First fracture characteristics of main roof plate structure with goaf (coal pillar) on both sides and elastic-plastic foundation boundary

In order to study the fracture position and engineering significance of the main roof plate structure under the condition of goaf on both sides (coal pillars), the double plasticized foundation boundary mechanical model of the main roof plate structure considering the elastic-plastic deformation of coal and the width and support capacity weakening of coal pillar on both sides is constructed. Based on the finite difference algorithm and the principal moment breaking criterion, the shape characteristics, location attributes and overall position characteristics of the main roof fault line above the asymmetric coal pillars area and the long side solid coal area are systematically calculated, and the new conclusions and important engineering significance of the new model are clarified by comparing with the traditional models from seven levels and four pairs of areas in transverse and longitudinal directions. The conclusions are as follows: ① The asymmetric coal pillars parameters on both sides have little influence on the main roof principal bending moment and fracture position above the long side solid coal area, but significantly affect the principal bending moment, position and fracture shape of the main roof above the coal pillar areas respectively. There are three types of evolution patterns of the main roof fracture line above the coal pillar areas on both sides (strong/wide coal pillar area + weak/narrow coal pillar area). With the increase of main roof thickness and elastic modulus, while coal pillars width, coal pillar bearing capacity and working face span decrease, its evolution law is as follows: asymmetric “continuous single arc + continuous single arc”→ “continuous single arc + open discontinuous double short arc”→ asymmetric “open discontinuous double short arc + open discontinuous double short arc”. ② The fracture line of the main roof above the long side solid coal area mainly has three types of location attributes. With the increase of main roof thickness and elastic modulus, while the plastic zone width and plasticization degree of solid coal and working face span decrease, its evolution law is as follows: the fracture line is above the plastic coal area (C-S type) → elastoplastic coal boundary area (C-TS type) → elastic coal area (C-T type). ③ With the increase of coal pillars width, coal pillar bearing capacity and working face span decrease, and considering the location attribute of the fracture line, the fracture mode and evolution law of the whole area of the main roof are as follows: the mode of C-S ()→the mode of C-TS ()→the mode of C-T ()→ mode of C-T ()→the mode of C-T (). Aiming at the three kinds of mechanical models for studying the fracture of the main roof plate structure with goaf (coal pillar) on both sides, the important differences of the three kinds of models are compared from seven levels, and its important engineering role is expounded from four transverse areas (front and rear of the mining area, coal pillar areas on both sides) and four longitudinal areas (asymmetric left coal pillar underlying and underlying mining space output/input coal pillar/body)

Determination of coal pillar width for gob‐side entry driving in isolated coal face and its control in deep soft‐broken coal seam: A case study

Abstract Large deformation control of surrounding rock in deep soft‐broken coal roadway has always been a key scientific and technological problem restricting the deep coal resources exploitation. Taking the gob‐side entry driving (GED) in isolated coal face under deep soft‐broken coal as research object, the prominent difficulties of surrounding rock control were clarified. Through numerical simulation, the evolution laws of deviatoric stress and plastic zone of GED under different coal pillar widths were studied. Combined with rational calculation, reasonable coal pillar width was determined to be 7.0 m. The study shows that severe disturbance distance in the front section of the coal face is 22 m, and makes it clear that coal pillar and roof are the key control areas. A subregional asymmetric combined support technology named anchor cable truss in roof and coal pillar + anchor cable in solid coal + single prop reinforced support in 25 m ahead of the coal face was proposed. The field application and ground pressure observation results show that good control effect has been achieved after adopting subregional asymmetric support technology under 7 m coal pillar width. The research results have important reference significance for safe mining in deep coal resources

Coal Pillar Size Determination and Surrounding Rock Control for Gob-Side Entry Driving in Deep Soft Coal Seams

In response to the large-scale instability failure problem of designing coal pillars and support systems for gob-side entry driving (GSED) in high-stress soft coal seams in deep mines, the main difficulties in the surrounding rock control of GSED were analyzed. The relationship between the position of the main roof breaking line, together with the width of the limit equilibrium zone and a reasonable size for the coal pillar, were quantified through theoretical calculations. The theoretical calculations showed that the maximum and minimum widths of the coal pillar are 8.40 m and 5.47 m, respectively. A numerical simulation was used to study the distribution characteristics and evolution laws of deviatoric stress and plastic failure fields in the GSED surrounding rock under different coal pillar sizes. Theoretical analysis, numerical simulation, and engineering practice were comprehensively applied to determine a reasonable size for narrow coal pillars for GSED in deep soft coal seams, which was 6.5 m. Based on the 6.5 m coal pillar size, the distribution of deviatoric stress and plastic zones in the surrounding rock of the roadway, at different positions of the advanced panel during mining, was simulated, and the range of roadway strengthening supports for the advanced panel was determined as 25 m. The plasticization degree of the roof, entity coal and coal pillar, and the boundary line position of the peak deviatoric stress zone after the stability of the excavation were obtained. Drilling crack detection was conducted on the surrounding rock of the GSED roof and rib, and the development range and degree of the crack were obtained. The key areas for GSED surrounding rock control were clarified. Joint control technology for surrounding rock is proposed, which includes a combination of a roof channel steel anchor beam mesh, a rib asymmetric channel steel truss anchor cable beam mesh, a grouting modification in local fractured areas and an advanced strengthening support with a single hydraulic support. The engineering practice showed that the selected 6.5 m size for narrow coal pillars and high-strength combined reinforcement technology can effectively control large deformations of the GSED surrounding rock

