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Roof caving when the working face advances to 58 cm (a), 99 cm (b), 128 cm (c), 139 cm (d), 155 cm (e), 221 cm (f), and 246 cm (g), respectively.
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Retaining a waterproof coal pillar is the most effective water conservation method for coal seam mining close to a reservoir, and determining a reasonable width for the waterproof coal pillar has been a common problem among mining scholars for a considerably long time. In case of mining a 4 ⁻² coal seam close to the Changjiagou Reservoir in the Zha...
Citations
... To prevent water inrush disasters, it is common practice in mine development to leave a certain width of coal pillars between adjacent working faces, as shown in Fig. 2, to block the accumulation of water in the gob Chen et al. 2022a;Galav et al. 2022;He and Huang 2022;Fang et al. 2023). The stability of these waterproof coal pillars has long been one of the critical research topics in coal mine development (Wang 2006;Hu et al. 2019;Li et al. 2021;He et al. 2023). ...
The failure of waterproof coal pillars under the coupled effects of mining, excavation and water seepage is a significant factor contributing to sudden water inflow accidents in underground roadways. Investigating the instability characteristics and optimal width of waterproof coal pillars holds vital significance for water control and resource protection in mines. This study focus on the rational width of waterproof coal pillar at Dongzhuang Coal Mine in Shanxi Province. Using FLAC3D, a fluid–structure interaction numerical model of waterproof coal pillar was established, revealing the coupling characteristics of stress fields, plastic zones, and seepage zones within coal pillars under the influence of mining, excavation and water infiltration weakening. Furthermore, the stability characteristics of waterproof coal pillars with different widths were compared. The results are as follows: (1) Under the combined action of overlying strata pressure and water pressure from the gob, the coal mass on the water-inflow side of coal pillar is the first to fail. Additionally, with the infiltration of water, the elastic modulus, cohesion, and friction angle of the coal mass in the seepage zone decrease. (2) The lifecycle of waterproof coal pillar can be divided into three stages: working face mining, water infiltration from the gob, and roadway excavation. Based on this, the connectivity between plastic zones and seepage zones serves as the critical condition for the stability of waterproof coal pillar was proposed. (3) When the width of waterproof coal pillar is 3 m and 5 m, plastic zones become connected, forming a water-conducting channel. When the width of waterproof coal pillar is 7 m, 9 m, and 11 m, seepage zones and plastic zones are not connected, and the coal pillar exhibits load-bearing and water-barrier properties.
... Finally, they determine the width and optimize the size of the coal pillar. Hu et al. [13] summarized that retaining a water-resisting coal pillar is the most effective water-saving method for coal seam mining, and proposed the conditions for determining the critical width of the water-resisting coal pillar. Ma et al. [14] proposed that when mining roadway excavation encountered fault fracture zone, permeability and porosity increased with time, and the granular structure of broken surrounding rock was easy to form water channel, which may cause water gusher hazard. ...
... Dong et al. [18] established the mechanical model of the main roof rock beam, deduced the limit span and limit deflection of the rock beam fracture, and verified the reliability of the thickness of the water-resisting coal pillar. Through theoretical analysis and simulation analysis, Hu et al. [13] provided a theoretical basis for the rationality of the width of the water-resisting coal pillar. It was reported that the maximum width of the water-resisting coal pillar is the critical width, Water 2023, 15, 1002 3 of 22 and the coal pillar was divided as mine pressure affected zone, effective waterproof zone, and water level affected zone with respect of water resistance. ...
