Main Roof Research Articles

Surrounding rock deformation and stress evolution in pre-driven longwall recovery rooms at the end of mining stage

Two case studies were conducted in the Shennan mining area of Shaanxi Province, China to evaluate the surrounding rock deformation and stress evolution in pre-driven longwall recovery rooms. These studies mainly monitored the surrounding rock deformation and coal pillar stress in the recovery rooms of the N1206 panel of 2−2 coal seam at Ningtiaota Coal Mine and the 15205 panel of 5−2 coal seam at Hongliulin Coal Mine. The monitoring results showed that the surrounding rock deformation of the main recovery room and the coal pillar stress in the N1206 and 15205 panels began to increase significantly when the face was 36 m and 42 m away from the terminal line, respectively. After the face entered the main recovery room, the maximum roof-to-floor convergence in the N1206 and 15205 panels was 348.03 mm and 771.24 mm, respectively, and the coal pillar stresses increased more than 5 MPa and 7 MPa, respectively. In addition, analysis of the periodic weighting data showed that the main roof break position of the N1206 and 15205 panels after the longwall face entered the main recovery room was − 3.8 m and − 8.2 m, respectively. This research shows that when the main roof breaks above the coal pillar, the surrounding rock deformation of the main recovery room and the coal pillar stress increase sharply. The last weighting is the key factor affecting the stability of the main recovery room and the coal pillar; main roof breaks at disadvantageous positions are the main cause of the support crushing accidents.

Numerical study of the effect of console length of the main roof hanging on the geomechanical parameters of the mine face

This article presents the results of scientific research aimed at establishing the effect of console length of the main roof hanging above the gob on geomechanical parameters in the vicinity of the mining face. According to the results of numerical modeling of the stress-strain state of the rock massif, the nature of the distribution of vertical displacements and stresses in the vicinity of the face was established with a consistent increase in the console length of the hanging main roof, the dynamic loads on the roof support sections, the hydraulic props settlement, the inrush area of roof rocks and the coal sloughing from the seam surface in the face. The results of the conducted research allow the preventive measures to be developed, the implementation of which will ensure the stable and safe operation of the working faces during underground coal mining in difficult mining and geological conditions.

Field study on the load transfer mechanics associated with longwall coal retreat mining

The two-entry longwall panel system is frequently used in underground coal mines in China. One of these entries serves only the current longwall panel while the other serves both current and subsequent longwall panels. The latter entry is built to survive the continuously high mining-induced stress generated by mining the current panel throughout the longwall mining process. The failure of the entry is controlled by the load transfer mechanics associated with the advancement of the longwall face. This paper presents a field study of the response of the entry during driving and longwall retreat phases. Extensive field data including entry convergence and changes in principal stresses are obtained for the Pingshu coal mine, China. A careful analysis of these data shows that the influence zone of the front abutment load on the trail entry can reach as far as 120 m outby the longwall face during panel mining. Bed separation can occur in competent fine-grained sandstone, which is 7.4 m deep into the trail entry roof. It is also found that the periodic collapse of the main roof above the mined-out area can continue when the longwall face has advanced 120 m away.

Research on the Transition Section Length of the Mixed Workface Using Gangue Backfilling Method and Caving Method

Abstract Aiming at the problem of the sudden increase of the working resistance of the support in the transition section of the mixed workface, this paper adopts the physical simulation to study the fracture and movement characteristics of the overburden strata in the backfilling section and the caving section after analysis of the system layout of mixed workface. Then, the definition of the transition section of the mixed workface is given. Next, a numerical model of the transition section length is established based on the geological conditions of the Ji15-31010 mixed workface of Pingdingshan No.12 Coal Mine. In the numerical simulation, it is used to study the influence of the advancing length of the mixed workface and the length of the backfilling section on the transition section length. The results show that with the increase of the advancing length of the mixed workface and the decrease of the backfilling section length, the transition section length increases continuously and eventually stabilizes. The range of variation is 6.2~13.5 m, corresponding to 4~9 hydraulic supports with a width of 1.5 m. Based on the above conclusions and the price of transitional hydraulic supports, the 4 transitional hydraulic supports used in the transition section of the Ji15-31010 mixed workface is determined. Filed measurement shows that as the mixed workface is advanced from 10 m to 40 m, the transition section length increases from 2 supports to 4 supports, and decreases to 2 supports again when advanced to 60 m. This indicates that the main roof of the mixed workface between 40 and 60 m experiences the initial caving. The maximum length of the transition section is 6.0 m, corresponding to 4 hydraulic supports.

