Entry Retaining Research Articles

Study of An Innovative Approach of Roof Presplitting for Gob-Side Entry Retaining in Longwall Coal Mining

Gob-side entry retaining (GER) is a hot issue with regard to saving resources and reducing the drivage ratio in longwall mining. This paper investigates an innovative approach of roof presplitting for gob-side entry retaining (RPGER). RPGER uses the directional cumulative blasting to split the roof in advance. The rock roof within the presplitting range caves in gob after mining. The caved gangue can become the natural rib of the gob-side entry and expands to be the natural supporting body for resisting the upper roof movement. A numerical model of RPGER was established by the discrete element method (DEM), which showed that the supporting effect by the expanded gangue was well functioning. The gob-side entry was in pressure-relief surroundings and featured in the lesser deformation. The roof presplitting design method was presented and validated with a field test. The test illustrated that RPGER reduced the mining pressure on the retained entry side. The expanded gangue on the entry side was gradually compacted. It is the compaction process that played the role of reliving mining pressure, and the compacted gangue became the effective rib of the gob-side entry. The retained entry in the pressure-relief surroundings would stabilize a lagging distance behind the working face. The gob-side entry after stabilization met the entry retaining and the safety production requirements. This work illustrates the mechanism of RPGER and validates its feasibility and efficiency.

Study on Three-Dimensional Stress Field of Gob-Side Entry Retaining by Roof Cutting without Pillar under Near-Group Coal Seam Mining

In order to explore the distribution law of stress field under the mining mode of gob-side entry retaining by roof cutting without pillar (GERRCP) under goaf, based on the engineering background of 8102 and 9101 working faces in Xiashanmao coal mine, the stress field distribution of GERRCP and traditional remaining pillar was studied by means of theoretical analysis and numerical simulation. The simulation results showed that: (1) in the front of the working face, the vertical peak stress of non-pillar mining was smaller than that of the remaining pillar mining, and it could effectively control stress concentration in surrounding rock of the mining roadway; the trend of horizontal stress distribution of the two was the same, and the area, span and peak stress of stress the rise zone were the largest in large pillar mining and the minimum in non-pillar mining. (2) On the left side of the working face, the vertical stress presented increasing-decreasing characteristics under non-pillar mining mode and saddle-shaped distribution characteristics under the remaining pillar mining mode respectively. Among them, the peak stress was the smallest under non-pillar mining, and compared with the mining of the large pillar and small pillar, non-pillar mining decreased by 12–21% and 3–10% respectively. The position of peak stress of the former was closer to the mining roadway, indicating that the width of the plastic zone of the surrounding rock of the non-pillar mining was smaller and bearing capacity was higher. In the mining of the large and small pillar, the horizontal stress formed a high stress concentration in the pillar and 9102 working face respectively. In non-pillar mining, the horizontal stress concentration appeared in solid coal, but the concentration area was small.

Entry Deformation Law in the Full Life Cycle Under Entry Retaining with Roof Cutting

In roof cutting pressure releasing gob-side entry retaining (RCPRGER) technology, the full life cycle of the retained entry includes two main parts: the entry retaining and retained entry reuse. The existing studies all focus on the process of entry retaining, while the studies of the whole life cycle and reuse process are relatively deficient. To compensate for the deficiency in the existing research and explore the deformation law of the entry in the whole life cycle under entry retaining with roof cutting, this paper takes the 8304 and 8305 working faces of the Tashan Coal Mine as an example, and the entry deformation evolution in the retaining and reuse processes is studied. First, this paper summarizes the technological process of RCPRGER and reuse and then deduces the stress evolution process of entry surrounding rocks, specifically including the analysis of the roof cutting short beam structure model. Subsequently, the monitoring schemes of the entry deformation and mine pressure in the working face within the retaining and reuse processes are proposed. Finally, through the practical examples of test working faces, it is found that the two entry sides move symmetrically between the upper and lower parts. The main deformation in the early stage of the entry retaining is the displacement of the gangue wall, and the main deformation in the later stage and reuse stage is the displacement of the coal wall. The roof and floor move asymmetrically between the left and right parts, the deformation of the roof cutting side is larger at each stage, and the roof subsidence is the main source of deformation.

