Roof Subsidence Research Articles

Deformation characteristics of a roadway in steeply inclined formations and its improved support

Mining roadways in steeply inclined rock layers commonly experience severe deformation in form of large asymmetric deformation, roof subsidence and floor heave, which significantly threaten the stability of a roadway. Using a coal exploration roadway in Anhui Province, China as a case study, the deformation characteristics of a roadway in steeply inclined rock formations and the corresponding improved support scheme is analysed by numerical simulation. The simulation is performed using a cohesive element to consider the influence of geologic bedding plane around the roadway. The results show that: due to the spatial relationship of the roadway and the two bedding planes (i.e. interfaces), rocks above the roadway move downward along the interface, while rocks below the roadway move upward along the interface, the maximum deformation occurs along the upper left and lower right sides of the roadway surface, which accounts for the large leaning asymmetric deformation of the roadway (right inclination). An improved support scheme for the roadway is proposed, using long cables to restrain severe deformation, short cables and enclosed U-shaped steel sets to improve the ground stress state. The proposed scheme can effectively restrain floor heave, manage large asymmetrical deformation compared to the original scheme.

Mechanism and Prevention of Rockburst in Steeply Inclined and Extremely Thick Coal Seams for Fully Mechanized Top-Coal Caving Mining and Under Gob Filling Conditions

The steeply inclined and extremely thick coal seams (SIETCS) under the condition of gob filling frequently suffer from the occurrence of rockbursts. Figuring out the mechanisms of rockbursts under this condition for taking targeted measures to mitigate rockburst hazards in SIETCS is of great significance. Using the typical SIETCS with an average dip angle of 87° at Wudong Coal Mine (WCM) as a case study, a mechanical model and elastic deformation energy (EDE) function of a “steeply inclined suspended roof structure” was developed, and the influence factors were analyzed by theoretical analysis. Simultaneously, the rockburst risk assessment was carried out based on the theory of a rockburst start-up. The pressure relief measures are optimized by comparing the pressure relief effects of three kinds of destress blasting schemes. The results indicate that the damage characteristics of rockburst are mainly floor heave, the sidewall’s inward deformation and roof subsidence. The damage degree of headentry on the roof side is more severe than that of tailentry, and the resultant impacts showed the directionality from the roof side to the coal side. The steeply inclined and suspended roof breakage is one of the main causes for the occurrence of rockbursts. The EDE of the roof increases with an increasing dip angle of the coal seam from 0° to 72.6° and then decreases as the dip angle increases. Furthermore, that increase is accompanied by the decrease of the lateral pressure coefficient and the supporting force coefficient. The EDE stored in the roof is sufficient to cause roof breakage and induce rockburst after the complete roof exceeds a certain length. The mechanism of rockburst in SIETCS for fully mechanized top-coal caving mining under gob filling conditions was proposed, i.e., “high compressive stress concentration plus breakage of the suspended roof-induced stress” rockburst, and this is further verified by ground destruction, microseismic (MS) monitoring and numerical modeling. The results also indicate that alternate deep and shallow hole-blasting modes are more suitable for pressure relief in SIETCS.

Self-bearing Characteristics and Control Strategy of a Cataclastic Regenerated Rock Mass in a Roadway Roof with Top Remains: A Case Study

A roadway roof has compound and cataclastic features when tunneling a roadway under a gob with top remains in downward mining. To identify the self-bearing characteristics and to develop a control strategy for a regenerated roadway roof with top remains (RRRTR), mechanical and numerical models of a RRRTR were built on the basis of a structural model of a compound and cataclastic roof. Through a theoretical analysis and numerical simulations, the formation and instability criterion of a three-hinged arch in a RRRTR were analyzed. Meanwhile, the self-bearing characteristics of a RRRTR with various crack spacing values in the top remains were studied. On this basis, a bolt-net-belt (BNB) support scenario was proposed to control the RRRTR and an on-site roof subsidence monitoring task was conducted. The theoretical and numerical analyses both indicate that a three-hinged arch exists in the roadway top remains, and it has a certain bearing capacity. The compressive failure of the hinge points is an instability trigger of a three-hinged arch that would trigger the entire failure of the RRRTR. The crack spacing values of the top remains influenced the formation and bearing capacity of the three-hinged arch. The three-hinged arch is easier to establish and has a better bearing effect when the top remains have a larger crack spacing value. The BNB support scenario could not only control the roof subsidence but also facilitate the potential self-bearing capability of the RRRTR.

