Voussoir Beam Research Articles

Controlled reconstruction and application research of lateral overlying strata structure in high-gas mines with thick-hard roofs for a fully-mechanized top-caving face

Due to the large mining area, the fully-mechanized top-caving mining with thick-hard roof is easy to form cantilever structure on the lateral roof of the working face, which on the one hand causes high stress level of adjacent roadway and serious deformation of roadway, on the other hand causes gas accumulation in corners, which brings severe challenges to safe and efficient mining of the mine. In this study, a mine facing such problems in Jincheng, China was taken as the research object. Based on the mechanical characteristics of coal and rock, the characteristics of overlying strata activity in the mining process of working face are mastered, and the dual effects of controlled transformation of lateral overlying strata structure on stress field and gas field were revealed. On this basis, roadway reinforcement and gas drainage schemes were put forward and applied. The results showed that the strength of the hard rock stratum was high in the triaxial stress environment, and it was not easily destroyed. However, once the strata exceed their strength threshold, they break down. In addition, the strength of coal is relatively low, and it is continuously deformed when the force exceeds its strength. The overlying strata structure after thick-hard roof fully-mechanized top-caving mining evolves in the following manner: “long cantilever length formed by the main roof being broken in the initial stage, voussoir beam formed by the upper hard roof being broken in the middle stage, and double cantilever beam formed by overlying strata compaction.” The stress carried by upper hard rock stratum is transferred to coal pillars, which is the main reason for the high stress environment of multi-purpose roadway with large coal pillars. The controlled transformation of lateral overlying strata structure by pre-splitting and roof cutting can realize the "transfer-unloading" of coal pillar stress and the "plugging and driving" of corner gas. Based on the hydraulic fracturing reconstruction of lateral overburden structure, the grouting reinforcement scheme of roadway and dynamic gas drainage scheme were put forward. The results demonstrated that after roof cutting, the maximum deformation of the surrounding rock in the multi-purpose roadway was reduced by approximately 90 %, and the maximum concentration of corner gas was decreased by 15.28 %. This approach successfully achieved a collaborative control effect on roadway surrounding rock stability and gas emission well within the safety limits.

Investigating the Mechanism of Strong Roof Weighting and Support Resistance Near Main Withdrawal Roadway in Large-Height Mining Face

Abstract Aiming at investigating the strong roof weighting when the large height mining face is nearing the main withdrawal roadway, the 52,304 working face (WF) nearly through the main withdrawal roadway mining in a colliery of Shendong coalfield was taken as the research background. The ground pressure, roof structure, and superposition effect of stress in the last mining stage were studied by field measurement, physical simulation, and numerical calculations. The obtained results demonstrated that the main roof formed the “long step voussoir beam” structure under the influence of the main withdrawal roadway. The superposition effect of the front abutment pressure of the WF and the concentrated stress of the main withdrawal roadway caused the stress asymmetrical distribution on the two sides -level hard rock straof the main withdrawal roadway, and the stability of the pillar on the mining side decreases. The initial average periodic weighting interval was 20.7 m. While the WF approaches the main withdrawal roadway, the pillar near the WF of the main withdrawal roadway collapsed, the main roof was broken ahead of the WF, and the actual roof control distance of support and the periodic weighting interval increased by 2.56 and 1.26 times the normal state, respectively. Consequently, the “static load” of the immediate roof and the “dynamic load” of the sliding unsteadiness of the long step voussoir beam increased. The structural model of the “long step voussoir beam” under the superposition of “static and dynamic load” was established concerning those results, and an expression was proposed to compute the support resistance. Meanwhile, the mechanism of strong roof weighting was revealed when the WF was nearly through the main withdrawal roadway. The research conclusion is expected to provide a guideline for the safe withdrawal of the large-height mining faces under similar conditions.

