weighting. The lag movement distance in the roof with horizon of 30 m is two periodic weighting intervals, which are about 26 m. (2) The first weighting interval of the working face is 32 m. The roof first break has obvious step sinking phenomenon, and the measured surface appears at a position 45 m away from the transport slot. It is statistically concluded that the periodic weighting interval is 9.5~16.5 m, the average weighting interval is 13 m, which is equivalent to the periodic dynamic crack spacing of the surface. (3) The results of field measurement and physical simulation show that the breaking angle of the roof of the No. 22201 large mining height is about 66°, and the periodic stepping distance of the T-junction suspension area is 6~8m. Along the strike of the working face, the roof breaking is mainly arc arched. The research results ensure the safe and green mining of shallow coal seam.

Study on deformation and failure characteristics of overlying strata with thick loose layers and thin bedrock based on Hoek-Brown criterion

Study on deformation and failure characteristics of overlying strata with thick loose layers and thin bedrock based on Hoek-Brown criterion

Study on Overlying Strata Motion Rule of Shortwall Mining Face of Shallow Seam with Simulation Experiment

Taking Shendong mining area as a research object, the roof strata moving law in the shallow buried deep thin bedrock was studied with 3D simulation experiment. The results showed that the surface displacement and roof pressure decrease little, and the roof does not appear the phenomenon of full thickness cutting. As the advancing distance of the working face increases, the displacement of the surface and roof increases. The old roof breaking is not easy to form a hinge structure, the roof when the pressure of mine pressure appearance is very intense.

Analysis of Surrounding Rock Vibration and the Influence of Soft Rock Mechanical Parameters During the Tunnel Blasting with Thin Bedrock Roof

The blasting construction of Shuidun Tunnel, China (a thin bedrock roof tunnel) is the engineering background in this paper. The numerical simulation method is adopted and combined with the verification of field measurement. The longitudinal, circumferential and radial distribution of the surrounding rock near the tunnel face is analyzed. The effects of various physical mechanical parameters of overlying soft rock on surrounding rock vibration are also discussed. The results may provide reference for construction safety control of similar projects. The main conclusions are as follows: The maximum value of the significant coefficient of cavity effect appears in the range of 2–4 m behind the tunnel face. The data like vault settlement within this range should be monitored emphatically. In the circumferential direction, the closer to the tunnel wall, the more significant the circumferential characteristic of the vibration velocity vector of surrounding rock is, and the maximum vibration velocity appears at the contact point between the rock layer interface and the tunnel contour wall. In the radial direction, the peak vibration velocity of surrounding rock decreases rapidly in the range of 1 m, while the attenuation tends to be gradual in the range of 2–4 m. The peak vibration velocity of surrounding rock near the tunnel face is positively correlated with the bulk density and poisson’s ratio of overlying soft stratum, and negatively correlated with its elastic modulus and internal friction angle of overlying soft stratum.

Feasibility of Modifying Coal Pillars to Prevent Sand Flow Under a Thick Loose Layer of Sediment and Thin Bedrock

This paper focuses on the first work face under a thick loose layer of sediment and thin bedrock in the No. 8 Mining District of the Baodian Coal Mine in the Yanzhou mining area of China. Due to mining, the hydrogeological conditions in the aquifers at the bottom of the thick, loose sediment layers have been greatly changed in the shallow areas in this area. The main problem is a low-pressure, water-rich aquifer. However, this problem could be remedied by modifying the coal pillars to prevent sand flow instead of water seepage. To do so, measurements of the first work face were obtained from drilling data, and the stress distribution and plastic zone were determined by examining the mining of the upper part of the no. 3 coal seam using FLAC3D numerical analysis software. The simulated results were also compared with the empirically calculated results. Comprehensive analysis indicates that modifying the coal pillars from preventing water seepage or inrush to preventing sand flow appears feasible, and provides an important reference for increasing mining areas and recovery of coal resources.

Study on the failure mechanism of clay layer overlying thin bedrock in coal seam mining

The water-blocking property of clay at the bottom of the Cenozoic overburden is an important factor for mining safety and protecting underground latent water resources in thin bedrock coal seam mining. The failure mechanism of such a clay is studied based on the actual engineering background of the SanYuan Coal Mine. The failure of the overlying clay layer is due to the reduction in the supporting space of the clay layer with the progression of coal mining, and the overlying clay layer will subside due to the self-weight load. Therefore, the vertical stress, horizontal stress and shear stress of the soil change during subsidence, and the change in these stresses determine whether the clay layer fails. Then, the failure criterion of soil expressed by the vertical stress, horizontal stress and shear stress is derived based on Mohr–Coulomb shear failure theory. Next, the function relating the stress and subsidence magnitudes are established by fit analysis. Finally, a failure criterion expressed in terms of soil subsidence amount is presented. Based on the failure criterion expressed by soil subsidence amount, a method for soil failure discriminant is proposed, in which only the subsidence amount is necessary to judge the soil failure state. This method is applied to the engineering of the SanYuan Coal Mine to get the failure subsidence amount curve of clay layer I. The results show that the failure subsidence amount curve plots below the subsidence amount curves of clay layer I at any advancing distance; that is, the clay layer did not fail. An investigation of the source and amount of water inflow to the 3301 working face also verifies that the clay layer does not fail, and the failure discriminant method used to judge the failure state of the soil is appropriate and convenient for use in engineering applications.

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

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

Evolution Law of Overburden Longitudinal Connected Fissures in a Shallowly Buried Coal Face with Thin Bedrock

Longitudinal connected fissures in a shallowly buried coal face with thin bedrock are the main factor causing sand‐burst accidents, water‐burst accidents, and abnormal increases in water inflow. To understand the evolution of longitudinal connected fissures and propose method for controlling such fissures, 3‐1 Coal of the Jinjie Coal Mine in Shendong Coal Group, China, was used as a case study. Physical simulation, numerical simulation, field measurements, and other measures were carried out to analyze thoroughly the opening and closure of fissures. At the same time, the stage characteristics of fissures evolution process are also obtained. The results indicate that when periodic weighting occurs, a longitudinal connected fissure starts to open. As the coal face moves on, the fissure expands gradually with the dynamic changes in horizontal force and rock dislocation; when the expansion reaches its limit, the key rock falls and the longitudinal connected fissure rapidly closes. With the advent of the next periodic weighting, the longitudinal connected fissure compacts further and the next longitudinal connected fissure appears. The formation of longitudinal connected fissures is described with reference to three factors: the advancement speed of the coal face, the holding power of the hydraulic support, and the filling degree of the goaf area, all of which can be easily controlled by engineering means. This study provides a basis for control of sand‐burst accidents, water‐burst accidents, and abnormal increase in water inflow in a shallow coal face with thin bedrock.