Search Results (16)

In order to examine the fracture development law of overlying strata in goafs and to reasonably lay out a high gas-drainage roadway under gob-side entry retaining with roadside filling, the 91–105 working face of the Wangzhuang Coal Mine was selected as the engineering case study. The failure laws and fracture development characteristics of the overlying strata in both the strike and dip directions using gob-side entry retaining and roadside filling were studied through rock mechanic tests and PFC numerical simulations. The optimal layout of the high gas-drainage roadway was determined through theoretical analysis and coupled Fluent–PFC numerical simulations, and on-site monitoring was conducted to evaluate the extraction effects. The results indicate that the first weighting interval of the 91–105 working face was 40 m, while the periodic weighting interval was approximately 14 m. The height of the falling zone was 14.4 m, and the height of the gas-conducting fracture zone was 40.7 m. In the dip direction, compared with coal pillar retaining, gob-side entry retaining with roadside filling formed an inverted trapezoid secondary breaking zone above the retaining roadway. Using this method, the span of the separation zone increased to 30 m, and the collapse angle decreased to 52°, resulting in a shift in the separation zone—the primary space for gas migration—toward the goaf. It was determined that the optimal location of the high gas-drainage roadway was 28 m above the coal roof and 30 m horizontally from the return air roadway. Compared with the 8105 working face, this position was 10 m closer toward the goaf. On-site gas extraction monitoring data indicate that, at this optimized position, the gas concentration in the high gas-drainage roadway increased by 22%, and the net gas flow increased by 18%. Full article

The non-uniform deformation and failure phenomena encountered in steeply inclined coal seams during roof-cutting and gob-side entry retaining operations demand urgent resolution. Taking the haulage roadway of the 3131 working face in Longmenxia South Coal Mine as the research background, the theoretical analysis method is adopted to explore the mechanical state of the main roof in inclined coal seams and the design of roadside support resistance. According to the structural evolution characteristics of the main roof, it is divided into four periods. Based on the elastic theory, corresponding mechanical models are established, and the mechanical expressions of the main roof stress and deflection are derived. The distribution characteristics of the main roof’s mechanical state in each zone and the influence law of the coal seam dip angle on the main roof’s mechanical state are studied. This study reveals a critical transition from symmetric to asymmetric mechanical behavior in the main roof structure due to the coal seam dip angle and roof structure evolution. The results show that, in the absence of roadside support, during the roadway retaining period, the upper surface of the main roof is in tension, and the lower surface is under compression. The stress value increases slowly from the high-sidewall side to the middle, while it increases sharply from the middle to the short-sidewall side. Under the inclined coal seam, as the dip angle of the coal and rock strata increases, the component load perpendicular to the roof direction decreases, and the roof deflection also decreases accordingly. On this basis, the design formula for the roadside support resistance of gob-side entry retaining with roof cutting in inclined coal seams is presented, and the roadside support resistance of the No. 3131 haulage roadway is designed. Building upon this foundation, a design formula for roadside support resistance in steeply inclined coal seams with roof-cutting and gob-side entry retaining has been developed. This formula was applied to the No. 3131 haulage roadway support design. Field engineering tests demonstrated that the maximum roof-to-floor deformation at the high sidewall decreased from 600 mm (unsupported condition) to 165 mm during the entry retaining period. During the advanced influence phase of secondary mining operations, the maximum deformation at the high sidewall was maintained at approximately 193 mm. Full article

Steep surrounding rock significantly challenges tunnel stability by affecting the stress distribution and deformation behavior. The angle of dip in surrounding rock greatly influences these factors, heightening the risk of instability along bedding planes, particularly under high ground stress conditions. This paper presents a comprehensive analysis of steep rock strata mechanical properties based on a railway tunnel in Yunnan Province, China. It incorporates long-term field monitoring and various laboratory tests, including point load, triaxial, and loose circle tests. Using experimental data, this study simulated the failure processes of steep surrounding rock and tunnel structures with a custom finite element method (FEM) integrated with the volume of fluid (VOF) approach. The analysis summarized the deformation patterns, investigated the causes of inverted arch deformation and failure, and proposed countermeasures. The findings reveal that increasing the rock dip angle results in greater deformation and accelerated failure rates, with the surrounding rock’s loose zone stabilizing at approximately 8 m once deformation stabilizes. At a surface deformation of 8 cm, the failure zone extends to 6 m; however, this extension occurs more rapidly with higher lateral pressure coefficients. Additionally, failure zones develop more quickly in thin, soft rock on steep slopes compared to uniform rock formations. The rise of the tunnel floor is attributed to the steeply inclined, thin surrounding rock. To enhance bottom structure stiffness, this study recommends incorporating an inverted arch structure and increasing both the number and strength of the anchor bolts. Full article