Instability mechanism of overlying strata in a shallow interval goaf and structural model of the “W-shaped voussoir beam”

Abstract To study the failure mechanism of overlying strata (OS) in shallow insufficient mining areas, with a combination of such research approaches as field investigation, theoretical analysis, similarity simulation, and numerical simulation, this paper studies the temporal and spatial evolution of the failure mechanism of overlying stratum structure in a shallow-buried interval goaf. The results show that the creep failure of temporary coal pillar (TCP) in the interval goaf is a primary reason for the failure of the basic roof. With the failure of the basic roof, stress arches in the OS of the mining section become unstable, which expands the damage range of the overlying strata. Consequently, adjacent stress arches overlap with each other, forming a “trapezoidal–semi-elliptical arch” collapse shape. Thick soil layers gradually collapse to the ground surface, and the OS collapse as a whole into a trapezoid-like shape. Rotary failure appears in the basic roof of mining section, forming a W-shaped voussoir beam hinge structure. In this study, a structural model of a W-shaped voussoir beam in the OS is established, and the mechanical characteristics of rock blocks in the basic roof of overlying strata during different mining stages of the interval goaf are analyzed. Also, with a discrete element UDEC program, this study performs a simulation to verify the rationality of the W-shaped voussoir beam structural model of overlying stratum movement in the shallowly buried interval mining section.

A Disturbed Voussoir Beam Structure Mechanical Model and Its Application in Feasibility Determination of Upward Mining

China is endowed with a large quantity of residual coal resources that require upward mining. The stability of interburden strata structures and accurate determination are crucial for safe mining. Therefore, we established a mechanical model of disturbed voussoir beam structures of interburden strata in upward mining. The model was solved, and stability analysis and instability mechanism analysis were conducted. Based on this model, a new method for determining the feasibility of upward mining was proposed and applied to the upward mining of coal seam No. 7 in Baijiazhuang Coal Mine. A physical simulation experiment and numerical simulation were conducted to validate the method. Through research, it was found that the model had two instability mechanisms: rotation instability and sliding instability. When the disturbance load crossed the critical block of the structures, the model was most likely to experience sliding instability. When the disturbance load acted entirely on the critical block, rotation instability was more likely to occur. The result of the determination, performed using the new method, showed that there was no rotation instability or sliding instability in the interburden strata structures of coal seam No. 7, indicating that the coal seam could be mined upward. This result was consistent with the determinations using the statistical method, “three-zone” method, and balanced surrounding rock method. Physical and numerical simulations revealed that the upward mining of coal seam No. 7 caused the subsidence, rotation, and separation compaction of the interburden strata structures but that the structures remained stable. The results indicate that the proposed model and method have accuracy and applicability, being able to guide the practical feasibility determination of upward mining.

Stability control of overburden and coal pillars in the gob-side entry under dynamic pressure

To improve the recovery ratio of coal resources and stress environment of the roadway, gob-side entries have been widely implemented to numerous coal mines in China. Traditional gob-side entry is generally excavated after the gob of the adjacent longwall face is stabilized. To achieve a balance between the seam mining and roadway excavation, numerous mine coals use the method of driving a roadway along the next gob, which heads adjacent to the advancing coal face, and maintaining a narrow coal pillar. Consequently, the dynamic pressure occurs at the gob-side entry; accordingly, the difficulty of roadway maintenance is increased. However, the traditional gob-side entry stability analysis is not applicable to a gob-side entry under dynamic pressure. In this study, the overburden structural model and coal pillar stress model of a gob-side entry under dynamic pressure were established considering the compression load of the immediate roof and gangue and influence law of the cantilever beam length of main roof. Subsequently, thickness, hardness, and deformation of the immediate roof on the stability of the gob-side entry were studied, and the theoretical criterion for the stability of “voussoir beam” structure and coal pillar of the gob-side entry under dynamic pressure were proposed. Based on the geological conditions in tailgate # 110505 of the Yushuling coal mine, reasonable measures were taken considering the aspects of roof cutting and pressure relief, coal pillar design, and surrounding rock reinforcement. After applying the proposed measures, the width of the coal pillar was reduced from 15 m to 4 m. Meanwhile, the grouting cable beam was used to strengthen the roof, and the narrow coal pillar was reinforced by a bidirectional grouting anchor cable was developed. As a result, after the roadway was stabilized, the roof-to-floor convergence and two-side convergence were 268 mm and 105 mm, respectively. Moreover, the stress concentration factor of the coal pillar was 1.20. The stability characteristics of the overburden and coal pillar of the gob-side entry under dynamic pressure were effectively controlled, and remarkable economic benefits were obtained in this coal mine. The techniques proposed for maintaining the stability of the 4 m wide coal pillar and ensuring the bidirectional grouting reinforcement have been successfully applied in a coal mine for the first time, which have high innovation and application values. This study provides a reference for the design and maintenance of the gob-side entry under dynamic pressure.