Key Parameters of Roof Cutting of Gob-Side Entry Retaining in a Deep Inclined Thick Coal Seam with Hard Roof

Gob-side entry retaining by roof cutting (GERRC) employed in a deep inclined thick coal seam (DITCS) can not only increase economic benefits and coal recovery, but also optimize surrounding rock structure. In accordance with the principles of GERRC, the technology of GERRC in DITCS is introduced and a roof-cutting mechanical model of GERRC is proposed to determine the key parameters of the depth and angle of RC. The results show that the greater the RC angle, the easier the caving of the goaf roof, but the length of cantilever beam increases. The depth of RC should account for the dip angle of the coal seam when the angle is above 20°. Increasing the coal seam dip angle could reduce the volume of rock falling of the goaf roof, but increase the filling height of the upper gangue to slide down. According to numerical model analysis of the stress and displacement of surrounding rock at different depths and angles of RC, when the depth of RC increased from 9 m to 13 m, the distance between the stress concentration zone and the coal side is increased. When the angle of RC increased from 0° to 20°, the value of roof separation is decreased. GERRC was applied in a DITCS with 11 m depth and 20° RC angle, and the field-measured data verified the conclusions of the numerical model.

Mechanical Analysis of Weakly Cemented Roof of Gob-side Entry Retaining in Fully-Mechanized Top Coal Caving Mining

To investigate the mine pressure behavior of weakly cemented roof of gob-side entry retaining (GER) in fully-mechanized top coal caving mining, the structural characteristics of lateral immediate roof in 4-1101 working face of Awuzisu Coal Mine were studied by using of numerical simulation, theoretical analysis and in-site measurement. Mechanical model of “equivalent block” composite structure of immediate roof was established, and verified by field test. The results indicate that weakly cemented roof has a large caving range, and it is easy to form the “equivalent block” composite structure in mechanized top caving mining. This structure affects roof breaking, and provide vertical constraint to the roof of GER. It explains the phenomenon that, the roof pressure of GER increases with the advancing of the working face, but the growth rate decreases gradually before the main roof breaking. This research can provide reference to roadside support design and surrounding rock control of GER with weakly cemented roof.

Distribution and Evolution Characteristics of Macroscopic Stress Field in Gob-Side Entry Retaining by Roof Cutting

Non-pillar mining technology plays an important role in sustainable exploitation of coal resources. Gob-side entry retaining by roof cutting (GERRC) is a new technique regarding a non-pillar mining method based on the “cutting cantilever beam theory”. In order to study the distribution and evolution characteristics of macroscopic stress field of surrounding rocks in GERRC, and find out differences from traditional pillar retained mining method (PRMM), some key issues about abutment pressure and stress concentration shell were analyzed by numerical simulations. Results show that: (1) distribution characteristics of lead abutment pressure for the two mining methods are basically the same during the primary mining stage; (2) during the secondary mining stage, peak stress in front of mining face of the two modes is close, while the abutment pressure can be transferred more evenly to coal and rock mass far away from goaf when using GERRC, and that is more likely to cause high stress concentration in the coal mass near the goaf when mining with PRMM; (3) for the PRMM, lateral abutment pressure will produce a higher stress concentration in section coal pillar and a high residual stress will be maintained in it when coal masses on both sides of the pillar are extracted; (4) adjacent goafs can be connected to form a wide range of pressure relief region when using GERRC, and shape of distribution of the maximum principal stress appears a semi-space ellipsoidal shell in three-dimensional space, and it is a single arch shape in the section perpendicularly to the mining direction, however, it looks like a “m” shape in that section when mining with PRMM because of the high stress concentration in the coal pillar.