The construction of multiple tunnels across inland rivers has had a significant influence on the improvement of the transportation infrastructure. The technology for constructing tunnels is progressing towards the development of larger cross-sections, longer distances, and the ability to withstand high hydraulic pressure in complex hydrogeological conditions, including high-permeability strata. In order to ensure the face stability of shield tunnels under high hydraulic pressure that crosses a fault fracture zone, it is necessary to study the progressive failure mechanism of shield tunnel faces induced by high hydraulic pressure seepage. This paper employs finite element numerical simulation software to methodically examine the variation in the characteristics of the water seepage field, limiting support force, and face stability failure mode of shield tunnels passing through fault fracture zones with high hydraulic pressure under varying fault fracture width zones. The results show that the formation hydraulic gradient will progressively widen when the tunnel face is located within the undisturbed rock mass and is advanced towards the area of fault fracture. This will raise the likelihood of instability in the shield tunnel and progressively raise the limiting support force on the tunnel face. Moreover, as the tunnel face nears the region of fault fracture within the undisturbed rock mass, the damage range increases gradually. In addition, due to the increase in seepage force, the angle between the failure area and the horizontal plane becomes more and more gentle. On the contrary, as the tunnel’s face moves closer to the undisturbed rock mass from the region of the fault fracture, the damage range gradually decreases, and the dip angle between the damage area and the horizontal plane becomes steeper and steeper due to the decreasing seepage force in the process. The study findings presented in this work are highly significant, both theoretically and practically, for the design and management of safety. Full article

A fault disrupts the continuity of the rock strata in a mining area. To study the law governing the fracture of overlying strata in mining areas under discontinuous boundary conditions, the overlying strata were redefined and grouped based on the activity characteristics of each rock layer during the overall movement of the overlying strata. The activity patterns of different layers of the fault were obtained through the movement and failure forms of each group of rock layers. The relationship among the size of the coal pillar at the boundary of the fault, the dip angle of the fault, and the movement angle of the rock strata was considered. A model of the spatial relationship between the overlying rock movement zone of the quarry and the fault surface was established. The limit equilibrium equations of the key layer in the fault zone before breaking were established based on the tensile strength of the rock layer. In addition, the mechanical slip instability criterion and the deflection instability criterion of the discontinuous-boundary rock mass are given herein. Based on a field case, a double criterion was used to determine the initiating activated rock layers of the fault in the cases where the fault dip was smaller than the rock movement angle. Rock movement during excavation was simulated by similar simulation tests, and different levels of rock movement patterns in the boundary fault zone were focused on monitoring and analyzing. The stress and displacement changes in different rock layers in the fault zone were analyzed with numerical simulation results. The results show the following: if the dip angle of the fault is smaller than the movement angle of the rock layer, the delamination space of the fault surface is mainly distributed in the bending and sinking zone of the overlying rock; with an increase in the working-face advancement distance, the vertical pressure of the upper part of the fault gradually decreases, and the stress-concentration area in the middle and lower part of the fault gradually increases; the rock layer of the upper part of the fault, which is mainly composed of the key stratum, is the main area of activation of the fault. Full article