Experimental research on the instability characteristics of the overlying strata structure that characterizes shallow interval goaf mining

Abstract Here, we analyze the instability characteristics of the overlying strata structure that characterize shallow-depth seams under insufficient goaf in Yushenfu mining area. To simulate the No. 20107 longwall interval working face situated in Nanliang Coal Mine, physical simulation, an acoustic emission (AE) monitoring system, a stress acquisition system and a total station were used. The results indicate that during interval goaf formation, which is correlated with mining, the immediate roof collapses, and the main roof strata remains stable. Gradually, the stress that acts on the temporary coal pillar (TCP) gradually exhibits the ‘uniform increase–accelerated increase catastrophe instability’ change characteristics. Due to the concentrated load of the overlying strata, the bearing capacity of the TCP gradually deteriorates until the catastrophic instability occurs, and the unstable roof strata forms a ‘W-shaped voussoir beam’ structure. The research results provide evidence for the strata control that is associated with shallow-seam mining.

A Simplified Approach to Estimate Anchoring Capacity of Blocky Rock Mass with Pressure Arch Theory

In this paper, a simplified method for predicting rock mass resistance against tensile forces from rock anchors (anchor) is presented. The interaction of anchors and rock mass was investigated using three-dimensional discontinuous numerical modelling. Several patterns of rock discontinuities were assumed in the numerical modelling while a single anchor is embedded in it. The numerical results show that the existence of a discontinuity set sub-parallel to the anchor significantly improves the rock mass resistance against the tensile force from the anchors. This phenomenon is due to the rock block interlocking at the sub-parallel discontinuity set. Rock block interlocking generates a zone of stress concentration inside the rock mass which has an arch shape (i.e., a pressure arch), resisting against the anchor's tensile force. The load-bearing capacity of the pressure arch plays a significant role in the resistance of the rock mass against the forces from the rock anchor. A voussoir beam analogy was utilised to study the load-bearing capacity of the pressure arch. A simplified analytical approach was developed to assess the load-bearing capacity of the voussoir beams. Then, it was used in combination with the weight of the mobilised rock mass by the anchor to assess the maximum anchoring resistance of the rock mass (anchoring capacity). The suggested method was calibrated by numerical modelling and relevant published pull-out test results. The technique developed in this paper shows the significance of rock block size, shear behaviour of rock discontinuities, Young's modulus of the rock mass, and uniaxial compressive and tensile strength of the intact rock in anchoring capacity of rock masses.

Experimental investigation on mechanical characteristics of red sandstone under graded cyclic loading and its inspirations for stability of overlying strata

The horizontal stress in the fractured blocks of a fractured zone is in a stress relief status under repeated mining disturbances. To investigate the effect of the horizontal stress in fractured blocks on the stability of the overlying strata, uniaxially graded cyclic loading and unloading experiments on red sandstone with different initial stress level were carried out. First, the evolution of the loading and unloading elastic modulus, irreversible strain and load-unload response ratio were analyzed. Then, the strain energy evolution and damage variable characteristics were examined. Finally, the stability of a voussoir beam structure under repeated mining disturbances was discussed. The results showed that with increasing cycle number, the elastic modulus showed a nonlinear decreasing trend, the load-unload response ratio always fluctuated at approximately 1, and the irreversible strain showed a rapid and steady increasing trend. The proportion of elastic energy tended to increase linearly with increasing cycle number, while the proportion of dissipated strain energy tended to decrease. Moreover, the damage variable, based on the dissipation strain energy definition, showed a parabolic growth trend with increasing cycle number. The irreversible strain and damage variables showed an “increasing–decreasing-increasing” trend as the initial stress level increased. Finally, a stability criterion of the voussoir beam structure based on the effect of repeated mining disturbances on horizontal thrust was proposed. This research has important implications for overlying strata stability.

Theoretical research on reasonable shield support capacity in close-multiple coal seams with the coordinated mining: A case study of Qianjiaying coal mine.

