Gob-side Entry Research Articles

Application of Gob-Side Entry Driving in Fully Mechanized Caving Mining: A Review of Theory and Technology

China has abundant coal resources, and the distribution of coal seams is complex. Thick coal seams account for more than 45% of all coal seams. Fully mechanized top coal caving mining has the advantages of large production, high efficiency, and low cost. In fully mechanized caving mining, especially in fully mechanized caving mining of extra-thick coal seams, the mining space is ample, the mine pressure is severe, and the roadway maintenance is complex. As a result, it is necessary to summarize and discuss the gob-side entry driving of fully mechanized caving in theory and technology, which will help to promote the further development of fully mechanized caving gob-side entry driving technology. First, in recent years, the research hotspots of gob-side entry driving have focused on the deformation mechanism and the control method of the roadway surrounding rock. Secondly, this paper discusses the theoretical models of the “triangle-block” and “beam” for the activity law of the overlying strata in gob-side entry driving, including the lateral breaking “large structure” model, compound key triangle block structure model in the middle and low position, the high and low right angle key block stability mechanics model, elastic foundation beam model, low-level combined cantilever beam + high-level multilayer masonry beam structure model, and the vertical triangular slip zone structure model. It introduces the “internal and external stress field theory” and the “stress limit equilibrium zone model”. Thirdly, it summarizes several numerical simulation analysis methods in different conditions or research focuses and selects appropriate constitutive models and simulation software. Finally, it introduces surrounding rock control technology, including two ribs, the roof, and under challenging conditions. It provides a method reference for support in similar projects.

Overlying main roof breaking characteristic and its effect on the stability of gob-side entry

In this study, to figure out the deformation mechanism, a particle flow model was used to simulate the deformation of surrounding rock in a deeply buried entry. The fracture evolution, stress, and porosity of surrounding rock in the process of the main roof breaking, meanwhile, the displacement after entry being excavated were recorded and analyzed in the simulation. The results show that the main roof breaking is different from the ideally elastic–plastic simply supported beam for it has two more plastic zones on both sides of the beam and its support points tend to move. In addition, the stress reduction zone in the coal seam is about 10 m where the coal body and surrounding rock are almost completely broken and porosity is surprisingly increasing from 0.01 to 0.39 in coal and 0.23 in the main roof. The displacement concentrated on the ribs of entry, especially on the coal pillar. The continuous and large deformation that appears in the stress reduction zone results from the nearly completely broken state of the surrounding rock. The tradeoff between the stress reduction and the broken state of the surrounding rock should be considered when choosing the location of the gob-side entry.

Simulation of narrow coal pillar width in gob-side entry driving

Numerical simulation of coal pillars with different widths in goaf excavation at 1102 working face of Huacaoan Coal Mine is carried out to study the influence of different widths of coal pillars on the stability of surrounding rock in goaf excavation. Finally, the width of coal pillars is determined to be 5m, which ensures the safety of roadway excavation, improves the utilization rate of coal extraction and responds to the call of green mining.

Dynamic Loading Mechanism and Stability Control of Gob-Side Entry Retaining with Thick and Hard Roof: Insights from Numerical Simulation and Field Test

Dynamic Loading Mechanism and Stability Control of Gob-Side Entry Retaining with Thick and Hard Roof: Insights from Numerical Simulation and Field Test

Study on Floor Heave Characteristics and the Control Method of Gob-Side Entry Driving in Weakly Cemented Soft Rock