The Stability of Gob‐Side Entry Retaining in a High‐Gas‐Risk Mine

There are series of problems faced by most of the coal mines in China, ranging from low‐coal recovery rate and strained replacement of working faces to gas accumulation in the upper corner of coalfaces. Based on the gob‐side entry retaining at the No. 18205 working face in a coal mine in Shanxi Province, theoretical analysis, numerical simulation, and engineering practice were comprehensively used to study the mechanical characteristics of the influence of the width of the filling body beside the roadway and the stability of surrounding rock in a high‐gas‐risk mine. The rational width of the filling body beside the roadway was determined, and a concrete roadway‐side support with a headed reinforcement‐integrated strengthening technique was proposed, which have been applied in engineering practice. The stability of the filling body beside the roadway is mainly influenced by the movement of the overlying rock strata, and the stability of the surrounding rock can be improved effectively by rationally determining the width of the filling body beside the roadway. When the width of the roadway‐side filling body is 2.5 m, the surrounding rock convergence of the gob‐side entry retaining is relatively small at only 5% of the convergence ratio. It has been shown that the figure for roof separation is relatively low, and strata behaviors are relatively alleviated and gas density do not exceed the limit, which are the best results of gob‐side entry retaining. The results of this research can provide theoretical guidance for excavation of coal mines with similar geological conditions and have some referential significance to safety and efficient production in coal mines.

The Blocking Mechanism of the Vertical Feeding System of Roadside Support Body Material for Backfilling Gob‐Side Entry Retaining

Reliable operation of the feeding system plays a crucial role in ensuring the safe and efficient production of the working face of backfilling gob‐side entry retaining (GER). In the process of vertical feeding of the roadside support body material, the problem of blocking of the feeding shaft has occurred to the test mine, which seriously affects the production safety in mines. In this paper, based on the theoretical analysis, a fluid‐solid coupling numerical model was established. The change rules of the speed of sacked gangue, pressure of air below it, and speed vector distribution with different vent diameters were obtained. The blocking mechanism of the feeding system was revealed. The results show that if the exhaust vent of the stock bin was shut, the speed of gangue in the mine increased and then decreased and finally blocked in the feeding shaft. If the exhaust vent of the stock bin was opened for pressure discharge, with the increase of diameter of the exhaust vent, the maximum speed and ending speed of sacked gangue increased, pressure differential reduced, and speed vector was uniformly distributed. The energy criterion of blocking of the feeding shaft was further obtained. Based on the engineering conditions of the test mine, when the feeding shaft is blocked, the critical value of diameter of the exhaust vent is 30 mm. The research results provide basis for the design of key parameters of the vertical feeding system, ensuring the safe and efficient production of gob‐backfilled GER working face.

Key Technologies and Application Test of an Innovative Noncoal Pillar Mining Approach: A Case Study

The waste of coal resources, a complicated production process and slow mining speed seriously restrict the rapid development of longwall mining. To achieve effective mining, an innovative noncoal pillar mining approach (i.e., Gob-side Entry Retaining by Roof Cutting (GERRC)) was introduced. The mechanism of the GERRC approach and its three key technologies (i.e., roof support technology, directional presplit cumulative blasting technology and surrounding rock control technology) were studied by theoretical analysis, numerical simulation, laboratory and field experiments. The new approach was finally tested under medium-thick coal seam and compound roof conditions. The results show that the directional presplit cumulative blasting technology can effectively control the damage evolution in the roof rock, maintain the integrity of the entry roof and contribute the gob roof to the cave in time. The support technologies in different roof movement stages can control the entry surroundings, and the final section of the retained entry met the safety production requirements. The test results suggested that the proposed approach for coal effective mining is feasible, and the introduced key technologies and design methods potentially produce reasonable values for applications of pillarless mining in similar projects.

Multiple and Long-Term Disturbance of Gob-Side Entry Retaining by Grouped Roof Collapse and an Innovative Adaptive Technology

The demand for gob-side entry retaining (GSER) technology is extensive in Chinese coal mines because of its outstanding advantages. However, due to the occurrence of long-term roof movement disturbances, maintaining the conventional GSER over long distances is difficult. The urgent demand for this technology and its difficult maintenance are prominent contradictions faced in its application. In this paper, two characteristics of strata movement after mining that have great influence on GSER are determined in a physical simulation experiment. One is that the roof layers, which are dominated by the key strata (KS), collapse in multiple groups to form two-directional periodic pressures and superposed disturbances. The other is that the movement of the main roof will experience three stages of deformation, as follows: bending subsidence before collapse, sinking deformation at collapse and compressive deformation after collapse. In particular, the gradual compression of the bulging gangue in the gob extends the disturbance cycle. Therefore, it is difficult to maintain the GSER over long distances because of the long-term and multiple breaking disturbances of the roof layers. Mechanical models of KS breaking and GSER are established, and a method to determine the timing and strength of KS disturbances are proposed. Making use of the characteristics of staged collapse and fluctuating weighting of the overlying strata, an innovative technology named short-staged GSER is proposed. This technology maintains the maximum length of GSER within the optimal length, which ensures that the entry avoids superposed disturbances, and reduces the maintenance difficulty. The method for determining the key technical parameters is discussed, and an engineering case in panel 24202 of the Shaqu Mine in China is presented. From a back-calculation of measurements, the engineering practice demonstrates that the surrounding rock mass is stable, and the deformed entry size is safe when the length of the short-staged GSER does not exceed 100 m.