In order to assess the rationality of the rated shield support capacity (RSSC) experienced selection and guide the reasonable RSSC selection for the subsequent working faces of each coal seam, the coupling relationship between shield and roof strata was revealed during each coal seams mining. According to whether the fractured rock blocks generated by the main roof are articulated and whether the upper coal seam has been mined and influenced on the lower coal seam, two roof structure mechanical models of the rock blocks generated by the thick main roof and two calculation methods of a given load on the rock blocks are proposed. In addition, a selection method of roof structure model for maximum shield support capacity (MSSC) of close-multiple coal seams with the coordinated mining is put forward. Three roof structures to calculate the MSSC are established. Based on a case study of close-multiple coal seams with the coordinated mining in the Qianjiaying coal mine, the MSSC is calculated and analyzed in each coal seam combined with roof structure characteristics description, theoretical analysis, and field measurement. No.7, No.12-1, and No.5 coal seams mining are applicable to a voussoir beam balanced structure. No.8 coal seam mining is applicable to a balanced structure with a given load of loose body. No.9 coal seam mining is applicable to a voussoir beam balanced structure with a given load of loose body. Through the calculation, the MSSC of No.7, No.8, No.12-1, No.9, and No.5 coal seam is 3948.55kN, 4018.32kN, 4101.63kN, 3560.03kN, and 4015.30kN, respectively. And the RSSC suggested selection of each coal seam is 4500kN, 4300kN, 4300kN, 4000kN, and 4300kN, respectively. By field measurement, the RSSC experienced selection of each coal seam in the Qianjiaying coal mine is unreasonable with low support load utilization. However, after adopting the RSSC suggested selection in each coal seam, the support load utilization increased by 29.07%, 9.6%, 8.57%, 15.33%, and 11.39%.

Theoretical analysis of the reasonable support capacity based on voussoir beam structure for a given load of the loose body under an extremely close goaf

Given the growing popularity of fully mechanized longwall mining, hydraulic shields have emerged as the most crucial pieces of equipment whose rated support capacities are a significant assurance for safe extraction of coal seams. Based on the extraction in the 1,692 panel of the No.9 coal seam in Qianjiaying coal mine, a detailed study of the reasonable shield support capacity under an extremely close goaf was conducted using field measurements, theoretical analyses, and system development. The results show that the ZY4800-13/32 shield employed in the 1,692 panel is unreasonable owing to its large surplus coefficient of support capacity and low support utilization rate. The voussoir beam structure is established for a given load of the loose body to calculate the shield support capacity when the lower coal seam is extracted under an extremely close goaf. The calculated shield support capacity required for the No.9 coal seam was 3,560.03 kN, so the rated support capacity should be 4,000 kN. A calculation and analysis scheme for the shield support capacity under the given load of loose body was developed using a GUI in Matlab, and this scheme was used to examine the influence degrees of the relevant parameters of the immediate roof, main roof, and loose body on the shield support capacity. The thickness of the main roof was found to have the greatest influence on the shield support capacity. Finally, presplitting the main roof was proposed as an effective method for controlling the breaking length and reducing the shield support capacity based on existing research.

Research of the broken roof structure and supporting capacity of a shield in a deep and thick coal seam

We determine the key issues of the reasonable supporting capacity of a shield and the shield selection in the panel face in a deep and thick coal seam. The No.232204 panel face of the Meihuajing mine in the Yuanyanghu mining area is taken as the research background. Using theoretical analysis, numerical simulation, and field measurement, the fracture characteristics and instability forms of roof structure of the panel face were studied. A mechanical model of roof fracture structure was established to calculate the reasonable supporting capacity of the shield, which guided the shield selection for the panel face. Also, a calculation and analysis system of the supporting capacity of the shield in a deep and thick coal seam was developed to realize the dynamic calculation and analysis of the supporting capacity of the shield. The results are as follows. 1) when the first weighting of the main roof appears, the broken rock block is hinged to form a “three-hinged arch” balanced structure. When the periodic weighting of the main roof appears, the broken rock block forms a “voussoir beam” balance structure. Also, the supporting capacity of the shield is stable between 8,900 and 9,600 kN. 2) The theoretical calculation showed that the supporting capacity of the shield in the No.232204 panel face was 9,581.04 kN, and the ZY10000/28/62D shield with supporting capacity of 10,000 kN is selected in the No.232204 panel face. 3) Through self-developed calculation software, the thickness and bulk density of the immediate roof had little influence on the supporting capacity of the shield, and the main roof thickness and bulk density have considerable influence on the supporting capacity of the shield. 4) The field measurement showed that the maximum supporting capacity of the shield in the panel face was distributed between 9,000 and 10,000 kN, which accounted for 77.58%. The loading utilization rate was more than 90%, and therefore the selected shield was reasonable. The research results provide a theoretical basis for the selection of shield supports for a deep buried and thick coal seam in the Yuanyanghu mining area.