Roadway rock pressure appearance law of roof-cutting and pressure-relieving retaining roadway in medium thick seam

Through long-term monitoring of the displacement of the roof and the pressure changes of various supporting methods along the retaining section of No. 9 coal seam in the Dianping coal mine, the mechanical characteristics of the displacement of the roof and various supporting methods were analyzed. It has been found that the top layer of the roadway section has a longer aging effect and a lower value than the roof subsidence. The maximum pressure and average pressure of the side support side of the roadway are slightly lower than the pressure of the left side of the upper roadway. The initial pressure and cycle of the roadway side are slightly higher than the roadway side of the upper roadway. The constant resistance large deformation anchorage is more adaptable to the fluctuation of the roof pressure in a wide range than the door bracket.

3D modelling of the strength effect of backfill-rocks on controlling rockburst risk: a case study

Rockburst is a typical form of dynamic disasters in coal mines. Reducing the risk of rockburst is an important measure to ensure safety and safe production from coal mines. According to the geological conditions of Hengda coal mine, the deformation deflection equation of the roof and the deformation energy equation of the surrounding rocks are derived from the elastic thin plate theory model. Based on these equations, the criterion of the elastic energy density was updated. Then, the roof movement, stress distribution, and elastic strain energy evolution law of the working face when employing different strengths for the backfill-rocks were analyzed using FLAC-3D. A method aiming to reduce the risk of rockburst by quantitatively adjusting the strength of refilled rocks was proposed. The results show that increasing the strength of refilled rock can effectively control the roof subsidence of the working face and improve the concentrated distribution of stress around the goaf during backfill mining. At the same time, it can effectively reduce the degree of the influenced area and the stress concentration in front of the working face and the elastic strain energy of the roof to minimize the risk of rockburst.

Surrounding rock failure analysis of retreating roadways and the control technique for extra-thick coal seams under fully-mechanized top caving and intensive mining conditions: A case study

Most retreating roadways in extra-thick coal seams, are affected by multiple mining activities under fully-mechanized top caving mining conditions. Surrounding rock deformation and stress evolution of the roadways are complex under intensive mining activities due to interlayers occurrence in top thick coal roof developed in fissures and fractures in coal seams, and large cross-section of the roadways. If no appropriate support techniques and optimization measures are taken under such mining conditions, consequences such as severe bedding separation, large deformation of surrounding rock, and large-area top caving disaster, can occur. Therefore, the investigation of surrounding rock deformation and failure mechanism of retreating roadways and the control technique for extra-thick coal seams under fully-mechanized top caving and intensive mining conditions, is the key for stability control for this type of roadway. Various research methods were adopted comprehensively in this study including field investigation, monitoring and statistics, experimental tests, physical similarity simulation, and field testing. Taking the return airway # 5110 in the main mining coal seam # 3–5 with the thickness of 14.7 m of Tashan coal mine, Datong Coal Mine Group as the case, failure characteristics of roof supporting structures of the return airway # 5110 undergoing retreating mining of the upper coal panel # 8109 and the current mining panel # 8110 were investigated. By employing bedding separation acquisition monitoring device and force-meters, surrounding rock deformation, carrying the load of rock bolts and cables, and bedding separation, were monitored and analyzed. The monitoring was done by the surrounding deformed rock where loading characteristics of the supporting structures in the roadway # 5110 were obtained under intensive mining conditions for the extra-thick coal seam. Stress evolution during the process from roadway stability to instability was characterized experimentally under different lateral pressure conditions by using physical similarity simulation. On this basis, the single mechanical yieldable prop with high constant resistance was proposed to support the surrounding rock of retreating roadways in extra-thick coal seams. The analysis was carried on the structure and on the working mechanisms of the prop set where its working resistance under the height of 3 m, support patterns, and support techniques, and then field tests were conducted in the retreating roadway # 5110. The typical monitoring results showed that the contraction area of the trial roadway ahead of the working face decreases significantly, and that the largest displacement between two coal sides is no more than 428 mm (the largest roof subsidence is no more than 663 mm). The test results show that good control effect on surrounding rock deformation and roof support has been achieved. The study can be applied for the entire Tashan coal mine, and has significant implication for engineering projects with similar geological and mining conditions.