Control Techniques for Gob-Side Entry Driving in an Extra-Thick Coal Seam with the Influence of Upper Residual Coal Pillar: A Case Study

In multi-seam mining, the residual coal pillar (RCP) in the upper gob has an important influence on the layout of the roadway in the lower coal seam. At present, few papers have studied the characteristics of the surrounding rock of gob-side entry driving (GED) with different coal pillar widths under the influence of RCP. This research contributes to improving the recovery rate of the extra-thick coal seam under this condition. The main research contents were as follows: (1) The mechanical parameters of the rock and coal mass were obtained using laboratory experiments coupled with Roclab software. These parameters were substituted into the established main roof structure mechanics model to derive the breakage position of the main roof with the influence of RCP, and the rationality of the calculation results was verified by borehole-scoping. (2) Based on numerical simulation, the evolution laws of the lateral abutment stress in the lower working face at different relative distances to the RCP were studied. FLAC3D was used to study the whole space-time evolution law of deviatoric stress and plastic zone of GED during driving and retreating periods with various coal pillar widths under the influence of RCP. (3) The plasticization factor P was introduced to quantify the evolution of the plastic zone in different subdivisions of the roadway surrounding rock, so as to better evaluate the bearing performance of the surrounding rock, which enabled a more effective determination of the reasonable coal pillar width. The field application results showed that it was feasible to set up the gob-side entry with an 8 m coal pillar below the RCP. The targeted support techniques with an 8 m coal pillar could effectively control the surrounding rock deformation

Control Techniques for Gob-Side Entry Driving in an Extra-Thick Coal Seam with the Influence of Upper Residual Coal Pillar: A Case Study

In multi-seam mining, the residual coal pillar (RCP) in the upper gob has an important influence on the layout of the roadway in the lower coal seam. At present, few papers have studied the characteristics of the surrounding rock of gob-side entry driving (GED) with different coal pillar widths under the influence of RCP. This research contributes to improving the recovery rate of the extra-thick coal seam under this condition. The main research contents were as follows: (1) The mechanical parameters of the rock and coal mass were obtained using laboratory experiments coupled with Roclab software. These parameters were substituted into the established main roof structure mechanics model to derive the breakage position of the main roof with the influence of RCP, and the rationality of the calculation results was verified by borehole-scoping. (2) Based on numerical simulation, the evolution laws of the lateral abutment stress in the lower working face at different relative distances to the RCP were studied. FLAC3D was used to study the whole space-time evolution law of deviatoric stress and plastic zone of GED during driving and retreating periods with various coal pillar widths under the influence of RCP. (3) The plasticization factor P was introduced to quantify the evolution of the plastic zone in different subdivisions of the roadway surrounding rock, so as to better evaluate the bearing performance of the surrounding rock, which enabled a more effective determination of the reasonable coal pillar width. The field application results showed that it was feasible to set up the gob-side entry with an 8 m coal pillar below the RCP. The targeted support techniques with an 8 m coal pillar could effectively control the surrounding rock deformation

Control Mechanism and Support Technology of Deep Roadway Intersection with Large Cross-Section: Case Study

Conventional bolt–shotcrete support technology is usually single-layered, which does not meet the requirements of strength and stiffness for roadway support. Therefore, in this paper, new combined support technology, including a multiple-layered staggered dense arrangement of bolts, multiple-layered laying of steel meshes, multiple-layered pouring of shotcrete, strengthening support of long cables, and full cross-section grouting, is proposed. Specifically, the following new combined support technology process is proposed: first layer of shotcrete (80 mm), first layer of mesh, first layer of bolt, second layer of shotcrete (50 mm), second layer of mesh, second layer of bolt, reinforced cable, third layer of shotcrete (50 mm), and grouting. The results show the following: (1) In the system of a superimposed coupling strengthening bearing arch, compared to a cable bearing arch, changing the support parameters of the bolt bearing arch can significantly vary the bearing capacity. A range of bolt spacing between 0.4 m and 0.7 m is more conducive for a high performance of the bearing capacity of the superimposed coupling strengthening bearing arch. (2) With the increase in the single-layer shotcrete thickness (from 50 mm to 100 mm), the bearing capacity of the shotcrete structure increased rapidly in the form of a power function. (3) After the multi-level bolt–shotcrete support structure was adopted, the ring peak zone of the deviatoric stress of the surrounding rock at the roadway intersection was largely transferred to the shallow part, and the plastic zone of the surrounding rock of the roadway was reduced by 43.3~52.3% compared to that of the conventional bolt–shotcrete support. The field practice model showed that the final roof-to-floor and rib-to-rib convergences of the roadway intersection were 114 mm and 91 mm after 26 days, respectively. The rock mass above the depth of 3 m of the roadway’s roof and sides was complete, the lithology was dense, and there was no obvious crack. The new technology achieves effective control of a deep roadway intersection with a large cross-section

