Stope Roof Research Articles

Assessing prestressed anchor cable support as ore pillar alternatives in stope span expansion: A case study

In large diameter long-hole (LDL) stopes, ore pillars are often set up in the rock drilling chamber to expand the span. However, after the ore pillars are blasted along with the stope, the stability of the roof is difficult to control, and collapse often occurs. This study introduced the idea of employing PAC support technology to replace the traditional ore pillar support and conducted site investigations, theoretical analysis, numerical simulations, and field tests to develop a technology for expanding the span of LDL stope based on prestressed anchor cable (PAC) support. Following a detailed investigation of pillar-supported LDL stope roof collapse events and geological conditions, a mechanical model for the stress arch considering the lateral stress coefficient was developed combining the failure characteristics and deep conditions, and the inverse relationship between the lateral coefficient of ground stress and the height of the stress arch above the opening was determined. Numerical simulation results demonstrate that the elliptical parabolic stress arch mechanical model has been verified, and PAC effectively controls roof stability, as evidenced by displacement and plastic zone response. Field tests were carried out utilizing PAC support, and monitoring results affirm the PAC support system's efficacy in stabilizing the roof. Using PAC to replace ore pillars resulted in a remarkable 29.8 % improvement in blast hole drilling efficiency and reduced ore dilution rate within the stope from 22.3 % to 14.2 %. This study serves as a reference for addressing analogous challenges in other mining operations.

Study on Deformation Response Law of Surrounding Rock in Solid Backfill Mining Stope

To fully control the roof and surface and reduce the ecological damage caused by mining, it is necessary to understand the deformation response law of the surrounding rock in solid backfill mining. The similarities and differences of overburden movement characteristics between natural caving method and solid backfill method are analyzed and compared by means of on-site investigation and monitoring, theoretical derivation and analysis, numerical simulation and effect verification. The control mechanism of backfill mining on the roof is clarified, and the relationship between stope roof displacement and support pressure of backfill body is established. Furthermore, the mechanical conditions of immediate roof in equilibrium state during backfill mining are established. The FLAC3D simulation indicates that the solid backfill mining face has a concentrated stress peak located 15 m ahead of the coal wall. Within the range from 0 to 5 m, the abutment pressure is reduced gradually. However, within the range from 5 to 15 m, the abutment pressure is increased evidently. The stress concentration is located 35 m behind the working face. With continuous backfilling of the goaf, the maximum displacement occurs in the backfill body near the working face. The total deformation of the surrounding rock in the goaf is reduced by 58%. The total deformation of coal wall is reduced by ∼37.5%. The subsidence coefficient is reduced from 0.59 to 0.013 by using the backfill mining technology. The research results provide theoretical guidance and reference for roof control and ecological protection of backfill mining.

Synergy Prediction Model of Information Entropy Based on Zone Safety Degree and Stope Roof Weighting Step Analysis

During the underground mining of coal resources, the rock pressure emerges acutely and the mine geological disasters occur frequently. It is of great significance to grasp the manifestation law of rock pressure in time to guide the safety production and operation in the pit. In this research, the calculation equations and concept of information entropy based on zone safety degree are primarily defined, and the synergetic theory of the maximum information entropy principle is combined to put forward the synergy prediction model of information entropy based on zone safety degree. In the meantime, the synergy prediction model of information entropy based on zone safety degree is employed to calculate and predict the first weighting step and the periodic weighting step of the main roof of the 9203 working face of Hengsheng Coal Mine in China’s Shanxi Province, as well as verifying the validity and reliability of the synergy prediction model of information entropy based on zone safety degree by the comparison of similar simulation test results, which has presented a scientific basis for the effective control of rock pressure and roof management.