A Discussion of Stability and Engineering Verification of Thin Immediate Roof under Uniform Load of the Rectangular Coal Roadway

Roadway roof is a key factor in roadway stability. At present, the analysis of roof stability is mainly based on numerical calculations and field measurements with a relatively weak theoretical basis and inadequate research studies on the loading mechanism of the roof. In this paper, a mechanical calculation model of immediate direct roof under uniform load of rectangular coal roadway is established. The stress distribution and roof subsidence in the roof are calculated theoretically and verified by the numerical calculation, physical tests, and engineering applications. Based on the classical beam solution theory, the stress distribution and sinking of the thin immediate roof under uniform load are obtained and verified by numerical calculations. The results are highly consistent. Two types of thin direct roof failure under uniform load are analyzed: the failure of the normal cross section caused by the bottom tensile stress and the failure of the inclined section caused by the combined effect of the tensile stress in the abdominal area and compressive stress. The stability of thin immediate roof under uniform load of the rectangular coal roadway was tested in 1905s mining roadway of Great Wall Number #3 coal excavation field. This research can further fill in blanks of the loading mechanism of the roadway roof and provide theoretical and pragmatic values to control the roadway pressure and rock stratum as well as the scientific references to the design of the anchor bolt supporting system.

Presplitting Blasting the Roof Strata to Control Large Deformation in the Deep Mine Roadway

As the mining depth increases, under the influence of high ground stress, the surrounding rock of deep mine roadways shows soft rock characteristics. Under the influence of mining disturbance at the working face, large deformation of the roadway has occurred. To control the large deformation of the roadway, many mines have adopted the form of combined support, which has continuously increased the support strength and achieved a certain effect. However, since the stress environment of the surrounding rock of the roadway has not been changed, large deformation of the roadway still occurs in many cases. Based on the theoretical basis of academician Manchao He’s “short cantilever beam by roof cutting,” this paper puts forward the plan of “presplitting blasting + combined support” to control the large deformation of the deep mine roadways. Without changing the original support conditions of the roadway, presplitting blasting the roof strata of the roadway, by cutting off the mechanical connection of the roof strata between the roadway and gob, improves the stress distribution of the roadway to control the large deformation. Through field tests, the results show that after presplitting blasting the roadway roof, the roadway roof subsidence is reduced by 47.9%, the ribs displacement is reduced by 45.7%, and the floor heave volume is reduced by 50.8%. The effect is significant.

Secondary Gob-side Entry Retaining Technology with Double Side Roof Cutting and Pressure Relief in Thin Coal Seam

In order to solve the problems of stress concentration and large deformation of surrounding rock of secondary gob-side entry retaining in thin coal seam caused by hard overhanging roof, the method of cutting off overhanging roof on both sides of roadway by manual pre-cutting seam is adopted. It can transfer high stress to goaf on both sides of roadway and reduce roadway deformation. The mechanism of roadway formation, the conditions of double side overhanging roof subsidence and the stress reduction of roadway side support of secondary gob-side entry retaining after double side roof cutting are given. This paper studies the pressure relief law of secondary gob side entry retaining after double side roof cutting in thin coal seam. After double side roof cutting, the stress of roadway side support is reduced by 76%, the maximum vertical displacement of roof is reduced by 84%, and high stress is transferred to goaf. The influence of different double side roof cutting thickness and angle on the pressure relief effect is analyzed. On the premise that the collapsed gangue can fill the goaf, the smaller the thickness of double side roof cutting is, the better the pressure relief effect is, and the greater the double side roof cutting angle is, the better the pressure relief effect is. It has been successfully applied in the left five roadway of No. 69 coal seam in Xinqiang coal mine, and the deformation of roof and floor is reduced by 47% and the deformation of two sides is reduced by 56% after double side roof cutting. It has important reference value for the future secondary gob-side entry retaining.