Aiming at the problems of large deformation, long duration, and difficult control of floor heave in gob-side entry driving in weakly cemented soft rock, this paper takes the weakly cemented soft rock mining area in Western China as the engineering background, and studies the characteristics and mechanism of floor heave in gob-side entry driving in weakly cemented soft rock by means of a field investigation, physical component analysis, mechanical property tests of the surrounding rock, and the stress monitoring of the surrounding rock. The classification control method of floor heave is put forward, and field tests are conducted. The results show that: (1) The floor heave characteristics of the dynamic change in the floor heave peak position of gob-side entry driving from the coal pillar side to the mining side are obtained through field observation. (2) Based on the analysis of field data and laboratory test data, it is concluded that the stability time of the overlying strata in gob-side entry driving is about 8 to 12 months. The main internal cause of roadway floor heave is the low load resistance of weakly cemented soft rock. High stress and strong disturbances are the main power sources of strong floor heave. The mechanism of floor heave affected by dynamic lateral abutment pressure is summarized, and the classification control method of floor heave is proposed. (3) The gob-side entry driving support technologies of “adjusting excavation deployment” and “surrounding rock pressure relief and improving support” are proposed. Through field tests, the floor heave can be effectively controlled.

Ground Response of Non-Coal Pillar Mining Panel

The mining-induced ground response (MIGR) has a critical impact on safety management, the mining plan, and entry support. A clear understanding of the characteristics is the foundation of the MIGRs scientific control. This study is the result of the MIGRs development of the non-pillar mining panel with gob-side entry by roof cutting (GSERC). Comprehensive research of the in situ measurements, numerical simulation, and theoretical analysis to determine the ground response characteristics, including mining panel and GSERC, were implemented. The results indicate that the MIGR presents the characteristic of asymmetric development and that the ground response near the non-roof cutting side is more significant than that near the roof cutting side. The development stage of the entry convergence of GESRC can be divided into seven stages; the primary rapid development stage should be paid more attention to in the support process. The entry convergence rapidly increases to 275 mm, 380 mm, 410 mm, and 525 mm, respectively, for the roof cutting rib to the virgin coal rib, the roof near the virgin coal side, the roof of the middle section, and the roof near the cutting side. The hydraulic support end cycle resistance at the roof cutting side and the middle section of the mining panel with the value of more than 30.8 MPa is greater than that at the non-roof cutting side with the value of less than 26 MPa, which presents the asymmetric feature. The numerical simulation results regarding vertical stress development, vertical displacement, and horizontal displacement also presents the asymmetric feature. The MIGR division is divided into five divisions. Division II (the middle section of the panel) and division IV (the entry range near the roof cutting side) should be paid more attention to in the panel mining process. The results of this study can provide technical guidance and theoretical reference for similar engineering practices.

Effect of Fly Ash and Steel Fiber Content on Workability and Mechanical Properties of Roadway Side Backfilling Materials in Deep Mine

The stable bearing capacity of roadside backfill body (RBB) is the key to control the surrounding rock deformation of gob-side entry retaining (GER) in deep mining. In this paper, slump and uniaxial compressive strength tests were conducted to study the changes of the slump and RBB mechanical properties of RBB fresh mixture under the influence of different percentages of fly ash (FA), river sand, and amounts of steel fiber (SF) admixture. The results show that (1) the slump first increases and then decreases with the increase of the FA proportion, while 20% of FA proportion is the critical point. In addition, SF has little influence on slump. However, when the proportion of FA is 60% and the content of SF is 0.4%, the slump reaches the minimum value of 130 mm. (2) The peak strength and residual strength increase with the increase of the FA proportion, and the strength gain starts to decrease when the FA proportion exceeds 40%. Moreover, SF only has a small effect on the peak strength when the dosage is 0.8%. (3) The post-peak strain difference decreases with the increase of the FA proportion and increases with the increase of the SF content. The addition of SF can make up for the reduced capacity of RBB to post-peak deformation caused by the addition of FA to a certain extent. (4) The addition of fly ash does not change the brittle failure mode of RBB. Furthermore, after the addition of SF, RBB changes from brittle to ductile failure mode. This study can provide experimental and theoretical basis for improving the bearing capacity of RBB and maintaining the stability of GER.