Research into the influence of the thin ore body occurrence elements and stope parameters on loss and dilution values

Purpose. Development of a methodology for determining the level of actual losses and dilution of ore based on the identified patterns of the influence of occurring ore body elements and the stope parameters when mining thin slope deposits using a system for delivering ore by the blasting force. Methods. The task set is solved using an integrated approach, including the analysis of literary sources and the existing practical experience on the issues of losses and dilution, conducting experimental-industrial experiments in the conditions of the Akbakai deposit to assess the recommended method effectiveness for determining the values of excess losses and dilution of ore, geomechanical assessment of the mass using the methods of limit equilibrium, numerical and probabilistic analyses. Findings. A methodology for determining losses and dilution when mining thin slope deposits using a system for delivering ore by the blasting force is proposed, which makes it possible to predict excess losses and dilution coefficients arising from stope roof caving and incomplete ore delivery. The actual losses and dilution of ores in the Akbakai deposit have been determined based on the proposed methodology and instrumental surveys. The developed methodology for determining the values of excess losses and dilution makes it possible to take preliminary measures to prevent the stope roof caving with the complete ore mass delivery using the blasting force. Originality. New dependences have been revealed for the conditions of the Akbakai deposit: logarithmic – the value of dilution depending on the angle of the ore body occurrence; polynomial – the stope maximum span depending on the ore body dip angle; exponential – ore losses depending on the angle of the ore body occurrence. Practical implications. The practical significance, confirmed in the course of pilot-experimental work, is in minimizing the percentage of loss and dilution of the useful component when mining thin slope ore deposits using a sublevel blast-hole stoping system with an open stope space, which makes it possible to reduce the cost of the produced mineral.

Evaluation of Stope Stability and Displacement in a Subsidence Area Using 3Dmine–Rhino3D–FLAC3D Coupling

Underground stope stability has important implications for the safety and efficiency of metal mining, especially when there are subsidence zones above. To analyze the stope stability around the subsidence area of the Hongling ore body, this paper innovatively adopts the 3Dmine–Rhino3D–FLAC3D coupled modeling numerical method to model and predict the excavation process of the stope around the Hongling subsidence area; it monitors and analyzes the displacement and stress-distribution changes in a selected stope roof, and its left-hand side and right-hand side walls. This paper realizes the separation and extraction of the special ore body and surface topographic features of the Hongling ore body; it discusses the stope stability characteristics around a subsidence area based on the real topography of the mining area and the occurrence form of the ore body. The study found that the influence of the upper collapse area on the undesired displacement of the downhole stope is mainly reflected in the roof displacement. As the stope position moves away from the collapse area, the roof displacement decreases significantly, the maximum displacement is at the center of the stope roof, and the maximum principal stress occurs at the corner of the stope.

Development of 3D top coal caving angle model for fully mechanized extra-thick coal seam mining

During high-intensity, fully mechanized mining of extra-thick coal seam, the top coal would cave to a certain 3D form. Based on the data collected during drilling, a 3D model of top coal caving surface space was established to determine the relationship between the location of the stope roof and the caving surface, enabling the mathematical computation of the top caving angle (φ). The drilling method was employed to measure the top caving angle on two extra-thick fully mechanized coal caving faces under the conditions of three geological structures, namely, no geological structure, igneous rock structure, and fault structure. The results show that the value of top caving angle could be accurately estimated on-site with the 9-parameter 3D top coal caving surface model built with the drilling method. This method is a novel on-site measurement that can be easily applied. Our findings reveal that the characteristics of the coal-rock in the two mining faces are different; yet their caving angles follow the rule φigneous rock structure < φno geological structure < φfault structure. Finally, through the data fitting with two indexes (the top coal uniaxial compressive strength and the top caving angle), it is found that the relationship between the two indexes satisfies an exponential decay function.

Evolution Characteristics of Overburden Instability and Failure under Deep Complex Mining Conditions

Aiming at the problems of intensified deformation and serious stress damage of stope overburden during mining in deep typical geological environment, taking the deep mining face of a mine in the northwest as the research background, the characteristics and influencing factors of overburden collapse in deep geological environment are analyzed, a comprehensive evaluation system affecting overburden instability and deformation is established, and the temporal and spatial evolution law of deep stope roof failure is obtained by using the “three-zone” distribution theory of overburden, analytic hierarchy process, and numerical simulation method. The results show that high ground stress has the most significant impact on the geological environment of deep stope, followed by the properties of surrounding rock, hydrogeological conditions, and unloading disturbance. The damage degree of the plastic zone at the top of the overburden is greater than that at the bottom. With the increase of mining time, the damage range of the roof and floor gradually increases. In the process of coal seam mining, the increasing range and sinking trend of roof stress are larger near the working face, and the farther away from the working face, the increasing range decreases continuously. The research conclusion provides an important reference for coal seam safety mining and surrounding rock control in complex environments such as deep mining, high stress, and strong disturbance.