Mechanism and control technology of strong ground pressure behaviour induced by high-position hard roofs in extra-thick coal seam mining

This work aimed at revealing the mechanism of strong ground pressure behaviour (SGPB) induced by high-position hard roof (HHR). Based on the supporting structures model of HHR, a modified voussoir beam mechanical model for HHR was established by considering the gangue support coefficient, through which the modified expressions of limit breaking span and breaking energy of HHR were deduced. Combined with the relationship between the dynamic-static loading stress of supporting body (hydraulic support and coal wall) and its comprehensive supporting strength, the criteria of ground pressure behaviour (GPB) induced by HHR were discussed. The types of I1, I2, Ⅱ1, and Ⅱ2 of GPB were interpreted. Results showed that types I1 and I2 were the main forms of SGPB in extra-thick coal seam mining. The main manifestation of SGPB was static stress, which was mainly derived from the instability of HHR rather than fracture. Accordingly, an innovative control technology was proposed, which can weaken static load by vertical-well separated fracturing HHR. The research results have been successfully applied to the 8101 working face in Tashan coal mine, Shanxi Province, China. The results of a digital borehole camera observation and stress monitoring proved the rationality of the GPB criteria. The control technology was successful, paving the way for new possibilities to HHR control for safety mining.

Expanding application of the voussoir beam analog to horizontally bedded and passively bolted flat-roof excavations using the discrete element method

The voussoir beam analog is a well-established analytical method that has been successfully applied to the design of flat-roof excavations in discontinuous rockmasses. However, application of voussoir beam theory is limited to very specific conditions. Previous authors have attempted to account for more realistic loading and geometric conditions, but have not presented systematic or verified methods to account for variations in inelastic material behavior or supported roof geometry. This study presents multiple original numerical models that methodically assess the effects of increasing material and geometric complexity on voussoir beam mechanical behavior and develops adjustments to the Diederichs & Kaiser (1999) analytical solution to account for variations in post-peak material behavior and the interactions between multiple passively bolted layers. A step-by-step guide is presented for implementing these adjustments.

Evolution Law of Gas Discharge of Carbon Monoxide in Mining Extra-Thick Coal Seam of Datong Mining Area

In order to reveal the evolution law of gas discharge of carbon monoxide in mining an extra-thick coal seam of the Datong mining area by the numerical simulation and field monitoring test, the 8202 working face and 8309 working face in the Tongxin coal mine are chosen as the test sites. The results show that the seepage flow of carbon monoxide gas reaches 1.854 × 10 − 8 m 3 / s in the #1 fracture after the #3 key stratum in the far field breaks in the 8202 working face, the seepage flow of carbon monoxide gas reaches 1.307 × 10 − 7 m 3 / s in the #2 fracture, the seepage flow of carbon monoxide gas reaches 4.276 × 10 − 7 m 3 / s in the #3 fracture, the seepage flow of carbon monoxide gas reaches 4.192 × 10 − 7 m 3 / s in the #4 fracture, and the seepage flow of carbon monoxide gas reaches 1.623 × 10 − 7 m 3 / s in the #5 fracture. The initial caving of the #3 key stratum in the far field occurs and collapses to the gob, when the working face in the #3-5 coal seam advances to 180 m, and the voussoir beam forms in the #3 key stratum. Besides, a shower shape was formed by the seepage flow of carbon monoxide gas, and the maximum flow in the working face reaches 4.562 × 10 − 4 m 3 / s . When the 8309 working face advances from 521.2 m to 556.4 m, the air pressure at the working face gradually rises and reaches the maximum magnitude and then begins to decrease; when the working face advances to 556.4 m, the air pressure at the working face reaches the maximum magnitude of 91.35 kPa. The gas discharge disaster of carbon monoxide in mining the extra-thick coal seam of the Datong mining area is effectively controlled by the dynamic balance multipoint control technology. The research results can be treated as an important theoretical basis for the prevention and treatment for carbon monoxide discharge disaster in mining the extra-thick coal seam of the Datong mining area.