In situ investigations into mining-induced hard main roof fracture in longwall mining: A case study

To better control strata behavior, it is necessary to understand the mining-induced hard main roof movement and fracture in longwall mining. In situ investigations were carried out on the 78,121 working face at the Jiangjiawan Coal Mine, Datong Coalfield, China. The longwall-mining-induced hard main roof movement and fracture were investigated by using digital panoramic imaging and a single-position extensometer. Then, geometric computational models were developed to estimate some parameters to describe the hard main roof fracture. Field observations indicate that after the mining of the 78,121 working face, the hard main roof was broken not only near the coalface regions but also near the side of the longwall panel. In accordance with in situ surveys and geometric computational models, the lateral fracture span, strike fracture span and final rotation angle of the broken main roof of the 78,121 working face were 15.56 m~23.97 m, 16 m~24 m, and 4.02°, respectively. In addition, field measurements found that the periodic roof weighting interval is equal to the strike fracture span and the lateral fracture span. This method provides a possible way to provide advance estimates of the above two parameters.

Oodi – Helsinki Central Library: Making the architectural vision a reality

ABSTRACTThe Helsinki Central Library is a landmark project, delivered as part of the “Finland 100” programme to celebrate 100 years of the independence of Finland. ALA architects delivered the winning entry to the architectural competition held in 2013. In 2014 Ramboll Finland won the structural design contract for the project, taking the project from conceptual architectural design through to completion of construction in 2018.The main structural system is fundamentally a steel frame solution designed to Eurocode. Two EXC4 classified, 109m span, tilted arches carry the two main floors and roof of the building. Post‐tensioned cables tie the bases of the arches to provide a tied‐arch structure. Other post‐tensioned cables in the floor system and concrete cores hold the tilting arches in position. Transverse trusses allow the cantilevering balcony to be delivered. 2400t of steel have been used to deliver the main frame structural solution.With careful and considered design, fabrication and construction, high quality to good tolerances and stringent requirements have been successfully achieved.An overview of the bridge‐like structural system that has been conceived and developed for the project shall be presented. Some of the challenges encountered and resolved in the design shall be discussed, as well as an overview of key decisions made to handle project delivery budget and timetable requirements.

Stress analysis of longwall top-coal caving face adjacent to the gob

ABSTRACT In longwall top-coal caving (LTCC) mining, progressive failure process of top coal is closely related to the mining-induced stress. In order to achieve failure mechanisms of the top coal, stress distribution of the LTCC face adjacent to the gob is analysed in this study with field investigation and numerical simulation. The concentration and rotation phenomenon of the principal stress are recognised. Particular emphasis is placed on rotation characteristics of the principal stress in the top coal adjacent to the gob. In addition, the influence provided by periodic rupture of the main roof on the mining-induced stress is discussed.

Mechanism and Control Methods of Roof Deformations in Gob-Side Entry Retention by Roof Cutting Under Medium-Thick Coal Seams

In order to ensure the successful implementation of gob-side entry retention by roof cutting under medium-thickness coal seams and compound roof conditions in Guandi Coal Mine, a mechanical model was established to analyze the factors affecting the roof deformation, and the roof fracturing height and roof fracturing direction were researched by way of numerical simulation and field experiments. The theoretical analysis and numerical simulation showed that the rotation angle of the upper main roof is the main factor leading to roof deformation, that increasing the support strength in the roadway cannot reduce the roof deformation, and that the roof deformation can be effectively controlled by increasing the roof fracturing height and the fracturing direction. The results confirmed that the roof fracturing height was 7 m, the roof fracturing direction was 15°, and the charging structure was 3 + 3 + 2 + 1. The roof was strengthened using a constant resistance and large deformation anchor, a single hydraulic prop reinforcement support in the lane, and a U-shaped steel retaining gangue support. The retained entry was stable at 200 m behind the working face and can meet the demands of reuse for the next mining face.