Mechanical Mechanism Analysis of Roof Fracture Evolution in Stope with Variable Length Based on Elastic-Plastic Structure Theory

Stope with variable length is formed by length change of mining face due to the irregular distribution of geological rock pillars, which is a typical representative of complex coal seam mining. According to the geometric characteristics and mechanical boundaries of each mining stage of stope with variable length, the roof structure models with four boundary conditions were established and solved successively by using the small deflection thin plate bending theory. Combined with the simulated images of MATLAB and FLAC3D, the fracture laws and corresponding engineering phenomena were analyzed. According to the characteristics of roof rock pressure zoning, the overburden structure pressure model of “three stopes, three areas, and three structures” is constructed. The research shows that the traditional “O-X” fracture occurs in the roof of small mining stope. For the cracks generated by prolonged “O-X” fracture and drift “O-X” fracture in mutative mining stope are similar to the crack development characteristics of large mining stope, so they are integrated into full-scale mining stope. The full-scale mining stope roof is broken in “X-O” shape, and the crack continues to develop to produce extended fracture, forming the roof fracture theory of “two stopes and two laws.” The research conclusion strongly reveals the failure law of roof from tension instability to plastic fracture and abnormal ground pressure during mining in stope with variable length. It provides a basis for exploring the essence of overburden migration in stope with variable length and strengthening the roof prevention and control theory under the occurrence conditions of deep complex coal seams.

Research on Roof Structure and Determination of Working Resistance of Shallow Buried Single Key Stratum Based on Grid Drillhole Field Method

Abstract The roof structure of the stope is the core of revealing the weighting mechanism and determining the working resistance of support. The Shenfu Dongsheng coal field of China has abundant reserves of shallow coal seam. The phenomena of strong ground pressure and step subsidence caused by the mining of shallow buried single key stratum (SBSKS) poses a serious threat to the fragile environment. Revealing the shape and movement process of roof structure of SBSKS is the primary prerequisite for ensuring safe mining of working face. Firstly, the field measurement technology of the space grid-like drill field was developed to construct vertical holes and incline holes in the auxiliary headgate gateway and tailgate airway ahead of the working face and to obtain the measured data of the broken position and vertical displacement of the roof by the drilling peep and multipoint displacement. Secondly, the key parameters of the stope roof were analyzed by the grid drillhole filed method, for example, the inclined roof break angle, the rotation angle, the thickness of equivalent immediate roof (EIR), and the roof structure-articulated level and shape. Then, we combined with the mining face comparison of compression laws revealing the time and space relationship between the movement of the roof structure and the roof weighting, and three more dangerous states of the roof were determined. Finally, the roof structure of SBSKS is moved upward through the field research on the 22201 working face, the “high step rock beam” structure and the advanced breaking position of the roof are obtained, and the calculation equation of the support resistance in large mining height face is given. The research results provide scientific guidance for the safety of SBSKS coal seam and provide technical support for green mining in ecologically fragile mining areas.

Mechanism and Control of Rockburst Induced by Draining Spatial Islands and Squaring

A small-pillar gob-side roadway showed rockburst appearance during the mining of a gob-side working face located in the Shaanxi-Inner Mongolia mining area. This study examines the 2202 gob-side working face of a coal mine in Inner Mongolia as a case study. A stress evolution model was built for the static-stress spatial islands formed by drainage regions and goafs based on the spatial relationships between drainage regions and goafs. The average microseismic frequency and energy of the high-stress zone of spatial islands were at least 1.37 times of those of other zones, validating the presence of spatial islands. The dynamic and static load effects of working face squaring were obtained based on the evolution of the stope roof as well as changes in microseismic data. Microseismically active zones were advanced to 200 m–300 m on working faces. The rockbursts induced by high static loads and dynamic and static loads formed by spatial islands and squaring were calculated. According to calculation results, the critical stress concentration value under high static loads was 3.27; the critical static stress concentration value under dynamic and static loads was 1.81. The superposition of drainage boundary stress, goaf lateral stress, and lead stress might reach the critical stress concentration under dynamic load disturbances, causing rockbursts. A stress adjustment scheme was established, including overall hydraulic fracturing of the external roof of the drainage region, reduction of stoping speed, and pressure relief of large-diameter boreholes. The stress adjustment scheme was implemented on-site and supplemented by monitoring and early warning methods to safely advance by the first squaring region.