Through field observations, theoretical analysis, and a calibrated numerical model, a study of stress redistribution due to the extraction of longwall panels at depths ranging from 580 to 660 m with a 30° dip angle at Tangshan coal mine is presented in this paper. Conventional and new split-level longwall layouts are compared regarding their stress redistributions. The height of the caved zone is 21.7 m; angles of break of 55.6° on the left and 54.2° on the right side of the gob are observed using cross-measure boreholes. Structural models as well as numerical models are constructed based on the above field data to make the geometry of the gobs closer to the in situ situation and more realistic. Compared with the conventional layout, the theoretical analysis shows that the overall influence of the elevated split-level longwall gob on the lowest intact stratum increases by more than 5.07%, meaning that the split-level longwall layout is more likely to maintain the stability of the overlying strata. This is also corroborated by numerical modeling. Conventional longwall panels and split-level longwall panels with and without considering the gob are all simulated using FLAC^3D. Instead of only backfilling the height of the coal seam or the height of the coal seam and the immediate roof, as in many numerical modeling studies in the past, in this study, the whole caved zone is backfilled with “double-yield” material. It is found that along the floor, the split-level longwall gob assumes 23.4% more load than the conventional longwall gob, and the split-level longwall abutment bears 6.2% less load than the conventional longwall abutment; stress arches are developed within the gob; concave-down stress beddings are more evident at higher locations of the gob; a self-supporting structure develops within the gob and surrounding rock mass around the lower end of the gob, forming a protective localized intact destressed zone around the location where the split-level tailgate is situated; the yield zone in the floor of the curved section tends to extends toward the center of the curved part, where the curvature is the maximum; the upper stress concentration zone is within the coal seam, while the lower one is above the coal seam; the upper one is more concentrated. Full article

The low-gas permeability area of a fully mechanized up-dip working face was quantitatively studied using a physical similarity simulation test and theoretical analysis under varying dip angles of rock strata. Based on the theory of fractal geometry, this study obtained the fractal dimensions of the low-gas permeability area, the boundary area of the low-gas permeability region, and various layer areas of the low-gas permeability area by increasing the dip angle of rock strata. The findings reveal that the goaf’s high penetration area moved from a symmetrical shape to an asymmetrical one as the dip angle of rock strata increased. The high penetration area on the open-off cut side is notably larger than that on the working face side, due to the effects of advancement at the working face. In the goaf, the lateral length of the cavity decreases as the rock strata’s dip angle increases, while the longitudinal width expands and then contracts until it vanishes because of sliding. In the goaf, the lateral length of the cavity decreases as the rock strata’s dip angle increases, while the longitudinal width expands and then contracts until it vanishes because of sliding. In the goaf, the lateral length of the cavity decreases as the rock strata’s dip angle increases, while the longitudinal width expands and then contracts until it vanishes because of sliding. Moreover, the low-gas permeability area has a larger fractal dimension. The fractal dimension of the area with low gas permeability steadily decreased as periodic weighting emerged, ultimately reaching values of 1.24, 1.27, and 1.34. Moreover, the area’s fractal dimension was greater on the open-off cut side in comparison to the working face side. As the distance from the rock strata floor decreased, the fractal dimension of the area with low gas permeability increased. According to the gradient evolution law, the low-gas permeability area may be divided from bottom to top into three areas: strongly disturbed, moderately disturbed, and lowly disturbed. Based on the theory of mining fissure elliptic paraboloid zones and experimental findings, a mathematical model has been developed to analyze the fractal characteristics of low-gas permeability areas that are influenced by the rock strata’s dip angle. Finally, this study established a dependable theoretical foundation for precisely examining the development of cracks in the area of low gas permeability and identifying the storage and transportation region of pressure relief gas, which is affected by various dip angles of rock strata. It also offered assistance in constructing a precise gas extraction mechanism for pressure relief. Full article

In deep coal mine strata, characterized by high ground stress and extensive fracturing, predicting the strength of fractured rock masses is crucial for stability analysis of the surrounding rock in coal mine strata. In this study, rock samples were obtained from construction sites in deep coal mine strata and intact, as well as fissured, rock specimens were prepared and subjected to triaxial compression tests. A numerical model based on the discrete element method was then established and the micro-parameters were calibrated. A total of 288 triaxial compression tests on the rock specimens under different conditions of confining pressure, loading rate, fissure dip angle, and fissure length, were conducted to obtain the triaxial compressive strength of the fractured rock specimens under different conditions. To address the limitations of traditional back propagation (BP) neural networks in solving stochastic problems, a modified BP neural network model was developed using a random factor and an interlayer mean square error corrected network model evaluation function. The traditional and modified BP neural network models were then employed to predict the triaxial compressive strength of the fractured rock specimens. Through comparative analysis, it was found that the modified BP neural network prediction model exhibited smaller errors and significantly reduced overfitting, making it an effective tool for predicting the strength of fractured rocks in deep coal mine strata. Full article