Experiment Study on Coal Roadways Roof Monitoring Based on the Pre-stretched Optical Fiber Lay out at Fixed-points

The safety monitoring of coal roadways roof is the key to prevent roof accidents and ensure coal mines safety. However, the monitoring blind zones caused by traditional methods is one of the main reasons of frequent roof accidents. To ensure the safety of coal mines production, a novel roof monitoring method is proposed in this paper, which is based on the distributed optical fiber sensing (DOFS) and the pre-stretched optical fiber layout at fixed points (POFLF). The research of this approach has been carried out through analysis of DOFS optical fiber strain monitoring and POFLF theory, and with the help of indoor optical fiber strain experiment. Finally the approach was applied in the field monitoring of a coal roadway roof. Aforementioned verified the optical fiber strain distribution not only accurately indicate the roof settlement range, but also qualitatively and quantitatively demonstrate the roof subsidence displacement. The feasibility and validation of the approach as a supplementary or auxiliary resorts in roof safety monitoring were confirmed. The research in this paper not only has important reference significance for the safety monitoring of coal roadways roof under similar geological and production conditions, but be of helpful for improving the coal mine safety as well.

Characteristics of Roof Ground Subsidence While Applying a Continuous Excavation Continuous Backfill Method in Longwall Mining

Activities of traditional longwall mining will result in ground subsidence and therefore cause issues such as damages to buildings and farmlands, water pollution and loss, and potential ecological and environmental problems in the mining region. With advantages of the longwall backfill mining method, as well as the room and pillar mining method, a continuous excavation and continuous backfill (CECB) method in longwall mining is recommended to effectively control the ground subsidence. In this method, mining roadways (MRs) are initially planned in a panel, and then they are excavated and backfilled in several stages until the whole panel is mined out and backfilled. According to the geologic conditions of an underground coal mine, and the elastic foundation beam theory, a mechanical model was built to study the subsidence of the roof while using this new mining method. In addition, methods to calculate roof subsidence in various stages in CECB were also provided. The mechanical parameters of backfilling materials, which were used in the theoretical calculation and the numerical analysis for mutual check, were defined through analyzing the stability conditions of the coal pillars and the filling bodies. The control effect for the ground subsidence of using the newly proposed mining method was analyzed based on both simulation results and site monitoring results, including the ground subsidence, horizontal displacement, tilt, curvature and horizontal strain. This research could provide suggestions to effectively control ground subsidence for a mine site with similar geologic conditions.

Failure Mechanism and Deformation Characteristics of Gob-Side Entry Retaining in Solid Backfill Mining: A Case Study

In solid backfilling mining, gangue and other solid materials are backfilled into the gob to restrain the roof subsidence. As a result, instead of being subjected to periodic collapse, the overlying strata will be able to maintain its integrity and bend or sink as a whole with partial fracture and failure, thus facilitating the gob-side entry to retain. To study the mechanical deformation characteristics and failure mechanism of such gob-side supporting wall, this research analyzes the structure features of the surrounding rock in gob-side entry and builds mechanical damage model for gob-side supporting based on the hypothesis of planar slip surface in the theory of Coulomb earth pressure. The relationship among normal force acting on the contact face N, the angle between the slip surface of gangue and vertical face η and other relevant factors are discussed. The result suggests that η is mainly affected by δ, the friction angle between the backfill body and the supporting wall interface, and φ, the internal friction angle of backfill materials; η increases as δ increases and decreases as φ increases. N, mainly affected by δ, φ and uniformly distributed load q, has positive correlation with q and negative correlation with δ and φ. The research also reveals that compressive stress exists only when the width of wall exceeds 1.741 m; such stress will decrease sharply when the width of wall is between 2 m and 3 m, and become stable when the width goes beyond 3 m. According to numerical simulation and field test, the maximum values of roof-to-floor longitudinal deformation and lateral deformation between two sidewalls are 350 mm and 210 mm, respectively. The research results proved that the model employed can accurately predict and evaluate the deformation and damage of the gob-side entry.

Numerical investigation of failure evolution for the surrounding rock of a super‐large section chamber group in a deep coal mine