Research on the Design of Roof Cutting Parameters of Non Coal Pillar Gob-side Entry Retaining Mining with Roof Cutting and Pressure Releasing

With the increasing tension of current coal resources and the increasing depth of coal mining in China, Non Coal Pillar Gob-side Entry Retaining Technology has become a preferred coal mining method in underground coal mines. In the current study of the non coal pillar gob-side entry retaining mining with roof cutting and pressure releasing, the theory and design of roof cutting under near-level conditions have formed a certain system. However, there is a lack of systematic research on the design of the roof cutting and the connection requirement of the cutting seam and other aspects about the inclined coal seam. Therefore, choosing the concrete representative target mines as the research foundation, using mechanics analysis, geometric analysis, numerical simulation and other means, on the basis of considering the change of mining height, this paper mainly studies about the effect of coal seam inclination on roof slit depth and angle, then get the three steps of the cutting height design, the three considerations of the roof cutting angle design and the connection rate requirement of the roof cutting. At last the key roof cutting parameters of the three target mines are calculated accordingly. Based on the existing research, this paper puts forward the concept and design method of roof cutting connectivity rate. The design system of roof cutting seam parameters of non coal pillar gob-side entry retaining mining with roof cutting and pressure releasing is further improved, and the corresponding calculation method is deduced for the design of the key parameters of the inclined coal seam and the gently inclined coal seam, and proposing the second roof cutting scheme of the upper side gob-side entry retaining.

Key Parameters of Gob-Side Entry Retaining in A Gassy and Thin Coal Seam with Hard Roof

Gob-side entry retaining (GER) employed in a thin coal seam (TCS) can increase economic benefits and coal recovery, as well as mitigate gas concentration in the gob. In accordance with the caving style of a limestone roof, the gas concentration and air pressure in the gob were analyzed, and a roof-cutting mechanical model of GER with a roadside backfill body (RBB) was proposed, to determine the key parameters of the GER-TCS, including the roof-cutting resistance and the width of the RBB. The results show that if the immediate roof height is greater than the seam height, the roof-cutting resistance and width of the RBB should meet the requirement of the immediate roof being totally cut along the gob, for which the optimal roof-cutting resistance and width of RBB were determined by analytical and numerical methods. The greater the RBB width, the greater its roof-cutting resistance. The relationship between the supporting strength of the RBB and the width of the RBB can be derived as a composite curve. The floor heave of GER increases with increasing RBB width. When the width of the RBB increased from 0.8 m to 1.2 m, the floor heave increased two-fold to 146.2 mm. GER was applied in a TCS with a limestone roof of 5 m thickness; the field-measured data verified the conclusions of the numerical model.

Roof Deformation Characteristics and Preventive Techniques Using a Novel Non-Pillar Mining Method of Gob-Side Entry Retaining by Roof Cutting

A new non-pillar mining technology, gob-side entry retaining by roof cutting (GERRC), different from the conventional gob-side entry retaining formed by a roadside filling support, is introduced in this study. In the new technology, roof cutting is conducted so that the roof plate forms a short cantilever beam structure within a certain range above the retained entry, thus changing the stress boundary condition of the roof structure. To explore the deformation characteristics of the roof under this special condition, a short cantilever beam mechanical model was established and solved using energy theory and displacement variational methods. Meanwhile, a theoretical and analytical control solution for roof deformation was obtained and verified via field-measured results. Based on the aforementioned calculation, the relationship between the roof deformation and main influence parameters was explored. It was concluded that the rotation of the upper main roof and width of the retained entry had the most significant impacts on roof deformation. Bolt and cable support and temporary support in the entry had a non-obvious influence on the roof deformation and could not prevent the given deformation that was caused by the rotation of the upper main roof. Based on comprehensive theoretical analysis and calculation results, ideas and countermeasures to control short cantilever roof deformation—that is, designing a reasonable height of roof cutting and a controlled width of retaining entry—were proposed and tested. Field monitoring shows that the entry control effects were satisfactory.