Gob-side entry retaining by roof cutting, a pillarless mining technique, plays a critical role in maintaining continuous production, rapid connection, and enhancing the coal recovery rate in fully mechanized top coal caving working faces. This technique stands as a sustainable development method in coal mining. The present research, set against the backdrop of the Yitang Coal Mine 100602 top coal gob-side entry retaining by roof cutting, investigates the influence of roof-cutting borehole depth, borehole dip angle, mining height, and coal seam thickness on stability in an ultra-thick coal seam under 12 distinct mining conditions. A typical model of overburden structure post-roof pre-splitting was established to study the failure mechanism of the top coal roof. The results reveal that the dip angle and depth of the roof pre-fracturing borehole significantly impact the movement characteristics of the overlying strata. Optimal conditions are found when the dip angle and depth of the roof pre-fracturing borehole, the mining height, and the top coal thickness are 10°, 16 m, 4 m, and 4 m, respectively. Under these circumstances, the load transfer from the goaf to the gob-side entry can be effectively intercepted, mitigating the influence of roof fracture activities on the top coal gob-side entry. Field measurements confirm that suitable anchor-net support can stabilize the roof’s rock structure. This research underpins the significance of roof pre-fracturing for the promotion and application of top coal gob-side entry retaining by roof cutting in ultra-thick coal seams. Full article

Few researchers have looked at the mechanical characteristics of rocks that are composed of numerous layers of various kinds of rock. Most strata in practical engineering are composite strata, and fracture propagation is frequently to blame for engineering mishaps. The digital image correlation method (DIC) and acoustic emission (AE) equipment were used to observe the crack strike and strain field changes of specimens that resemble rocks with a constructed joint under uniaxial compression in order to study the crack growth process and failure mode in composite rock strata under uniaxial compression. The research focus of this paper is to conduct a quantitative and qualitative analysis of crack types based on the data obtained from the DIC test. The covariance matrix was introduced to quantify the strain field dispersion of samples with joint dip angles α = 0°, 15°, 30°, 45°, 60°, and 75°. The outcomes of the analysis were as follows: the displacement data of the two crack sides were quantified using the novel method, and the crack types were determined to be tensile crack (type I), shear crack (type II), and composite tension–shear crack (type I–II); the covariance matrix-based parameter V can be used to describe the crack creation and spread process; and according to the growth rate of V, the crack types were identified as tensile crack (0.12 × 10⁻⁴–0.49 × 10⁻⁴), shear crack (1.17 × 10⁻⁴–4.5 × 10⁻⁴), and composite tension–shear crack (0.72 × 10⁻⁴–0.99 × 10⁻⁴). Full article

Unlike land mining, the safety of seabed mining is seriously threatened by an overlying water body. In order to ensure the safety of subsea mining projects, it is of great importance to understand the failure characteristics and influencing factors of overlying strata deformation. Focusing on the Sanshandao Gold Mine, a typical submarine deposit in China, geomechanical model testing and numerical simulations were carried out. The results show that in the mining of a steeply dipping metal ore body, subsidence deformation mainly occurs on the hanging wall; the subsidence center is located on the surface of the hanging wall, and the uplift center is located on the upper surface of the ore body. The critical mining upper limit, which represents the minimum thickness of the reserved isolation pillar between the overlying seawater and the goaf, was determined to be 50 m in the Xinli mine; fault slip would occur if this critical value was exceeded. The dip angle and thickness of the ore body were negatively correlated with the vertical surface deformation. As the dip angle and thickness increased, the critical upper mining limit increased. When the fault was located in the footwall, the critical upper mining limit increased as the distance between the fault and the ore body increased, and the failure mode of the goaf was fault slip. When the fault was located in the hanging wall, the final failure mode of the goaf changed to a combined failure mode of overlying rock collapse as well as fault slip. These research results provide a theoretical basis for the selection of the reserved pillar height in the Xinli mining area, as well as a reference for safe mining practices under similar geological conditions. Full article