AbstractThe stability control of surrounding rock for a large or super‐large section chamber group is a difficult technical problem in deep mining conditions, and this stability control has become one of the most important factors restricting the safety of a large‐scale coal mine. Based on the in‐site geological conditions of a super‐large section chamber group that functions for a coal gangues separation system in the Longgu Coal Mine, we first researched the stress, deformation, and failure characteristics of the in‐site chamber group surrounding rocks by using the FLAC3D software. Simulation results showed that the maximum vertical stress, deformation, and failure range of the surrounding rock for a super‐large section chamber group are larger than those of an ordinary section chamber. In addition, the roof subsidence and the plastic zone radius at the intersection are obviously larger than that of other parts, which increase by approximately 50.8% and 44.4%, respectively. Therefore, the chamber group intersection should be taken as the key area for surrounding rock control. Then, the influences of two key parameters of the chamber group, spacing and angle, were researched in detail to help in the design of the chamber group. Results show that when the chamber spacing is 80 m, the interaction begins to occur. When the angle is 70°, the stress of the surrounding rock and roof subsidence reach the minimum. Thus, the optimum chamber group parameters are determined to be a spacing of 80 m and an angle of 70°. Finally, from the perspective of chamber group stability, the optimal chamber group parameters can better meet the normal use for the super‐large section chamber group. This research provides a reference for the design of a super‐large section chamber group under the same or similar conditions and provides a strategy for controlling the surrounding rock.

Survey on roof movement of the gangue backfill working face

This paper studied the roof movement characteristics of LW1302N-1 with gangue backfill mining in Shandong Xinjulong Company of China. The monitoring of roof subsidence of the gob and bearing stress of filled gangue was carried out by using the roof subsidence and gangue loading monitoring system. From data analysis, the result shows that: (1) Gob roof subsidence can be divided into six stages including slow subsidence, fast subsidence, very fast subsidence, subsidence slowing down, subsidence speeding up and subsidence becoming slow. (2) Filled gangue loading period can be divided into four stages including slow increasing resistance, fast increasing strength, strengthening and continuing increasing resistance, Similarly, the gob roof movement consists of four steps such as immediate roof caving, central roof flexure, main roof fracturing, and high strata flexure. (3) The immediate roof is 3.1 times the thickness of equal mining height, and the main roof is 2.7 times the thickness of equal mining height. The research results provide a reference to the analysis of the characteristics of overlying strata structure and roof movement for gangue backfill mining in deep coal mines.

Discussion on stability analysis and support technology of surrounding rock of gob-side entry retaining

In order to further analyze the stability and control mechanism of surrounding rock in the gob-side entry retaining, the slip instability coefficient k1 and compression failure coefficient k2 are proposed by the mechanical analysis of the established filling body mechanics model, and when both k1 and k2 are less than 1, the roadside filling body can keep stable. The mechanical model of main roof is analyzed by the static equilibrium method, and the formula for calculating the maximum roof subsidence is deduced. Based on the data of haulage gateway of No. 21201 coal face of Juncheng Coal Mine, using the formula to calculate the amount of roof subsidence at the junction point of filling body and the roof, and proposes two methods to reduce roadway floor heave by increasing the roadside filling width and transferring the surrounding rock stress to the gob. Finally, the corresponding surrounding rock control technology is proposed according to the surrounding rock deformation characteristics during roadway excavation, roadway retention and roadway reuse, and then carries out field monitoring and evaluation. The results show that after adopting the corresponding supporting technology, the surrounding rock of the gob-side entry retaining has been effectively controlled from the excavation to the reuse period with the coal face advancing, the roof-to-floor convergence of roadway solid coal side is less than 220 mm, the roof-to-floor convergence of filling body side is less than 280 mm, and the convergence on the both sides of the roadway is about 190 mm. At the same time, the calculated roof subsidence with the junction point of filling body and the roof is similar to the field monitoring results, which achieves the expected control effect, and the research contents provides a certain reference for the support technology of the gob-side entry retaining.

A Case Study on Large Deformation Failure Mechanism and Control Techniques for Soft Rock Roadways in Tectonic Stress Areas

Large deformation and failure of soft rock are pressing problems in the mining practice. This paper provides a case study on failure mechanisms and support approaches for a water-rich soft rock roadway in tectonic stress areas of the Wangzhuang coal mine, China. Mechanic properties of rock mass related to the roadway are calibrated via a geological strength index method (GSI), based on which a corresponding numerical simulation model is established in the Universal Discrete Element Code (UDEC) software. The failure mechanism of the roadway under water-saturating and weathering conditions is revealed by field tests and numerical simulation. It is found that the stress evolution and crack development are affected by weathering and horizontal tectonic stresses. The roadway roof and floor suffer from high stress concentration and continuous cracking, and are consequently seen with rock failure, strength weakening, and pressure relief. Unfortunately, the current support system fails to restrain rock weathering and strength weakening, and the roadway is found with serious floor heave, roof subsidence, and large asymmetric deformation. Accordingly, a new combined support system of “bolt–cable–mesh–shotcrete + grouting” is proposed. Moreover, numerical simulation and field testing are conducted to validate the feasibility and effectiveness of the proposed approach, the results of which demonstrate the capacity of the proposed new support method to perfectly control the surrounding rock. Findings of this research can provide valuable references for support engineering in the soft rock roadway under analogous geological conditions.