Analysis on Rockbolt Support Interaction with Roof Dilatancy Above Roadside Backfill Area in Gob-Side Entry Retaining

To overcome the challenge to the roof dilatancy deformation control above roadside backfill area of gob-side entry retaining for confining pressure decreasing, a mechanical model of rockbolt reinforcement to the roof above roadside backfill area was established. The shear strength of the shear sliding zone above roadside backfill area considering rockbolt support was deduced. Factors and effect of rockbolt reinforcement to the shear sliding zone of the roof above roadside backfill area based on the 3107 working face of Xinyuan coal mine were obtained. Moreover, a numerical model was established to analyze rockbolt support for roof dilatancy deformation control. Mechanical results show that the shear strength increment from rockbolt axial force is positively related with rockbolt arrange density and rockbolt pretension while the shear strength increment from rockbolt shear force is positively related with rockbolt diameter, rockbolt arrangement density. Numerical results indicate that rockbolt support improves the confining pressure environment around the gateway excavation and effectively restrains roof dilatancy deformation of lower confining pressure region. Field tests and field monitoring indicated that the gob-side entry retaining in the 3107 working face with a width of 2.0 m roadside backfill body and high-strength rockbolt support for the roof above roadside backfill area could meet the requirement for gas drainage and ventilation when 3107 working face was retreating.

Experiment on Mine Ground Pressure of Stiff Coal‐Pillar Entry Retaining under the Activation Condition of Hard Roof

In mining excavation, the retained entry with stiff coal pillar is situated in the strong mine ground pressure. Influenced by mining abutment stress and dynamic stress (the vibration signal) induced from the hard roof activation, the retained entry may be subjected to roof separation, supporting body failure, severe floor heave, and even roof collapse. Based on a 2D physical model, an experimental method with plane‐stress conditions was used to simulate the mechanical behavior of the rock strata during mining. In this experiment, three monitoring systems were adopted to reveal the characteristics of the strong mine ground pressure in the stiff coal‐pillar entry retaining. The results show that the hard roof undergoes bending down, fracture, and caving activation successively until it is able to support overlying loads. The abutment stress which is induced from the loading transfer in stiff coal pillar is larger than that in other rocks around the retained entry in amplification, and overlying loads above the worked‐out area have a loading effect on the unworked‐out area. When the hard roof is situated in the activation state, the dynamic stress is generated from the hard roof activation, which is verified by the great saltation of acoustic emission signals. The results of mining ground pressure in the physical model can clearly illustrate the mechanical behavior of the rock around the retained entry with stiff coal pillar.

An Experimental Investigation on the Mechanical Properties of Gangue Concrete as a Roadside Support Body Material for Backfilling Gob‐Side Entry Retaining

Development of a safe and economical roadside support body (RSB) material is the key to successful backfilling gob‐side entry retaining (GER). By means of laboratory tests, this paper studied the effects of the water‐cement ratio, aggregate content, and age on the contractibility and resistance increasing speed, compressive strength, and postpeak carrying capacity of the concrete with gangues as an aggregate. It also discussed the rationality and adaptability of gangue concrete as a RSB material for backfilling GER. The experimental results show that the compressive strength of gangue concrete increases with age, and that the strength of gangue concrete demonstrates a nonlinear decreasing trend with the increase of the cementing material’s water‐cement ratio. The water‐cement ratio in the range of 0.46–0.60 has the most significant regulation effect on the strength of gangue concrete. Mixing with a certain amount of coal gangue enhances the postpeak carrying capacity of concrete, preventing the sample from impact failure. The field experimental results report that as a RSB material, gangue concrete can meet the design and application requirements of GER with gangue backfilling mining. A RSB material featuring high safety, high waste utilization rate, fast construction speed, and low costs is provided.