Surrounding rock activity law and support optimization of double flexible formwork wall gob-side entry retention in stratified mining coal seams

To ensure gas control in the layered mining of extra-thick coal seams in Baijigou Coal Mine—in addition to reducing the loss of coal resources and amount of roadway driving—this article proposes the gob-side entry retention technology of flexible formwork concrete, and retains the 010202 working face four-layered return air roadway as the 010203 working face gas drainage lane. By combining theory and field practice, a flexible formwork concrete wall + anchor cable joint support scheme is proposed to strengthen small coal pillars. Further, the design scheme for the aforementioned framework at the bottom of the layered mining region of an extra-thick coal seam is proposed. Then, field tests are carried out, and the surrounding rock activity law of the double flexible formwork wall entry retention in a layered mining coal seam is obtained. The results show that (i) 010202 the whole process of overburden collapse to stability at the stratified mining face is completed 60--200 m behind the face and (ii) the roadway deformation of the extra-thick coal seam in the Baijigou Coal mine can be divided into four stages, and the mining pressure of the gas drainage roadway basically presents a dynamic change of “pressurized stage – peak stage – stabilized pressure stage”. Based on this, three mechanical stages of roadway deformation are derived: (1) δz< Nmz<< Nwz; (2) Nmz < δz << Nwz; and (3) Nmz < δz< Nwz. Three repeated mining support optimization schemes are provided, one for each stage. Finally, the deformation and damage of the surrounding rock are effectively controlled, and the requirement that the gas drainage roadway is still intact under the influence of the 010203 working face mining is realized. This study has certain reference significance for gob-side entry retention in the bottom layer of stratified mining in extra-thick coal seams.

Control Technology of Roof-Cutting and Pressure Relief for Roadway Excavation with Strong Mining Small Coal Pillar

In order to solve the problem of serious deformation and failure of surrounding rock and difficult maintenance of gob-side entries with strong mining-induced small coal pillars, taking the A110607 return airway of the Shanwenjiaba Coal Mine as the engineering background, the key parameters of roof-cutting and pressure relief control technology for roadway excavation with strong mining-induced small coal pillars were studied by using two-way concentrated blasting roof-cutting and pressure relief technology, combined with theoretical analysis, numerical simulation and a field industrial test. A collaborative control scheme of “roof-cutting pressure relief + anchor cable combined support” is proposed. The test results show that when the height of roof-cutting is 8 m, the angle of roof-cutting is 15°, and when the width of the coal pillar is 3 m, the effect of roof-cutting and pressure relief is the best. Through the field blasting test, it was determined that the blast hole spacing was 800 mm, 321 charge structure was used in the intact roof, and 221 or 211 charge structures were used in the broken roof and geological structure zones. During the driving and strong mining period, the roof and floor movement of the roof-cutting section of the roadway excavation was reduced by about 38% compared with the uncut section, and the deformation of the two sides of the roadway was reduced by about 44% compared with the uncut section. It shows that the collaborative control scheme of “roof-cutting pressure relief + anchor cable combined support” has a good effect on the roadway excavation driving of small coal pillars in strong mining.

Study on the failure law of surrounding rock in inclined coal seam with gob side entry

Aiming at the problem of stability control of surrounding rock in the process of upward mining of inclined coal seam gob-side entry retaining, the gob-side entry retaining of 5449 working face in Beipingdong coal mine is taken as the research background. Based on the study of theoretical model of gob-side entry retaining, the law of overburden movement and surrounding rock deformation in the process of gob-side entry retaining is obtained through similar simulation experiment and numerical simulation analysis, and the optimal optimization scheme of supporting structure is put forward. The main research results are as follows: in the process of coal seam mining, the center of gravity of the overlying strata on the whole working face will shift to the side of the goaf, and the supporting structure composed of roadside filling body and bolt effectively inhibits the deformation of the roadway, in which the roadside filling body plays a key role; the top bolt bears greater stress than the side bolt in the process of mining, so the top bolt mainly maintains the stability of the roadway, and the side bolt plays the role of strengthening the side and preventing the extrusion. The width of the roadside filling body should be about 2.5 m, and the plastic failure range can be well controlled under this width.