Control Technology of Soft Rock Floor in Mining Roadway with Coal Pillar Protection: A case study

This study considered the mining roadway with coal pillar protection in the fully mechanized caving face of the Dananhu No.1 Coal Mine, China. Theoretical analysis, numerical simulation, and field tests were conducted, and the stress environment, deformation, and failure characteristics of the mining roadway in the fully mechanized caving face were analyzed. The results revealed that the intrinsic cause for the large asymmetrical floor deformation in the mining roadway is the asymmetrical phenomenon of the surrounding rock’s stress environment, caused by mining. This also results in the non-uniform distribution of the mining roadway floor’s plastic zone. The degree of asymmetrical floor heave is internally related to the thickness of the caving coal. When the thickness of the caving coal was in the range of 5.9 m, the deformation of the asymmetrical floor heave, caused by the plastic failure in the floor, became more obvious as certain parameters increased. As the rotation angle of the principal stress direction increased, the maximum plastic failure depth position of the floor gradually moved toward the middle of the roadway. This caused a different distribution for the maximum deformation position. The control of the floor heave deformation was poor, and it was not feasible to use high-strength support under the existing engineering conditions. Hence, the control should mainly be applied to the floor heave deformation. When the thickness of the caving coal was more than 5.9 m, the main roof strata was prone to instability and being cut along the edge of the coal pillar; the rock stress environment surrounding the roadway tended to revert back to the initial geostress state. The proposed floor heave control strategy achieved good results, and as the deformation of the floor heave decreased, the workload of the floor heave was also greatly reduced.

Waste rock filling in fully mechanized coal mining for goaf-side entry retaining in thin coal seam

A large amount of waste rock is generated during thin seam coal mining operations underground. The waste rock material stacked on surface also adds to the environmental pollution. So it is important to reduce the amount of waste rock by exploring gainful utilization methods. This paper deals with an investigation undertaken to develop an alternate gob filling material in laboratory and evaluating its efficiency numerically as well as in field. A coal–waste rock separation system and a roadway layout system were designed, and a new technique of fully-mechanized goaf-side entry retaining and goaf filling back hydraulic supports was proposed. These methods helped realize the goal of green mining, which was featured by waste rock consumption in nearby region without any need to be elevated to ground surface. This study demonstrated that the two proposed mining methods, namely, caving mining method and mining with filling method, differed in mechanical environments where roadside filling body was located, deformation, and failure scope. The maximum principal stress concentration degree near the roadside filling body for caving mining method was high with a maximum value of 42.5 MPa because of key rock blocks in overburden. As a result, the surrounding rock of roadside filling body was severely deformed with a maximum displacement of 1117 mm. The entire roadside filling body moved toward the inside roadway and experienced plastic failure. After goaf filling, filling bodies in goaf, roof and floor strata, and roadside were combined to form an entire withstanding deformation. The motion space of overburden rock was limited, the adjustment range of surrounding rock stress near the roadside filling body was narrowed, the maximum principal stress decreased to 29.5 MPa, and the main roof did not rupture. Accordingly, the deformation of surrounding rocks of the roadside filling body was small, the maximum displacement of roof and floor was 457 mm, and the roadside filling body did not experience plastic failure. The study results enrich goaf-side entry retaining theory in thin coal seam and mechanized filling technology and provide a theoretical foundation for thin coal seam mining with difficulty in waste rock treatment.

Mathematical Modeling of Stress-Strain Evolution in the Rock Mass around a Mine Opening. Evaluation of the Steps of First Roof Caving at Different Thicknesses of the Main Roof

Intensive mining activity has highlighted the need to study the dynamics of rock mass elements and to assess the degree of their nonequilibrium. The roof, floor, pillars, gateways, and so on are in a highly nonequilibrium state due to difficult mining conditions and high face advance rates. A good tool for predicting the mechanical behavior of the rock mass during mining is mathematical modeling, which is based on the solution of dynamic problems taking into account the nonequilibrium and nonstationary deformation and failure of rocks. In this paper, dynamic modeling has been performed within an evolutionary framework to investigate the steps of first caving in a model rock mass during face advance. All other things being equal, the thickness of the main coal seam roof is varied in the calculations. The steps of first caving are evaluated in the conditions of highly nonstationary deformation of the rock mass. The fluctuation statistics of the stress-strain parameters of the rock mass is analyzed. It is shown that the first roof caving is preceded by a fall in the slope of the amplitude-frequency curve of stress fluctuation.

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.