Groundwater inrush hazard has always been a great threat to the construction of vertical shafts in coal mines. Generally, the failure of the water-resisting coal pillar under coupled stress-seepage conditions around the vertical shaft is the main reason for the generation of the water inrush channel. In order to understand the mechanical behaviors of the water-resisting coal pillar, the strength of typical coal affected by the size and water content was investigated, and the stress sensitivity of permeability was investigated by a stress-seepage coupling test. The stress field and deformation of the water-resisting coal pillar were investigated by numerical simulation, the stability of the water-resisting coal pillars with different widths was evaluated, and the reasonable width of the coal pillars under coupled stress-seepage condition was determined. Results show that the water content and coal pillar width have a great influence on the mechanical characteristics of coal samples. Under the conditions of lower water content and larger coal sample width, the coal sample presents higher strength, smaller axial deformation, smaller permeability and porosity, and weak sensitivity to stress. The simulation results show that the boundary of the main roadway at the end of the coal pillar is dominated by tensile stress, and fractures can significantly contribute to the destruction of coal pillars. With the increase in the width of the water-resisting coal pillar, the internal damage variable, maximum tensile stress, porosity, and average water flow velocity of the coal pillar decrease, which reduces the risk of water inrush and improves the safety of the water-resisting coal pillar. An evaluation model of the reasonable width of the water-resisting coal pillar under the stress-seepage coupling was proposed, and the model was verified by the shear slip law and experimental results. This study provides theoretical and experimental guidance for the risk management of groundwater inrush disaster during the construction of vertical shafts in coal mines.
... The increase in the mining of coal resources brings rising tensions to the Chinese resource situation. The roadway protection technology regarding small coal pillars in gobside entry driving has been gradually adopted by more and more mines, with the width of coal pillars reduced from the original 30~40 m to 5~8 m, which not only greatly improves the coal recovery rate, but also prevents the stress outburst caused by the traditional wide coal pillars [1][2][3][4]. However, various types of problems caused by small coal pillar mines under the condition of a hard roof are becoming more and more serious due to the large internal bearing deformation of small coal pillars, the run-through of the plastic zone, and the poor isolation effect caused by the long hanging hard roof, leading to secondary disasters, such as harmful gas leakage in the goaf [5][6][7][8]. ...
As a widely-used method of digging roadways in China, gob-side entry driving features specific advantages, such as a high recovery rate and good isolation effects. However, under the condition of hard overburden, the excessive bearing pressure of small coal pillars will easily cause serious internal damage in the coal and the run-through of the plastic zone, leading to harmful gas leakage in the goaf. Therefore, based on the engineering background of small coal pillars in the 18506 working face of Xiqu Coal Mine, this paper comprehensively adopts theoretical analysis, numerical simulation, industrial tests, and other methods, analyzes the evolution mechanism of isolated bearing and plastic fracture areas of small coal pillar under hard overburden, studies the influence law of hard overburden cutting parameters on the isolation and stability of small coal pillars, and puts forward the technology of actively cutting the top to weaken the stress concentration of coal pillars under hard overburden. With the reasonable cutting parameters determined, the controllable mechanism of hydraulic fracturing cutting under hard overburden further revealed, and the hydraulic fracturing cutting technology with “controllable cutting orientation of hydraulic fracturing with local stress field intervention” formed as the basic core, the stress situation on the roof is improved, realizing the stability control of the coal pillars for the roadway protection, and avoiding gas leakage and other disasters caused by small coal pillar destruction.
... For example, water may weaken the strength of coal pillars and promote roadway deformation (Poulsen et al. 2014). However, water may also exert positive effects in some engineering applications, such as water injection into coal seams or the roofs for pressure relief, rock burst prevention, working face dust reduction, etc. (Liu et al. 2017;Shi et al. 2019;Hu et al. 2019). ...
The mechanical properties and degradation caused by damage within coal pillar dams determine the safety and stability of underground water reservoirs. This study investigated the influence of moisture content on the mechanical properties and failure behaviors of coal samples by conducting a series of non-destructive water immersion, uniaxial compression, acoustic emission (AE), and computed tomography tests, with the objective of providing a reference for the design and maintenance of coal pillar dams in underground water reservoirs and other engineering problems (such as roadway support) in coal mines. The coal samples had low saturated water content because of poor porosity and few clay minerals (illite and kaolinite). The uniaxial compressive strength, elastic modulus, and strain softening modulus of the coal samples decreased almost linearly with water content. Four types of post-peak variation forms of the coal samples were summarized and quantitatively studied using the post-peak gradient. The crack propagation process of coal samples with different water contents was further divided according to the AE count and cumulative AE count. Finally, a new method was proposed to calculate the inclination angle of the failure surface of the coal samples fitted using the AE location. Low water content generally resulted in larger inclination angles (37°, 39°), and high water content generally resulted in smaller inclination angles (26°, 33°); these results were consistent with the 3D reconstructions of cracks in the coal samples. Overall, this study supports the use of the inclination angle of fitted failure surfaces to approximately determine the water content and damage of coal samples.