The damage of overlying strata and ground surface caused by the one-time mining space is relatively severe in steeply inclined extra-thick coal seams. The unique law of surface subsidence at these conditions is still missing. Taking Huating Dongxia Coal Mine as the research background, this paper reveals the law-governing effects on rock strata and surface movement and deformation caused by steeply inclined extra-thick coal seam mining with different coal seam dip angles and coal thicknesses by using the methods of surface measurement, theoretical analysis, and numerical simulation. Based on the characteristics of the surface inclination deformation, the surface is divided into four areas along the tendency section line—namely, an outcrop discontinuous deformation area, an overall subsidence area, a gradual subsidence area, and a slight subsidence area. The results show that the influence of the coal seam dip angle on surface subsidence zoning in steeply inclined and thick coal seams is mainly reflected in the affected area range and the form of damage. Coal thickness has a weak effect on the form of rock strata damage and surface movement. Utilizing the influence of the coal seam dip angle and coal seam thickness on the change in the surface subsidence zoning, the calculation formulas for each area range and zoning angle in relation to the coal seam dip angle, coal thickness, mining depth, and vertical stage height are established. The research results can provide a reference to evaluate the influence of mining, especially in steeply inclined extra-thick coal seams. Full article

The application of roof-cutting and pressure-relief gob-side entry retention plays a critical role in controlling the stability of the surrounding rock at the entry, easing continuity tension and improving resource recovery ratio. The excavation of the 360,803 airway in Xinji No. 1 Mine is affected by intense mining of the 360,805 working face. Hence, to address the stability problem of surrounding rock in the 360,803 airway, rock mass blast weakening theory was used in this study to analyze the blasting stress of columnar charged rock mass and obtain the radiuses of crushed, fractured, and vibration zones under uncoupled charging conditions. The reasonable array pitch, length, and dip angle of boreholes were determined according to the pressure-relief range of the blasting fracture. The migration laws of roof strata were explored based on a mechanical model of overlying roof strata structure on the working face. Subsequently, the horizon, breaking span, and caving sequence of hard roof strata were obtained to determine the roof-cutting height of this entry. On the basis of the theory of key stratum, the number of sequences at the roof caving limit stratum and hanging roof length in the goaf were calculated, the analytical solution to critical coal pillar width was acquired, the evaluation indexes for the stability of entry-protecting coal pillars were determined, and the engineering requirements for the 25 m entry-protecting coal pillars in the 360,803 airway were met. Moreover, various indexes such as roof separation fracture, displacement of surrounding rock, and loose circle of surrounding rock in the gob-side entry were analyzed. The stability and cementation status of surrounding rock in the 360,803 airway were evaluated, and tunneling safety was ensured. Full article

Sinking and horizontal movements are necessary parameters for assessing the potential impacts of surface subsidence in mining activities. Based on similarity criteria, the surface subsidence mechanism was studied using a physical model composed of similar materials such as sand, cement, and gypsum. With constant field geological parameters maintained in two angles of a coal seam, models of roof subsidence of composite rock were compared for different mining configurations. In accordance with observations from the physical model, it was concluded that subsidence and horizontal movement of strata near to and far from the coal seams were different and divided into five zones. The zone above a mined-out area underwent greater total subsidence compared to unexploited regions on both sides. Correlations between a subsidence curve and the height of a caving zone and the mining dip angle were obtained and verified from numerical model results. According to the roof’s position relative to the goaf, the area above the goaf of the composite rock layer was divided into three regions: a curving zone, a water-conducting fracture zone, and a falling zone, to which the subsidence and movement characteristics of each area could be proposed. Compared with the subsidence and movement characteristics observed from the physical and numerical model, the acquisition of subsidence characteristics and parameters in different areas can provide an idea for improvement, innovation or proposal of a theoretical formula for subsidence prediction of composite rock formations. Full article

The shape of a free surface is an important factor that determines the effect of bench blasting. The structural dynamics theory was applied to establish a structural failure model of the layered rock considering the impact of a blasting gas intrusion. Combined with the continuous-discontinuous element method (CDEM), the influence of rock strata on the failure mechanism of back-break was analyzed. The results show that structural failure characteristics of stratum with different dip angles are different. The bending failure characteristics of dipping-in-face stratum are stronger than that in dipping-out-of-face stratum. With the increase of the dip angle and height of rock stratum, the bending failure length of dipping-in-face stratum increases and the maximum value reaches 5.24 m. The trend of failure along the stratum surface towards the bottom increases, which is an important reason for the formation of an unfavorable shape of free surface. However, the failure depth of the gently dipping stratum and dipping-out-of-face stratum is relatively uniform; the average value is about 0.5 m. Finally, combined with the results of the bench blasting field test of the Changjiu (Shenshan) limestone mine, which is the largest in the production of sand and gravel aggregates, we verify the correctness of the theoretical analysis results. Relevant research results can provide a theoretical basis and technical support for controlling the bench blasting effect. Full article