The destruction of the water-resistant key strata is the root cause of water resource destruction in ecologically fragile areas and mining water hazards. Accurate and effective scientific monitoring ...of the stability of water-resistant key strata is an important basis for water-preserved coal mining and rock formation control. At present, drilling detection, theoretical analysis and numerical simulation are often used to calculate the height of water-conduction fractured zone to indirectly determine the aquifer permeability. With the development of optical fiber sensing technology, fiber Bragg grating (FBG) and distributed optical fiber (BOTDA) methods are used to monitor the stability of mining overburden, which provides a new research idea for water-preserved coal mining. The research show: In shallow seam, when the thickness of bedrock is only 60–67 m, the water-conduction fractured zone will develop directly to the ground surface, which will inevitably lead to the destruction of bedrock aquifer, the fracture line and its longitudinal cracks nearby prove to be the main seepage channel; the strain detected by optic fiber presents double peak characteristic due to the stress concentration of strata in fracture position, and the peak position corresponds to the fracture line. It's possible to prediction the fracture line position by optic fiber sensor; combining with the limit equilibrium theory calculation, the breaking limit strain threshold value of the water-resistant key strata based on optical fiber sensing detection is given as 2000 με. The FBG sensor can accurately monitor the deformation of rock mass at a specific location. The detection of instability position of the water-resistant key strata is basically consistent with the actual observation and distributed fiber optic detection results.
To overcome the limitation of the virtual extensometer method in measuring the crack opening displacement (COD) in the process of complex multi-crack propagation of rock, the measurement error of ...Digital Image Correlation (DIC) local deformation is theoretically analyzed. An improved virtual extensometer method for measuring the COD is proposed, which considers the temporal and spatial characteristics of crack development in the process of complex crack propagation. The accuracy of the proposed method is verified by the strain localization band numerical simulation test and indoor single crack simulation test. Furthermore, the method is applied to the two-dimensional similarity simulation test of simulating complex multi-fractures in rock stratum. The COD obtained by the traditional and improved methods is compared with the measured COD. The results show that in the case of multiple complex cracks, to obtain the COD accurately, the relative distance between the virtual extensometer measuring point and the crack should be greater than half of the sum of the width of the crack strain localization zone and the subset size. With the development of the crack, the relative distance between the virtual extensometer measuring point and the crack should increase with the increase of the width of the crack strain localization zone. The error of the COD measured by the traditional method increases with fracture development, and the maximum is 21.20%. The maximum relative error between the COD measured by the improved method and the measured crack opening is 3.61%. The research results improve the accuracy of the virtual extensometer in measuring the COD under complex multi-crack conditions.
•Strain coefficient of ordinary single-mode optic fiber polyurethane packaged with 2 mm diameter is 0.0497 MHz/με.•The law of the deformation of overlying strata caused by coal mining and the strain ...of fiber monitoring is obtained.•The main technical problems of distributed optical fiber in the test process are analyzed in detail.
The common utilization research methods in mining engineering include theoretical analysis, mechanical modeling, similarity model test, numerical simulation, and field test. And physical similarity model test is one of the main research methods for mining engineering problems. Underground engineering is often in the complex three-dimensional stress state. Compared with plane model, three-dimensional model can actually reflect the stress state of surrounding rock. Nevertheless, traditional measurement methods can’t achieve internal deformation of the model with multi-scale distributed monitoring. Optical fiber sensing technology provides a solution to these problems. The sensing fibers are arranged in the plane model with the size of 3000 × 200 × 1280 mm and the three-dimensional model with the size of 3600 × 2000 × 2000 mm, and the strain distribution of the model has been analyzed with the consideration of different positions relationship between the working face and the optic fiber. The results show: the strain coefficient of the test optic fiber with 2 mm diameter calibrated by the uniform strength beam experiment is 0.0497 MHz/με. The frequency shift of the optic fiber is positive when the fiber is under tension state, and the result is opposite when under compression; In physical model tests, when the working face is close to the sensing fiber, the strain curve shows a negative step change caused by the abutment pressure. When the working face cross the fiber, the strain curve shows a positive step change due to the downward movement of broken rock layer; after the working face away from the fiber, the re-compaction of the broken rock block induced by gravity load causes the optical fiber to be under pressure, and the sensing fiber generates compressive strain. The three-segment distribution area of the strain curve corresponds to the “three-zone” height range of the overburden caused by mining, respectively. The test results could provide theoretical guidance for the application of distributed optical fiber in the determination of the caving zone and fractured zone range of overburden induced by coal mining.