Study on mechanism of end face roof leaks based on stope roof structure movement under repeated mining

In view of the problem that the broken stope roof under repeated mining is prone to end face roof leaks, this paper’s research background is the close distance coal seams mining of a mine in Guizhou province, China. The impact of stope roof structure movement on the immediate roof was elucidated from the analysis of stope roof structure movement under repeated mining employing a combination method of field measurement, physical simulation, theoretical analysis, and numerical simulation. As a result, the mechanism of end face roof leaks under repeated mining is investigated, and the corresponding control measures are proposed. The results show that the cantilever beam structure instability of the stope roof under repeated mining is an important reason for the end face roof leaks and support crushing accident. The fracture forms of cantilever beam structure are divided into direct fracture and hinged structure fracture. The direct fracture is affected by the support working resistance. The cantilever beam hinged structure is divided into sliding and rotational instability. The periodic weighting interval is small and the fracture degree is large under repeated mining, which leads to the sliding instability. It is concluded that reasonable support working resistance is the important reason to keep the cantilever beam structure stability. Therefore, the stope roof leakage control countermeasures under repeated mining are proposed. The UDEC numerical simulation is used to verify the above analysis again, so as to provide the basis for the end face roof leaks prevention and control under repeated mining.

Simulations of goaf surface subsidence via filling control.

Xincheng Gold Mine is taken as an example to investigate the ground subsidence that results from filling. Both numerical simulations and simulation experiments are conducted to simulate the deformation process at the stope roof and bottom from excavation and filling. The assumption of macroscopic continuity from traditional continuum mechanics models is overcome. The simulation results demonstrate that the ground subsidence is slowed due to filling. The total trends of the top and bottom displacements are sinkable and upturned, respectively. Moreover, with an increased buried depth and lateral pressure coefficient, the displacements of the top and bottom of the stope increase as well. The characteristics and evolution of the displacement vector field of the rock mass are macroscopically and microscopically studied over the excavation progress. This provides technical support for stope safety production.

Application of Numerical Simulation Methods in Solving Complex Mining Engineering Problems in Dingxi Mine, China

In the mining industry, with numerical simulation analysis of stope roof stability, stope exposed area computation, and pillar buckling collapse simulation, backfill body creep damage mechanism research is becoming the most popular method in the field of backfill mining techniques. In this paper, we first summarized and analyzed the current application status and the existing problems of numerical simulation for solving mining engineering technical problems; then, based on the practical engineering problems of mining phosphate rock resources under high and steep rock slopes (HSRS), we carried out a true-3D numerical simulation study for different underground mining methods, to determine the appropriate mining method. Therefore, this paper, taking Dingxi Mine in China as an example, highlights the advantages of the backfill mining method with a high and steep slope; meanwhile, it also points out how to improve the accuracy of a numerical simulation and make it more consistent with the actual situation of the mining engineering application site. This paper only serves as a guide, in order to start a conversation, and we hope many more experts and scholars will become interested and engaged in this field of research.

Study on Stability of Stope Surrounding Rock under Repeated Mining in Close-Distance Coal Seams

In order to study the influence of roof stress and fracture distribution characteristics on the stability of stope surrounding rock under repeated mining in close-distance coal seams, the research background of this paper is the close-distance coal seam mining of a mine in Guizhou province, China. The dynamic evolution characteristics of stope roof caving and the stress environment change law under repeated mining are studied by using the method of similar simulation experiment, numerical simulation, and field verification. The results show that the roof stage under repeated mining can be divided into four stages: normal mining, roof deterioration, end face roof leaks, and support crushing. The tip-to-face distance, support height, and support working condition are the main influencing factors in end face roof control. The main reason for the support crushing is that the roof above the support is broken, the main roof cannot form the self-stable structure, and the support force is insufficient. The roof dynamic load is high, and the subsidence of the end face roof is large, which is prone to roof caving during the stope roof weighting. Moreover, the roof weighting is frequent under repeated mining, which leads to the broken roof and the support cannot be supported, resulting in the occurrence of the support crushing accident. The control strategies for stope roof under repeated mining are provided based on the thorough study findings. The working face 17101 roof is successfully controlled by the aforesaid procedures, which provides the foundation for the management of stope surrounding rock under repeated loading.