Roof structure of shallow coal seam group mining in Western China.

In order to realize roof control of shallow coal seam group mining in Western China, combining with engineering statistics, physical simulation and theoretical analysis, the roof weighting characteristics during lower coal seam mining were revealed, and the classification of shallow coal seam group was proposed. Based on this, mechanical models of roof structure were set up, and the calculation method of support resistance was determined. The results show that the roof weighting is closely related to the interburden thickness and the mining height of lower coal seam, considering the ratio of interburden thickness to the mining height, as well as the key stratum structure, the classification of shallow coal seam group was put forward. The first type is shallow coal seam group with no key stratum (SCSG-No), its roof pressure is mainly affected by caving roof of upper coal seam, and the interburden roof forms slanting pillar-beam structure. The second type is shallow coal seam group with single key stratum (SCSG-S), interburden roof represents step voussoir beam structure. The third type is shallow coal seam group with double key strata (SCSG-D), interburden roof can form double key strata structure, the lower key stratum forms slanting step voussoir beam structure, while the upper key stratum forms voussoir beam structure, besides, longwall face represents large—small periodic weighting. Through establishing the roof structure models, the calculation formulas of support resistance were determined, it can provide basis for roof control and promote safe mining in Western China.

Numerical Simulation of Stress Arching Effect in Horizontally Layered Jointed Rock Mass

Stress arching effect during the excavation of broken surrounding rock in underground engineering has an important influence on the stability of surrounding rock after underground excavation. To determine the stress arching effect in horizontally layered jointed rock mass, the stress arching characteristics of surrounding rock mass after excavation is analyzed in this study by using a series of numerical tests. The formation mechanism of stress arch is revealed through a comparison of the stress characteristics of a voussoir beam structure and theoretical analysis of multi-block mechanical relationship of jointed rock mass. The method for determining the boundaries of a stress arching zone is proposed, and the influence of various factors on a stress arch is further discussed. Results show that after the excavation of horizontally layered jointed rock mass, the stress arch bunch (SAB) is formed in the lower strata above the cavern, and the global stress arch (GSA) is formed in the higher strata, both of which are symmetrical arch stress patterns. The SAB is the mechanical manifestation of the voussoir beam structure formed by several low-level sandstone layers, and the GSA is caused by the uneven displacement between blocks. Compared with the GSA, the SAB is more sensitive to various influencing factors. The extent of stress arching zone decreases with the increase of an internal friction angle of the joint, lateral pressure coefficient, and overburden depth. In addition, the joint spacing of rock strata is conducive to the development of a stress arch. Results can provide technical support for deformation control and the stability analysis of broken surrounding rock in underground engineering.

Stability Analysis and Fracture Patterns of Hard Main Roof in Longwall Top Coal Caving with Large Mining Height

In order to study the fracture patterns of hard main roof in longwall top coal caving (LTCC) with large mining height, a two-dimensional physical similarity model was created to simulate the mining process of No. 8100 large mining height face in Tongxin coal mine, China. The results show that there are three positions of broken line in hard main roof presented with the advance of longwall face, and the underground pressure induced by hard main roof fracturing presents the effect of superposition of large and small periods. It is found that there are two fracture patterns of main roof during the mining process: composite structure of lower cantilever beam and upper voussoir beam with hard main roof and composite structure instability of lower and upper voussoir beam with hard main roof. The underground pressure induced by these two fracture patterns is also analyzed by building mechanical models. In the end, the hydraulic fracture technique is introduced to presplit the main roof and weaken the effect of dynamic loads induced by composite structures instability.