Mechanism and key parameters of stress load-off by innovative asymmetric hole-constructing on the two sides of deep roadway

Abstract Traditional dense large-diameter borehole stress load-off techniques reduce the stress levels in the shallow surrounding rock, weaken the bearing capacity of the shallow surrounding rock, and greatly deteriorate the shallow surrounding rock strength and supporting structure, which is not conducive to maintaining the long-term stability of the roadway. Therefore, to address the control problem for the pronounced extrusion deformation in the two sides of a roadway and the overall outward movement of the shallow surrounding rock supported by the sides bolts and anchor cables, as well as to comprehensively consider the on-site construction conditions of the two sides of a test roadway, stress load-off technology for asymmetric hole construction on the two sides of a roadway is proposed. The asymmetric stress load-off technique is a new method; while the shallow surrounding rock of the roadway sides is strongly anchored via a full anchor cable support form, a group of large stress load-off holes near the deep stress peak line of the roadway sides is excavated to relieve pressure and protect the roadway. This technology can transfer the peak stress area of the roadway side deeper into f the surrounding rock without deteriorating the shallow surrounding rock strength and damaging the supporting structure. A numerical simulation analysis of asymmetric stress load-off on the two sides of the roadway was performed, the stress load-off effect evaluation index was established, and the optimal field construction parameters were obtained. The stress load-off parameters obtained from the study are applicable to field engineering practice. Mine pressure data reveal that the test roadway remains intact and stable during the use period when the asymmetric stress load-off technique is adopted

An innovative destressing technology and key parameters determination in both sides of a deep roadway

AbstractThe general or single supporting theory and technology of the shallow surrounding rock of the roadway are not suitable for solving the problem of continuous large deformation of the both sides under the continuous migration of coal mass in the deep domain of the roadway side. Furthermore, the general destressing technology of dense drilling in the roadway destroys the shallow anchorage domain while releasing the stress. Therefore, this study proposes the “shallow supporting and deep destressing” synergism technology. This technology provides puissant supporting in the shallow domain of roadway sides, and at the same time, large destressing holes are excavated at the coal mass migration channel in deep stress peak domain far from the anchorage domain, conducting destressing regulation of roadway sides. This technology can shift stress peak domain of the roadway side to solid coal side of destressing hole without destroying the shallow anchorage domain, and at the same time, provide a buffer space for that coal mass in the deep domain of the roadway side continuously migrates to the anchorage surrounding rock, creating a beneficial stress circumstances for the roadway stability. The “shallow supporting and deep destressing” synergism technology can solve the contradiction between the shallow surrounding rock supporting and the continuous migration of coal mass in deep domain. The field application results show that the innovative destressing technology can effectively solve the problem of surrounding rock control in deep roadway.</jats:p

Control Techniques for Gob-Side Entry Driving in an Extra-Thick Coal Seam with the Influence of Upper Residual Coal Pillar: A Case Study

In multi-seam mining, the residual coal pillar (RCP) in the upper gob has an important influence on the layout of the roadway in the lower coal seam. At present, few papers have studied the characteristics of the surrounding rock of gob-side entry driving (GED) with different coal pillar widths under the influence of RCP. This research contributes to improving the recovery rate of the extra-thick coal seam under this condition. The main research contents were as follows: (1) The mechanical parameters of the rock and coal mass were obtained using laboratory experiments coupled with Roclab software. These parameters were substituted into the established main roof structure mechanics model to derive the breakage position of the main roof with the influence of RCP, and the rationality of the calculation results was verified by borehole-scoping. (2) Based on numerical simulation, the evolution laws of the lateral abutment stress in the lower working face at different relative distances to the RCP were studied. FLAC3D was used to study the whole space-time evolution law of deviatoric stress and plastic zone of GED during driving and retreating periods with various coal pillar widths under the influence of RCP. (3) The plasticization factor P was introduced to quantify the evolution of the plastic zone in different subdivisions of the roadway surrounding rock, so as to better evaluate the bearing performance of the surrounding rock, which enabled a more effective determination of the reasonable coal pillar width. The field application results showed that it was feasible to set up the gob-side entry with an 8 m coal pillar below the RCP. The targeted support techniques with an 8 m coal pillar could effectively control the surrounding rock deformation.</jats:p