Study on small coal pillar in gob-side entry driving and control technology of the surrounding rock in a high-stress roadway

The technology described as gob-side entry driving was adopted in the Zhaogu No. 1 mine to resolve the issues pertinent to mining succession tension, serious roadway deformation, and support difficulties. The key parameters of coal columns must be studied to ensure the stable support effect of along-air tunneling under high stress. A calculation for the vertical stress of a coal pillar is proposed, and the location of the breaking line in the top plate in the hollowed mining zone is determined by considering the breakage attributes of the basic uppermost large structure of the key layer, combined with the internal and external stress fields, and the limit equilibrium zone theories. FLAC3D software is employed to simulate the geological requirements of the mine, the characteristics of vertical stress of the coal pillar, and the surrounding rock of the roadway under the technical plan of leaving a 4.5-m coal pillar. The software is also used to verify and analyze the actual measurements. The outcomes suggest that the basic roof break is located at 2.65 m on the side of the coal pillar in the near mining area, and there is a trapezoidal internal stability region around the coal pillar that can provide lasting and stable support of the roadway. The displacement of the two helpers of the roadway is between 100 and 500 mm, and the displacements of the top and bottom plates are less than 300 mm, which ensures the roadway is surrounded by rock when the high-stress requirements occur. The safety of the roadway is determined under high-stress conditions.

Layout timing of mining roadways considering goaf and roof stability

Reasonable roadway layout timing can effectively reduce the deformation of the rock surrounding the roadway and relieve mining and excavation tension. To analyze the mine pressure behavior of the gob-side entry at the edge of a goaf and the roof stability along the strike direction in the middle of a goaf, field observations and particle flow discrete element numerical simulation (PFC) method were performed. The results showed that deformation of the surrounding rock mainly occurred because of roof-to-floor convergence, caused mainly by floor heave. The mechanical behaviors of the rock mass, such as elasticity, fracture, and post-peak softening, could be simulated using the model of a jointed rock mass generated by rigid block elements in the PFC method. Considering the length of the violent and reduced roof activity zones and the activity duration as the indices to determine the basic stability of the goaf, the basic stability distance of the tested goaf edge was 135 m after coal seam mining, and the basic stability time was 27 days. The basic stable distance in the middle of the goaf was 183.4 m after coal seam mining, and the basic stability time was 37 days.

Asymmetric Damage Mechanisms and Prevention Technology in Large-Section Gob-Side Entry Retaining

Based on the deformation and damage characteristics of the surrounding rock in a large-section gob-side entry retaining and combined with field observations and an analysis of the dominant factors, an overall deformation control plan is formulated. The plan mainly includes a structure of high strength, high stiffness and high ductility for roadside support; the reinforcement of the roadway-in support; and the setup entry hydraulic fracturing pressure release mechanism and advanced long, horizontal borehole “fracturing-jet” pressure relief technology. Industrial field tests were completed taking Tangan coal mine as the engineering background. The research shows that the large-section gob-side entry retaining has the typical “asymmetric” overall deformation and damage characteristics, including top coal sinking along the inner side of the roadside support, the whole roadside support structure skewing, and even splitting damage; floor heaving; and the coal-side bolt support structure basically losing support ability and bulging out from the coal side. The dominant factors of deformation damage are disintegration of the floor mudstone by water and deflection deformation under horizontal stress, splitting damage in the concrete roadside support under asymmetric load, damage and expansion due to the insufficient strength of the coal side support, strong dynamic pressure on the roof, and the mutual influence of support and pressure relief. The industrial test shows that the deformation control scheme optimizes the stress environment of the gob-side entry retaining space, the deformation control effect is remarkable, and the roadside fully meets the reuse requirements.