Study of Instability Mechanism and Roof Caving Mode of Cementing Filling Stope: The Case Study of a Nonferrous Metal Mine in China

The downward layered cemented filling method, which is generally used in the mining of high‐value metal mines with poor surrounding rock quality, is widely believed to not cause large‐scale instability of the roof strata in the mining area. However, a nonferrous metal mine in northern China, which has been using the downward cemented filling method, suddenly suffered a violent collapse accident of the stope roof, and the surface is accompanied by significant subsidence on a large scale. The accident revealed that the roof collapse mechanism still needed further research. In this paper, field investigation and numerical simulation were combined to study the mechanism of roof collapse. Based on the input data including in‐situ stress state, geological occurrence pattern, and mining steps, the particle flow code (PFC) was used to simulate the stress and displacement changes of the rock mass under mining disturbance. These results indicate that the failure process of the overlying rock mass can be divided into four stages due to the special geological conditions of the mine: pillar stability stage, pillar chain failure stage, roof filling caving stage, and gneiss plug settlement stage. In the early stage of mining, the pillars between the mined‐out drifts could effectively support the overlying rock mass due to the small exposed roof. As more drifts were mined, the vertical pressure on the pillars was added. When the number of mining drifts reached five, one of the pillars was firstly destroyed due to overloading, and then the pressure of the overlying strata was transferred to the surrounding pillars, leading to the subsequent failure of other pillars. When pillars were damaged, arch caving appeared inside the roof filling material. Finally, the vertical shear resistance capacity of the gneiss mass above is insufficient, owing to the steeply dipping joints. Finally, the gneiss above was subject to sudden plug settlement along the vertical joints. It should be noted that the stope mining management of the mine has a significant impact on production safety. In order to ensure the stability of the stope formed by cemented filling method, the dense distribution of simultaneous mining drifts should be avoided and the mine‐out areas should be backfilled in time.

Behaviors and Overlying Strata Failure Law for Underground Filling of a Gently Inclined Medium-Thick Phosphate Deposit

In this study, the No. 6 pit in the eastern mining area of the Jinning phosphate mine in China was taken as the research background. In order to reduce the cost of filling, an improved pillarless sublevel caving method is proposed. This method greatly improves the ore recovery rate by adding a recovery route. In addition, the combination of similar material simulation experiments and numerical simulations (discrete element and universal distinct element code) revealed the deformation and failure laws of the surrounding roof rock and the characteristics of the surface subsidence. The results indicate the following. (1) The similar simulation experimental results indicate that the deformation of the overlying rock layer originated from the direct roof of the goaf and gradually developed into the deep part of the rock layer. An irregular stepped caving zone formed in the goaf. The maximum surface subsidence was located above the phosphorus orebody, and it gradually decreased toward both sides. As the stope approached propulsion, the location of the maximum subsidence gradually moved toward the propulsion direction. (2) The numerical results revealed that the displacement of the overlying strata was nonlinear, and it decreased with increasing roof height. A support pressure concentration area was formed within a certain range of the stope roof. The numerical simulation results are basically consistent with the similar simulation experimental results.

Study on stability of filling wall under lateral large-span composite hinge fracture of hard critical block.

There is still a lack of mature researches on the stability mechanism, influencing factors and control technology of the gob-side filling wall, and systematic researches on the cracking forms and characteristics of the stope roof and the stability of the filling wall are rather insufficient. This paper is aimed at investigating the deformation law of the filling wall under the large-span composite hinge fracture of the hard critical block and solving the difficulty that the large-span critical block lateral fracture poses to gob-side entry retaining. Research methods such as theoretical calculation, mechanical analysis, numerical simulation and field test were adopted comprehensively in this study. When the large-span critical block B is divided into two or three parts, its force on the immediate roof decreases with the increase in the number of segments. Meanwhile, as the number of segments grows, the displacement and axial stress of the filling wall both decrease gradually; the tensile failure weakens relatively, while the shear failure changes slightly. Moreover, both the number of shear cracks and the number of tensile cracks in the filling wall are positively correlated with the strain. When the critical block divided into four parts, the amount of lateral displacement is about 190 mm, and the axial displacement reaches the minimum (about 235 mm). The stability of the filling wall along the gob-side entry is closely related to the lateral fracture span of the stope roof. Under the lateral fracture of the hard critical block, a smaller span of the lateral fracture of the critical block corresponds to a smaller force on the filling wall and a weaker damage to the filling wall. The field test result verifies that cleaving the large-span critical block into smaller segments is conducive to reducing surrounding rock and filling wall deformation.