The monitoring of mine pressure, division of vertical zoning of the overburden, discrimination of key stratum structure of the overburden and monitoring of advanced abutment pressure are still the ...main research problems in the field of coal mining. Therefore, the promotion of development of a monitoring technology of mining-induced rock mass deformation has important research value in the mining field. There are many problems to be solved in the application of optical fiber sensing (OFS) to deformation monitoring, such as the corresponding relationship between actual deformation and optical parameters, the coupling relationship between the optical fiber and rock mass and the reasonable division of vertical zoning of the overburden. In this study, a quantitative index of coupling action between the mining rock mass and optical fiber is put forward, and the coupling coefficient of different vertical zonings is quantitatively analyzed and discussed. Based on this, five different media in contact with optical fiber are proposed. The relationship between the strain curve form, the development height of the fracture zone and the activity of key stratum is established. It is of great academic value and research significance to establish a characterization system of displacement, deformation and structural evolution of overlying strata based on optical fiber sensing technology.
The impact ground pressure in coal mining is closely related to the fault structure, and the fault activation pattern is different when the working face advances along the upper and lower plates of ...the fault, respectively. In this paper, the F16 positive fault in the southern part of Yima coalfield is used as a prototype to carry out the physical similar model test simulating the process of the working face advancing from the upper and lower plates of the fault, and PPP-BOTDA optical fiber sensing technique is used to study the overburden deformation law and fault activation law when the working face is located in the upper and lower plates of the fault, respectively. The study shows that the key stratum breakage is closely related to the fault movement, and the shear stress concentration range occurs within the key stratum. The additional shear stress concentration at the fault surface caused by the working face advancing in the lower plate is much larger than that at the upper plate, which is the reason for the serious fault destabilization phenomenon at the lower plate. The upper rock layer on the fault face is affected by the mining action of the working face before the lower one, and the working face is affected by the fault in a larger range when advances in the lower plate than that in the upper plate, and the risk of fault activation instability occurs earlier when the working face advances in the lower plate than that in the upper plate. The distributed optical fiber sensing technology is used to verify the basic conclusions that the impact of the working face advancing from the lower plate is much greater than that from the upper plate, which is more likely to cause fault activation. The preferential placement of the working face in the upper plate in the fault area will be beneficial to mine pressure control. The results of the study provide an experimental basis for the application of distributed optical fiber sensing technology to the study of fault activation law.
•This paper explores the construction technology, monitoring effect and applicability of distributed optical fiber sensing technology in the ground surface monitoring of model test.•The comparative ...analysis of the model test results and the numerical simulation results reveals some basic laws of the relationship between the ground surface and the ultra-thick conglomerate movement and deformation.•The main technical problems of distributed optical fiber in the test process are analyzed in detail, such as space location, temperature compensation, layout mode, etc.
The purpose of this paper is to study the law of ground surface movement and deformation in deep and extra-thick coal seam mining under ultra-thick conglomerate. With the engineering geological conditions of Qianqiu Coal Mine as the background, the three-dimensional model with the size of 3600 mm × 2000 mm × 2000 mm (length × width × height) was constructed. The distributed optical fiber sensing monitoring network is set up in the model. The optical fiber sensing signal is collected in real time by NBX-6055 demodulator to characterize the surface movement and deformation. Then based on the Mohr-Coulomb criterion, the numerical calculation model is established by using UDEC 6.0 software to explore the internal relationship between overburden and surface movement and deformation in the process of coal mining. The test results show that the distributed optical fiber monitoring technology can effectively capture the movement and deformation of ground surface. The optical fiber strain distribution curve can accurately locate the formation area and stress state of surface subsidence basin. The surface is controlled by the key strata of ultra-thick conglomerate, and in the whole process of mining, the surface movement and deformation can be roughly divided into three stages: deformation initiation, slow deformation and sudden deformation. The conclusions indicate that the distributed optical fiber sensing monitoring technology has good applicability in the surface monitoring of model test. It provides a method for the monitoring information research of mining surface movement and deformation.
Abstract
The reserved thickness of top coal has an important influence on the stability of a large section open-off cut under gob in the thick seams slicing mining. The destabilization extremum ...conditions of the open-off cut top coal were derived from by elastic–plastic theory, and the optical fibre sensing technology was utilized to monitor the top coal deformation law with different thicknesses (3, 3.5, and 4 m) in the physical similar simulation experiment in the paper. The results show that the top coal thickness is greater than 3.4 m without tension cracks. In the vertical direction, the top coal of the large open-off cut is divided into mining and excavation disturbance zones under the influence of the upper slice coal mining and the excavation disturbance. In the direction of the span of the top coal can be divided into the roof fall risk zone and the warning zone. The deformation changes from exponential to linear to logarithmic in the roof fall risk zone, and it changes from linear to logarithmic in the roof fall warning zone as the number of excavations increases. The sinking amount in the two zones is smaller as the thickness of the top coal becomes larger. It is comprehensively determined that the thickness of the top coal of open-off cut is set as 3.5 m, the stability is moderate, and the field application shows that the integrity of the top coal is good after support, and the maximum off-layer value is 6 mm, which can satisfy the production requirements.