Effects of Aeolian Sand and Water−Cement Ratio on Performance of a Novel Mine Backfill Material

The gob-side entry retaining (GER) technique, as the family member of the pillarless coal mining system, is becoming popular, mainly attributed to its high resource recovery rate and significant environmental benefits. Seeking cost-effective backfill material to develop the roadside backfilling body (RBB) is generally a hot topic for coal operators and scholars. Except for its relatively high cost, the other shortcoming of the widely used high-water backfill material is also obvious when used in arid, semi-arid deserts or Gobi mining areas lacking water. The modified high-water backfill material (MBM) mixed with aeolian sand was recently developed as an alternative to conventional backfill materials. Some critical parameters affecting both the physical and mechanical properties of the MBM, including the amount of the aeolian sand and water-to-powder ratio of the high water-content material, have been experimentally investigated in the present research. Test results showed that the MBM featured high early strength and bearing capability after a large post-peak deformation. In particular, the adjustable setting time of the MBM through changing the amount of sand widens its application in practice. Unlike the high-water backfill material, the MBM is a typical elastoplastic material; the stress-strain curves consist of pore compression, elastic deformation, yielding, and total failure. Note that both the peak and residual strength of the MBM increased as the doping amount of aeolian sand increased, which is probably because of the impacted aeolian sand and the uniform reticular structure of the ettringite in the MBM. Compared with the high-water backfill material, only limited cementitious material and water resources are requested to cast the RBB, which provides more economical and environmental benefits for the application of the GER technique in the arid, semi-arid deserts or the Gobi mining areas.

Numerical Simulation Study on Mechanism of Roof Directional Breaking in Gob-Side Entry Formed Automatically Without Coal Pillars

Numerical Simulation Study on Mechanism of Roof Directional Breaking in Gob-Side Entry Formed Automatically Without Coal Pillars

Thick-Anchored Dual-Layer Locking Supporting Technique in Gob-Side Entry Driving with the Narrow Pillar: A Case Study

Roadway maintenance of the gob-side entry driving with narrow pillars is a key issue that needs to be considered in underground mining. In this paper, a thick-anchored dual-layer locking (TADL) supporting technique is proposed, in which, the flexible bolt and the cable are used as the main bodies of the foundation supporting layer and the deep reinforcement layer, respectively. In the instantaneous deformation stage after excavation, TADL is applied to form a thick-anchored bearing structure in time, aiming to achieve the following effects: (1) prevent the delamination of the roadway at the initial stage of excavation, (2) avoid the reduction of the bearing capacity of the surrounding rock structure, and (3) effectively control the plastic zone development and surrounding rock deformation of roof strata. Finally, the mining conditions of gob-side entry driving with narrow pillars are selected for engineering application and verification. The results of numerical simulation and field industrial tests show that the TADL support technique can effectively control the deformation of surrounding rock and drive away tensile stress. Compared with the original support scheme, the roof displacement of the roadway is significantly reduced by more than 16% after TADL support. This study provides a reference for the stability control of roadway roofs under similar conditions.

Mechanics Principle and Implementation Technology of Surrounding Rock Pressure Release in Gob-Side Entry Retaining by Roof Cutting

Improving coal resource mining rates has long been a focus of coal industry research. The gob-side entry retaining by roof cutting (GERRC) is a new coal mining technology that has gained popularity in China due to its high mining rate and safety. Based on the GERRC technology, the precise technical procedure is elaborated, and the fundamental idea of pressure relief is explored through creating a structural mechanics model of the surrounding rock in this paper. The results of mechanical analysis show that the primary mechanism of roof pressure release is to weaken the integrity of the roof, thereby reducing the ultimate bending moment of the rock stratum. In addition, an additional strategy for pressure release is suggested in this research, involving the weakening of the roof rock by the creation of dense pressure-released holes. The results of the engineering field experimentation demonstrate that the dense pressure-released holes can completely replace the conventional blasting technology of the past to achieve the effect of releasing the roof pressure, thus avoiding the use of hazardous and challenging-to-obtain explosives and demonstrating safety, reliability, and feasibility.