Experimental study of the shrinkage behavior of cemented paste backfill

Abstract Cemented paste backfill (CPB) is largely used in underground mine stopes worldwide. When a CPB is placed in a stope, an important task is to estimate the settlement associated with the shrinkage and self-weight consolidation of the CPB. This is closely related to the volume management to ensure the stability of barricades and tight contacts between the backfill and stope roof. Over the years, shrinkage studies were mostly performed on fine-grained soils (silts and clays), with only a few publications on the shrinkage behavior of uncemented tailings. No study has been published on the shrinkage behavior of CPB. To fill this gap, a series of shrinkage tests has been conducted on CPB with different cement contents, including zero cement content (uncemented paste backfill, uCPB). The results show that the shrinkage response of CPB is very different from that of uCPB. At a given initial water content, CPB exhibits a shorter normal shrinkage stage than uCPB. The unsaturation onset water content and void ratio, shrinkage limit and final void ratio of CPB are generally higher than those of uCPB. At a given cement content, the shrinkage behaviors of CPB and uCPB are significantly influenced by the initial water content.

Comprehensive safety factor of roof in goaf underdeep high stress

The safety factor of roof under deep high stress is a quantitative index for evaluating roof stability. Based on the failure mode of surrounding rock of stope roof, the mechanics model of goaf roof is constructed, and the internal force of roof is deduced by the theory of hingeless arch. The calculation method of roof safety factor (K) under the environment of deep mining is proposed in view of compression failure and shear failure of roof. The calculation formulas of shear safety factor (K1), compression safety factor (K2) and comprehensive safety factor (K) of roof are given. The influence of stope span and roof thickness on roof stability is considered in this paper. The results show that when the roof thickness remains constant, the roof safety factor decreases with the increasing of the stope span; when the stope span remains constant, the roof safety factor increases with the increasing of the roof thickness. The deep mining example shows that when the stope span is 30 m and the roof thickness is 10 m, the roof comprehensive safety factor is 1.12, which indicates the roof is in a stable state.

Study on Control Technology for Working Faces Passing through Long-Span Abandoned Roadways

The width of an abandoned roadway has a great influence on the roof stability of the working face. According to the coal seam conditions of the 30106 working face in the Sanyuan Shiku mine, the advance of a working face through an abandoned roadway was studied by using theoretical analysis, similar material simulation, numerical simulation, and field testing to determine the law of stope roof fracture migration, the stress distribution characteristics, and the variation in support resistance. Several conclusions are drawn: (1) The roof of the overlying strata is fractured at the edge of the abandoned roadway in front of the coal pillar and rotates downward due to the run-through of the plastic zone between the working face and abandoned roadway. (2) The hydraulic support working resistance gradually increases with decreasing coal pillar width between the working face and abandoned roadway, and the working resistance of the support tends to peak when the plastic zone extends to the coal, resulting in 3~4 times the normal recovery. Leakage occurred in front of the support in the caving zone. (3) The analysis of the relationship between the support and surrounding rock with the mechanical model for calculating the support load allows the derivation of the support working resistance formula for a working face passing through an abandoned roadway. (4) When the working face is excavated to expose the abandoned roadway, the shrinkage of the front column of the hydraulic support is significantly greater than that of the back column, and the stability is greatly reduced. This problem can be effectively solved when the uniaxial compressive strength of the backfill ≥2 MPa. (5) The engineering practice showed that the danger of leakage and roof fracture impact load was eliminated with the mining pressure reduction after reinforcement measures were taken in the abandoned roadway. The working face passed the abandoned roadway safely, providing the theoretical basis and guidance for coal remining under similar conditions.