Horizontal wells within the roof are an effective method to develop gas in broken and soft coal seams, and layer-penetrating fracturing is a key engineering method for the stimulating of horizontal ...wells within the roof of a coal seam. To understand the propagation law of fracture in the composite roof of coal seams, this study conducted research using numerical simulation and physical similarity simulation methods. Furthermore, engineering experiments were carried out at the Panxie coal mine in the Huainan Mining Area and the Luling coal mine in Huaibei Mining Area, to further validate this technology. The numerical simulation results indicated that fracture within the coal seam roof can propagate from the roof to the target coal seam, effectively fracturing the coal seam. Due to the coal seam’s plasticity being greater than that of the roof mudstone, the coal seam forms a broader fracture than the roof. With the increase in pseudo roof mudstone thickness and being under constant fracturing displacement, the energy consumed by the pseudo roof mudstone during fracturing causes a decrease in pore pressure when fracture extends to the coal seam, resulting in a reduction in fracture width. Therefore, the pseudo roof mudstone is an adverse factor for the expansion of hydraulic fracturing. Physical similarity simulation results demonstrated that when horizontal boreholes were arranged within the siltstone of the coal seam roof, were under reasonable vertical distance and high flow rate fracturing via fluid injection conditions, and if the coal seam had a thin pseudo roof mudstone, the fracture could propagate through the direct roof-pseudo roof interface and the pseudo roof-coal seam interface, extending to the lower coal seam. The fracture form was curved and had irregular vertical fractures, indicating that hydraulic fracturing can achieve production enhancement and the transformation of soft and hard coal seams. However, when the coal seam had a thick pseudo roof mudstone, the mudstone posed strong resistance to hydraulic fracturing, making it difficult for the fracture to propagate to the lower coal seam. Therefore, the pseudo roof mudstone plays a detrimental role in hydraulic fracturing and the production enhancement of coal seams. The engineering verification conducted at Panxie coal mine and Luling coal mine showed that by utilizing a construction drainage rate of 7.5 cubic meters per minute at Panxie coal mine, the maximum fracture length reached 218.3 m, with a maximum fracture height of 36.8 m. The maximum daily gas production of a single well reached 1450 cubic meters per day, with a total gas extraction volume of 43.62 × 104 cubic meters across 671 days. At Luling coal mine, utilizing a construction drainage rate of 10 cubic meters per minute, the maximum fracture length reached 169.1 m, with a maximum fracture height of 20.5 m. The maximum daily gas production of a single well reached 10,775 cubic meters per day, with a total gas extraction volume of 590 × 104 cubic meters for 1090 days. This indicated that the fracture within the roof of coal seams can penetrate the composite roof of coal seams and extend to the interior of the coal seams, achieving the purpose of transforming fractured and low-permeability coal seams and providing an effective mode of gas extraction.
The damage and degradation are the main influence factors of the instability of rock mass engineering. In this paper, the damage and deformation characteristics of the rock-like samples are ...investigated under the uniaxial compression test, and the advanced digital image correlation (DIC) device is devoted to full-field deformation data acquisition on the sample surface. Based on the full-field deformation data, a new damage variable is proposed by the principal strain standard deviation to characterize the uniaxial compression damage process of the rock-like samples. The results show that the newly presented damage variable can be utilized for the quantitative characterization of the sample damage. According to the characteristics of the damage variable, the damage evolution process of the rock-like specimens under uniaxial compression can be divided into four stages: initial damage closure stage, linear elastic damage stage, elastic-plastic damage stage, and plastic damage stage. From the stress-strain curve, the cut-off point from elastic to plastic deformation of the rock-like specimen is also the turning point from micro to macro damage; after the point, the apparent initial damage starts to occur on the sample surface; furthermore, the damage of the specimen is accelerated in the plastic damage stage. When the overall damage variable reaches 0.5 or the damage variable of strain localization zone reaches 0.8, the macro crack forms, and the bearing capacity of the rock-like specimen decreases rapidly. The findings are of great significance to the prediction of the damage process of rock mass engineering by digital image correlation.
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