Study on Size Optimization of a Protective Coal Pillar under a Double-Key Stratum Structure

Because of the problem that the size of a coal pillar is difficult to determine and it is easy to waste coal resources, taking the Sangou Xindu coal industry as the engineering background, this paper examined the roof cutting and pressure relief technology of small coal pillar gob-side entry driving by means of theoretical analysis, laboratory experiments, and numerical simulation. Through theoretical calculations, the coal pillar width should not be less than 7.15 m without cutting the top. According to the theory of key strata, there are two key strata in the overlying strata on the working face, namely the main key strata and the subkey strata. Through numerical simulation, the stress evolution characteristics of the coal at the side of the goaf under the double-key stratum roof cutting structure are studied. When the roof is not cut, the roof cutting height is 6 m and 12 m and the width of the lateral stress reduction zone in the goaf is 4 m, 8 m, and 10.5 m, respectively. Therefore, the cutting top height is determined to be 12 m and the hydraulic fracturing cutting plan is designed. After cutting the top of the main key stratum, the size of the coal pillar is optimized. Three schemes of coal pillar sizes of 6 m, 5 m, and 4 m are designed for simulation and the analysis shows that it is most reasonable to keep a 5 m wide coal pillar. After field application, the deformation of the surrounding rock in the roadway is within a controllable range and the roadway use is good.

Ground response mechanism of entries and control methods induced by hard roof in longwall top coal caving panel

This paper presents an integrated approach for theoretical analysis and numerical modeling to investigate the ground response mechanism of entries and control methods induced by hard roof in longwall top coal caving panel (LTCC). The test site is located in the city of Jinzhong, Shanxi Province, China. A roof mechanical model for the main roof's different breaking positions was created to study the ground response mechanism of gob-side entries. The theoretical analysis indicated that the closer the main roof fracture line is to the gob, the smaller the lateral cantilever length of the main roof above the gob, and the surrounding rock of the roadway are more stable. Therefore, an innovative approach is proposed t to reduce the lateral cantilever length of the main roof above the gob area and automatically retain entry. This approach involves applying the “directional roof split blasting technique for pressure relief and automatic retained entry” (DRSBPRRE) in the thick coal seam of the LTCC panel. Meanwhile, numerical modeling results verified the results of the theoretical analysis. The results of the numerical simulation and practical application suggest that the DRSBPRRE approach can eliminate the cantilever length of the main roof, significantly reduce the peak vertical stress in the solid coal rib by approximately 13.76 % and reduce the surrounding rock damage and deformation. Therefore, the DRSBPRRE approach is a safe and efficient mining method for pressure relief and entry retention. This research could provide an important reference for thick coal seam LTCC panel non-pillar coal mining technology.

Mechanism and Application of Roof Cutting by Directional Energy-Cumulative Blasting along Gob-Side Entry

In this paper, roof-cutting technology of directional energy-cumulative presplitting blasting is taken as the research object. Through the numerical simulation software Ansys/Ls-dyna3D, the process of energy-cumulative blasting and non-cumulative blasting is simulated and analyzed by using the ALE algorithm. Moreover, the evolution processes of tensile strain energy, detonation stress wave of explosives, stress state of rock mass, and rock crack damage cumulative are compared in two conditions. In the energy cumulative state, the detonation wave acts centrally on the hole wall in the energy cumulative direction to form an initial crack, and then under the action of the jet of energy cumulative, the crack continues to propagate until it runs through. In the non-energy cumulative state, the crack propagates uniformly around the hole wall, forming irregular short cracks. The simulation is verified by the field test, and the law of crack propagation is the same with the simulation. Therefore, directional energy-concentrated presplitting blasting has good practicability in a roof presplitting operation.