DYNAMIC EVOLUTION CHARACTERISTICS AND MECHANISM OF SURROUNDING ROCK FRACTURES DURING THE REPEATED MINING OF CLOSED DISTANCE DEEP COAL SEAM

The disturbance mechanics behavior and mechanics mechanism of repeated mining of coal seam group are the important theoretical basis for safe mining of deep coal seam group. For the characteristics of interaction among coal seams under the condition of combined mining of coal seam group, in this paper, the spatio-temporal evolution characteristics of 3D mining stress field and the law of strata activity in repeated mining of deep coal seam group were studied by using similar material simulation and numerical simulation methods. The results showed that during the first mining, overburden mining broke the initial stress balance; part of the surrounding rock stress was released and free space was formed; rock mass moved to the direction of goaf, which released the pressure and protected the rock mass; a caving arch was formed in the overburden strata of coal seam whose arch trace was composed of the boundary line of separated strata and the fracture line of rock beam. As the working face continued to advance, the exposed strata in the arch gradually transferred and expanded to the upper strata, and the separated strata also gradually propagated upward, thus forming the periodic failure process of the caving arch. During repeated mining, the upper coal seam underwent the support and compaction process of caving, natural filling, the integrity of the main roof of the underlying coal seam was destroyed, all the interlayer strata collapsed or broke, and were connected with the goaf of the upper coal seam working face, resulting in the fractured rock mass of the goaf directly transferring load downward, and then the periodic weighting of mining was relaxed. The results have certain reference value for the combined mining of coal seams under similar conditions.

Mechanisms behind strong strata behaviour in high longwall mining face-ends under shallow covers

Abstract Safe and efficient mining of shallow coal seams relies on the understanding and effective control of strata behaviour. Field measurements, theoretical analysis and numerical simulations are presented in this study to investigate the mechanism behind abnormal strata behaviour, such as roof collapse and severe roadway deformation, that occurs in high longwall face-ends under shallow cover. We observed that coal pillars with two sides being mined out become unstable when the cover depth exceeds a certain value. The instability of the coal pillar can alter the fracture line of the overlying strata, triggering a reversed rotation of the ‘curved triangle blocks’ that form after the breakage of the overlying main roof. The revolving blocks apply stress on the roof strata directly above the longwall face-end, resulting in roof collapse. The collapse of both the coal pillars and the roof also leads to the advancement and increase of the overlying abutment pressure, which further causes severe roadway deformation in front of the working face. The strong strata behaviour that occurs in high longwall face-ends with shallow cover is presented in this study and countermeasures are proposed, such as widening or strengthening the coal pillar, or implementing destress blasting. The countermeasures we proposed and the results of our analyses may facilitate the safe mining of shallow coal seams with similar problems in the future, and may improve the safety and efficient working of coal mines.

Study on Key Parameters of Roof Cutting and Pressure Release in Medium-Thickness Coal Seam

According to the characteristics of strata behaviors under the condition of roof cutting and pressure release, this paper takes the 5-200 working face of Dianping coal mine as the engineering background, and comprehensively adopts the theoretical analysis, numerical simulation and field measurement to study on key parameters of roof cutting and pressure release in medium-thickness coal seam. The results show that: (1) The theoretical analysis determines that the height of roof cutting is 8.3 m, the angle of roof cutting is 15°, and the distance between the blasting holes is 500 mm; (2) The peak value of stress under the condition of roof cutting is lower 38.8% than it been under the condition of non-cutting, and the maximum displacement of the roof is reduced from 2500 to 420 mm. The first weighting length of main roof on the side of cutting roof is increased by 6 m compared with the non-cutting side, the periodic weighting length is increased by 12 m, the peak stress is reduced by 29.3%, and the average stress is reduced by 20.9%. The size of the roadway is 2601 mm × 4673 mm, which is in line with the numerical simulation results. The roof cutting can significantly reduce stress, deformation, improve the retaining effect on roadway and enhance production efficiency.

Investigations into Mining-Induced Stress–Fracture–Seepage Field Coupling Effect Considering the Response of Key Stratum and Composite Aquifer