An Innovative Approach for Gob-Side Entry Retaining in Thick Coal Seam Longwall Mining

Gob-side entry retaining (GER) is a popular non-pillar mining technique regarding how to reserve a gateroad for the use of next panel mining. When used in thick coal seams, the conventional entry retaining method requires a huge amount of filling materials and may cause entry (gateroad) accidents. Thus, an innovative non-pillar longwall mining approach is introduced. First, structural and mechanical models were built to explore the mechanism of the new approach. The modeling results indicate that effective bulking of the gob roof and reasonable support of the entry roof were key governing factors in improving entry stabilities and reducing roof deformations. Accordingly, a directional roof fracturing technique was proposed to contribute to gob roof caving, and a constant resistance and large deformation anchor (CRLDA) cable was used to stabilize the entry roof. Subsequently, the evolutionary laws of the roof structure and stresses were explored using numerical simulation. It was found that the structure of the surrounding rocks around the retained entry changed significantly after roof fracturing. The stress-bearing center was transferred to the gob area, and the entry roof was in a low stress environment after adopting the approach. Finally, the approach was tested on a thick coal seam longwall mining panel. Field monitoring indicates that the retained entry was in a stable state and the index of the retained entry met the requirement of the next mining panel. This work provides an effective and economical approach to non-pillar longwall mining in thick coal seams.

Soft Roof Failure Mechanism and Supporting Method for Gob-Side Entry Retaining

To study the soft roof failure mechanism and the supporting method for a gateway in a gently inclined coal seam with a dip angle of 16° kept for gob-side entry retaining, and through the methodology of field investigation and numerical and analytical modeling, this paper analyzed the stress evolution law of roof strata at the working face end and determined that the sharp horizontal stress unloading phenomenon along the coal wall side did not appear after the working face advanced. Conversely, the horizontal stress along the gob side instantly decreased and the tensile stress produced, and the vertical stress in the central part of the roof had a higher reduction magnitude as well. An in-depth study indicates that the soft roof of the working face end subsided and seriously separated due to the effect of the front abutment pressure and the roof hanging length above the gob line, as well as certain other factors, including the rapid unloading of the lateral stress, tension and shear on the lower roof rock layer and dynamic disturbance. Those influencing factors also led to rapid crack propagation on a large scale and serious fracturing in the soft roof of the working face end. However, in the gob stress stabilized zone, the soft roof in the gob-side entry retaining has a shearing failure along the filling wall inside affected by the overburden pressure, rock bulking pressure, and roof gravity. To maintain the roof integrity, decrease the roof deformation, and enable the control of the working face end soft roof and the stabilization of the gob-side entry retaining roof, this study suggests that the preferred bolt installation angle for the soft roof situation is 70° based on the rock bolt extrusion strengthening theory.

Numerical Simulation of Gas Transportation in Mechanized Top-Coal Caving with Gob-Side Entry Retaining

To solve the problem of gas accumulation in thick-seam top-coal caving mining face, goaf gas migration equations were established based on seepage theory and the law of mass conservation. Taking No.7602 working face of Wuyang coal mine as the study object, gas migration rules of goaf in ventilation of U + I and Y+I were simulated by Fluent, and comparative analysis and verification were carried out by in-situ monitoring data. Test indicates that after gob-side entry retaining, average gas concentration value of No.7602 working face Return airway decreased by 39% compared with similar working face, decreased by 39% in gas emission roadway, decreased by 42% in upper corner. Ventilation of U + I can successfully resolve the problem of gas accumulation in upper corner and working face. The conclusion has a good instruction value to gas control in thick-seam top-coal caving mining face.

Application on Gob-Side Entry Retaining Technology with Coal Gangue Bag Packing in Medium-Thick Coal Seams

The key point of production smoothing in coalmine is regular continuing excavate, the gob-side entry retaining technology with coal gangue bag packing not only solve the continuing excavate problem, but also facilitate the meticulous mining. The selection of the waste filling technique and the determination of basic parameters of filling body are the keys to whether gob-side entry retaining technology success or not. Due to the thicker thickness in medium-thickness coal seam and the higher height of crossheading, it is propose that the gob-side entry retaining technology need to be improved much higher. Therefore, the determination of basic parameters of filling body is particularly important.