Cooperative Control Mechanism of Long Flexible Bolts and Blasting Pressure Relief in Hard Roof Roadways of Extra-Thick Coal Seams: A Case Study

The higher strength of a hard roof leads to higher coal pressure during coal mining, especially under extra-thick coal seam conditions. This study addresses the hard roof control problem for extra-thick coal seams using the air return roadway 4106 (AR 4106) of the Wenjiapo Coal Mine as a case study. A new surrounding rock control strategy is proposed, which mainly includes 44 m deep-hole pre-splitting blasting for stress releasing and flexible 4-m-long bolt for roof supporting. Based on the new support scheme, field tests were performed. The results show that roadway support failure in traditional scenarios is caused by insufficient bolt length and extensive rotary subsidence of the long cantilever beam of the hard roof. In the new proposed scheme, flexible 4-m-long bolts are shown to effectively restrain the initial expansion deformation of the top coal. The deflection of the rock beam anchored by the roof foundation are improved. Deep-hole pre-splitting blasting effectively reduces the cantilever distance of the “block B” of the voussoir beam structure. The stress environment of the roadway surrounding rock is optimized and anchorage structure damage is inhibited. The results provide insights regarding the safe control of roadway roofs under extra-thick coal seam conditions.

Pressure Relief Mechanism of Directional Hydraulic Fracturing for Gob-Side Entry Retaining and Its Application

In order to make clear the pressure relief mechanism and application effect of directional hydraulic fracturing for gob-side entry retaining, the directional hydraulic fracturing was carried out by 400 m in haulage gateway remaining along the goaf in 50108 working face of Hejiata Coal Mine. Taking this as the engineering background, a mechanical model of roof cutting was established and the pressure relief mechanism was clarified. The theoretical research shows that it is the moments of gravity FG of the curved triangular roof plate at the face end, the pressure q of the overlying soft rock, and the transverse force TCB in the “voussoir beam” structure to the left endpoint of the triangular block, that is, M F G , M q , and M T CB , which determines the roadside supporting resistance. Hydraulic fracturing can reduce the lateral cantilever length of the basic roof, thus greatly reducing the values of M F G , M T CB , and M q , and significantly reduce the roadside supporting resistance. The field test shows that the directional hydraulic fracturing technology can effectively improve the stress environment of the face end and reduce the deformation of the roadway, and it has a good application effect on the gob-side entry retaining.

Study on the bearing structure and stability of overlying strata: an interval gob in shallow buried coal mining of Northwest China

The bearing structural characteristics of overlying strata in overlying gobs are the main factor in inducing dynamic disasters in roof in close-distance coal seam group mining. In the present work, the bearing structure and stability characteristics of overlying strata in the shallow-buried interval gob (SBIG) of Yushenfu mining area in north Shaanxi Province, China, have been studied by numerical and physical similarity simulations and theoretical analyses. Research results showed that the instability of gob temporary coal pillars (TCP) led to the rotary instability of basic roof, increasing the stress arch in overlying strata with the expansion of the failure range of overlying rocks giving rise to increases of arch height and superimposition of adjacent stress arches. This way, “trapezoid-semicircular arch” caving of overlying strata was performed. “W-shaped voussoir beam” bearing structure was formed by the fracture of the basic roof of gob and “double arch bridge” bearing structure was created by interval coal pillar (ICP) and fracture rock. In this work, the bearing structure model of interval gob overlying rock “W-shaped voussoir beam + double arch bridge” was established. On the basis of “voussoir beam structure,” the stability of “double arch bridge” type bearing structure formed by the rotation of broken rock block on the top of the basic roof of the interval gob was analyzed. When the support capacity of coal pillar was weakened or the overlying strata were changed, the stability of asymmetrical stress inclined arch equilibrium structure was lost. The stress distribution of “double arch bridge” structure overlying the interval type gob was analyzed by numerical simulation, and the stress transfer law of pier column in floor was obtained. It was found that with the decrease of the width of pier pillar, the “double arch bridge” structure became asymmetric, and the change characteristics of stress inclined arch height were decreased. The arch roof transferring to pier side was divided into prevention and control areas of roof dynamic disaster in the mining of lower coal seams. The purpose of this paper was to provide theoretical guidance for dynamic pressure prevention and control of mining under shallow interval gob.