... The methods dealing with mine construction can be modified to limit this potential danger, such as dewatering, advanced drilling support, rock grouting, and the waterproof coal pillar setting 19,20 . In the above-mentioned measures, the reasonable waterproof coal pillar is considered an economical and efficient mean to prevent water inrush 21,22 . Several formulas pertaining to the strength and width of the waterproof coal pillar are proposed by scholars 23-25 . ...
Numerous field examples of coal seam mining show that when coal seams under confined water are mined close to faults, water inrush effects on complex mining surfaces occur. Obeying similarity rules, physical similarity models consisting of sand, lime, and plaster were used to investigate the water conducting process, along with stress and displacement measured by a combination of mechanical senor, total station, and video camera-. Comparing the physical model tests with the calculation results of elastoplastic limit equilibrium theory, the rationality of the model has been verified. Besides, a safe width of the waterproof coal pillar has been obtained. It can be demonstrated from the model observations that the coal seam in front of the mining can be divided into three areas with different characteristics of stress and displacement, namely, which are the fault-affected area, the elastic area, and the plastic yield crack area. A closed-loop water inlet and outlet pipeline composed of a water control platform that can provide stable water pressure, and water bags pre-buried in the fault was used to simulate the water conduction in the fracture zone. Integrate the development law of stress, displacement, and water conduction coming from the upper and lower walls of the fault to further determine the reasonable width of the waterproof coal pillar.
... e pouring interval is 10 days. It should be shortened as far as possible in the actual construction [17,18]. Table 3. ...
The key problem of mass concrete temperature control is to effectively control the maximum temperature inside concrete, the temperature difference between inside and outside concrete, and the temperature difference between surface and environment. The size of the main tower cap of No. 3 Jinsha River Bridge is 37 m × 23.5 m × 5.5 m, and the cubic volume of concrete reaches 4782.3 m3, which is poured in two times. In order to ensure construction quality of mass concrete structure, prevent the large mass concrete temperature stress, through the numerical simulation of the temperature control and optimization scheme, by optimizing the mixture ratio design, reducing the temperature of concrete pouring into the mold, cooling water cycling, insulation keeping in good health and a series of measures to effectively achieve the control goal, and eliminating the temperature cracks. The measured data show that the maximum temperature inside concrete, the temperature difference between inside and outside, and the temperature difference between surface and environment are qualified, but the temperature difference control of cooling water inlet and outlet has hysteresis effect, and the temperature difference between inlet and outlet will be greater than 10°C, which should be noticed.
... e Yulin-Shenmu coal mine area is known for the occurrence JingLe group Hipparion red clay and Lishi loess that form a clay aquiclude owing to their high clay mineral contents. is causes the soil to crack to a lesser degree, which suppresses the lead water cracking [28][29][30][31][32] and resistance of the clay layer bridge. e waterproof effect after mining recovery remains poorly understood. ...
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.
... Advanced Segment of 3205 Tailgate. During the mining period of panel 3205, 6 groups of "cross-shaped" measuring stations [21] are set in 3205 tailgate. Among them, 3 groups of "cross-shaped" measuring stations are set outside the influence of gob of upper protective layer and can be sequentially numbered group #1, group #2, and group #3, and the interval between two adjacent measuring stations is about 50 m; the other 3 groups of "cross-shaped" measuring stations are set within the influence of gob of upper protective layer and can be sequentially numbered group #4, group #5, and group #6, and the interval between two adjacent measuring stations is also about 50 m. ...