The effect of mining-induced stress–fracture–seepage field coupling (SFSC) is critical to the safety and productivity of the underground mining. This study investigated the spatiotemporal effect of SFSC at the Bulianta Colliery panel #31401 of the Shendong coalfield in China, characterized by ladder key stratum and composite aquifer. An integrated method including borehole and working face observation, in-site water leakage, and real-time video imaging was adopted to examine the behavior of overburden movement, water level change, and groundwater inrush. Simultaneously, the evolution of the SFSC was visualized by the FLAC3D–CFD (fluent) simulator with a developed non-Darcy model, and the dynamic characteristics of the SFSC were analyzed. The results indicated that the macro-stress shell, fractured field distributed beneath the macro-stress shell with a 48 m height difference in the lower and higher key stratum zone, and seepage field including initiation zone, acceleration zone, deceleration zone, and drainage zone were identified. The confined aquifer was affected by the fracture field with a height of 140–154 m, and groundwater invasion was triggered and developed beneath the lower key stratum. The confined aquifer was kept stable by protection of the higher key stratum by controlling the propagation of the fracture field at the height of 90–100 m. Highly permeable channels were distributed beneath the micro-stress shell as a result of the release of in-site stress, especially at the rupture region of the micro-stress shell between the lower and higher key stratum, with the maximum fluid discharge. Furthermore, the SFSC response resulted in large and medium groundwater invasion and strong variation of abutment stress peak, when creating rupture at the micro-stress shell and main roof. Finally, based on the mechanism of the SFSC, the developed optimization strategy could efficiently mediate the effect of the SFSC by lowering the mining height, increasing the advance rate, and enhancing the support resistance of the longwall mining face.

Analytical study to estimate rib spalling extent and support requirements in thick seam mining

In order to better understand the mechanism behind the poor stability of support and surrounding rocks at mechanized working faces with increased mining height (MWMH), a conceptual model of the support and rocks surrounding the MWMH, under different structural conditions of the main roof, is established in this study. On the basis of the mechanical model, the interaction among the coal wall, support and roof in MWMH is studied. The analytical analysis reveals that the coal wall and support interact more significantly in the MWMH than conventional approaches (e.g., conventional longwall mining). With the increase in the depth of rib spalling in coal wall, the support strength, needed to control the roof, increases linearly. While as the support strength increases, the depth of rib spalling in coal wall showed a hyperbolic-like decrease. Based on these results, the expression, which describes the relationship of the interaction between the rib spalling in coal wall and support strength in the MWMH, is proposed. Rib spalling is then controlled in real working face in a soft, thick coal seam. The field experiment demonstrates that the advanced long-hole hydrostatic pre-injection can control rib spalling in coal wall and guarantee the stability of support and surrounding rock, which enables a high-efficiency mining of the MWMH.

Neural Network Assisted Analysis for Longwall Gate Road Stability Using Measured Roof Convergence Data

Immediate roof of a longwall panel is composed of layered coal, shaly coal, shale and clay strata overlaying by a thick sandstone main roof. The average depth of the coal seam is about 420 m and it has dip of 10°. The gate roads of width 5.2 m and height 3.5 m are developed in either sides of the panel and the roof is supported by 2.4 m resin grouted rock bolts. A special resin grouted 60 tonne capacity, 6.1 m long bulb type cable bolt is also installed for effectively supporting 8 m width shield installation roadway and gate roads during retreat of the longwall panel. The paper emphasizes on the monitoring and analysis of roof convergence in gate roads by convergence recorder as well as double telescopic telltale devices. Based on the analysis results, design is modified to improve the stability of gate roads. Cumulative roof convergence (CRC) data during development of gate roads are found to vary as tangent hyperbolic function with passage of time and are analyzed using artificial neural network to estimate the key parameters of the function. Convergence increase rate along with the maximum CRC is then used to prepare a Trigger Action Response Plan for immediate notification of the ground control problems. Data analysis also suggests that during retreat of longwall panel, CRC increases exponentially if the longwall face approaches within 10 to 25 m from a measuring station installed along main and tail gate roads. In this paper, methodologies are described for the analysis of CRC data for better understanding of roof behavior and for taking timely decision for any unusual event.

Analysis of the Initial Fracture Characteristics for Main Roof Overlying Fully Mechanized Face with Large Mining Height

The fully mechanized mining technique with large mining heights has become the most widely adopted mining method for thick coal seams in Shendong Coalfield of China. Increases in the mining height result in rib spalling, support instability, and other disastrous accidents, in addition to abnormal roof weighting. Knowledge of the initial fracture characteristics of the main roof is critical in the control of ground pressure. Based on an analysis of the impacts of controlled caving on fractures in the main roof, a mechanical model of rock beams that considers the supporting effects of hydraulic support and caved gangue is proposed. The initial fracture characteristics of the main roof are analyzed. The results indicate that the initial step of fracturing of the main roof is dependent on the physical and mechanical properties of the roof, the external load, and the support provided by the hydraulic support and caved gangue; the initial step increases with increases in the supporting effect and the roof-control distance of the hydraulic support and caved gangue. In combination with in situ monitoring data, the characteristics of the main roof are verified. The support provided by the caved gangue has different effects on different positions of the main roof, and this effect is smaller for higher positions.