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.

Stability Control for Gob-Side Entry Retaining with Supercritical Retained Entry Width in Thick Coal Seam Longwall Mining

Taking gob-side entry retaining with large mining height (GER-LMH) of the 4211 panel in the Liujiazhuang coal mine as the engineering background, a numerical simulation was conducted to study the surrounding rock deformation, stress, and plastic zone distribution of GER-LMH with respect to retained entry width. The concept of critical retained entry width of GER-LMH was proposed. In view of the deformation characteristics of surrounding rock, an innovative approach to determine the critical width of GER-LMH based on the cusp catastrophe theory was proposed. The cusp catastrophe functions were set up by approximate roadside backfill body rib convergence and roof subsidence series with respect to different retained entry widths. The critical retained entry width of GER-LMH was 4.0 m according to bifurcation set equations. Surrounding rock stability control principle and technique of GER-LMH was proposed, including “rib strengthening and roof control”: road-in support with high pre-stress rockbolts and anchor cables, roadside backfill body construction technology with high-water quick consolidated filling materials and counter-pulled rockbolt, road-in reinforced support technology with hydraulic prop support and roof master. Field test and field monitoring results show that GER-LMH with supercritical retained entry width in the 4211 panel could meet the requirements for ventilation when the 4211 panel was retreating.

Experimental and Numerical Study of Rock Stratum Movement Characteristics in Longwall Mining

The roof fracture is the main cause of coal mine roof accidents. To analyze the law of movement and caving of the roof rock stratum, the roof subsidence displacement, rock stratum stress, and the rock stratum movement law were analyzed by using the methods of the particle discrete element and similar material simulation test. The results show that (1) as the working face advances, regular movement and subsidence appears in the roof rock strata, and the roof subsidence curve forms a typical “U” shape. As the coal seam continues to advance, the maximum subsidence displacement remains basically constant, and the subsidence displacement curves present an asymmetric flat‐bottomed distribution. (2) After the coal seam is mined, the overburden forms an arched shape force chain, and the arched strong chain is the path of the overburden transmission force. The farther away from the coal seam, the smaller the stress concentration coefficient is, but it is still in a high stress area, and the stress concentration position moves toward the middle area of the goaf. The stress concentration in front of the coal wall is the source of force that forms the abutment pressure. (3) Above the coal wall towards the goaf, a stepped fracture was formed in the roof rock stratum. The periodic fracture of the rock stratum is the main cause of the periodic weighting of the working face. Understanding the laws of rock movement and stress distribution is of great significance for guiding engineering practice and preventing the roof accidents.

Mechanical Modeling of Roof Fracture Instability Mechanism and Its Control in Top‐Coal Caving Mining under Thin Topsoil of Shallow Coal Seam

This study aimed to explore the safe and efficient top‐coal caving mining under thin topsoil of shallow coal seam (SCS) and realize the optimization of hydraulic support. Numerical simulation and theoretical analysis were used to reveal the stress distribution of the topsoil, the structure characteristics of the main roof blocks, and the development of the roof subsidence convergence. Step subsidence of the initial fractured main roof after sliding destabilization frequently existed, which seriously threatened the safety of the hydraulic supports. Hence, a mechanical model of the main roof blocks, where the topsoil thickness was less than the minimum height of the unloading arch, was established, and the mechanical criterion of the stability was achieved. The working resistance of the hydraulic support was calculated, and the reasonable type was optimized so as to avoid crushing accident. Findings of the present analysis indicated that the hydraulic support optimization was mainly affected by fractured main roof blocks during the first weighting. According to the block stability mechanical model based on Mohr–Coulomb criterion, the required working resistance and the supporting intensity were determined as 4899 kN and 0.58 MPa, respectively. The ZZF5200/19/32S low‐position top‐coal caving hydraulic support was selected for the studied mine and support‐surrounding rock stability control of thin‐topsoil SCS could be achieved without crushing accident.