To study the reasonable layout of upper protective layer for the prevention rock burst under coal seam group with close quarters conditions, a panel 3203 that belong to Zhongxing Colliery is taken as a typical engineering background. By means of on-site survey, theoretical analysis, numerical simulation, and on-site industrial applications, the reasonable layout of upper protective layer for prevention rock burst is studied. The results show that the overall stress environment of the floor under gob of upper protective layer is good, and the overall stress environment of the floor under the upper side of gob is also good, but the overall stress environment of the floor under the lower side of gob is bad. According to numerical simulation results, an L-shaped stress superposition area is formed in the lower end of panel 3203 under the original layout scheme conditions, and the maximum stress concentration coefficient is about 2.8 in stage I and 4.0 in stage II. A new stress superposition area is formed at the middle to lower end part of advance mining face of panel 3203 for the stage II under the optimal layout scheme conditions, and the maximum stress concentration coefficient is about 2.4; the original L-shaped stress superposition area is gone due the transfer and release of stress, and the optimal layout scheme has a very significant effect on the prevention and control of the subsequent rock burst accidents; the monitoring results of working resistance of hydraulic supports and surrounding rock deformation indicate that the overall pressure relief of the surrounding rock in advanced segment of 3205 tailgate can be effectively realized. The study conclusions provide theoretical foundation and a new guidance for preventing rock burst with similar engineering geological conditions.
... With the continuous improvement of coal mining methods and the enhancement of the mechanical equipment level, fully mechanised caving has become an important mining method for thick coal seam [1][2][3]. It can achieve high-yield and high-efficiency mining of thick coal seams, but roadways in mining area face challenges of surrounding rock control, for example, the large cross section, strong mining influence, and soft and thick coal roof and floor [4][5][6][7]. ...
Reasonable width of gob-side coal pillar can reduce the waste of coal resources and is conducive to roadway stability. According to the distribution of internal and external stress fields at the working face, a method for determining the width of gob-side coal pillar was proposed. The coal pillar and roadway should be set within the internal stress field, and support is provided through the anchored part and the intact part of the coal pillar. The method was used in the design of the coal pillar at No. 130205 working face of Zaoquan Coal Mine. The calculation results indicated that the width of a coal pillar suitable for gob-side entry is 6.0 m. It is reasonable to arrange the roadway and coal pillar in the low-stress zone with a width of 11 m. During tunnelling of roadway and stoping of the working face, the deformation of the roadway increased with a reduction in the distance from the working face. Even during stoping of the working face, there was an approximately 1.5 m intact zone in the coal pillar. This indicates that the proposed method of designing small coal pillar of gob-side entry driving is reliable.
... Coal resources threatened by water account for over 27% of the total proven coal reserves. Waterproof coal pillars of large scale need to be left to mine these coal resources using the conventional methods [4,5]. This implies considerable losses of the coal resources and, more importantly, the hidden danger of water burst of coal and rock masses and damage to the aquifer [6][7][8]. ...
Sand-based cemented backfill (SBCB) mining technology is instrumental in utilizing coal resources buried under the water bodies. SBCB is exposed to the long-term action of mining-induced stresses in the goaf and groundwater permeating via microcracks along the rock strata. Studying the permeability evolution of SBCB under varying stress states is crucial for protecting coal and water resources below the aquifer. This study is focused on the influence law of different stress states on the SBCB permeability exposed to groundwater, which was tested under different axial and confining pressures using a laboratory seepage meter, particle size analyzer, scanning electron microscope (SEM), and X-ray diffractometer (XRD). Best-fitting quadratic polynomials linking the SBCB permeability with confining and axial pressures, respectively, were obtained via statistical processing of test results. The permeability gradually dropped within the elastic range as the confining and axial pressures increased. Moreover, an increase in the confining pressure caused a more dramatic reduction in the SBCB permeability than the axial pressure. Finally, the SBCB seepage mechanism under different stress states was revealed based on the particle size analysis, XRD patterns, and SEM microstructure. These findings are considered instrumental in substantiating safe mining of coal resources below the water bodies and above the confined groundwater.
