Roof Stability Research Articles

Study on mining area of roof surrounding rock stability of mining fissure development

The “double end sealing device of drilling drilling water injection leakage” section, to the different depth of sealing hole drilling water injection test, observation in different directions and different drilling water Kong Shen missed development in mining fracture analysis at different depth of stratum loss, combined with different directions borehole sections of fissure, preliminary judgment the test site of mining roof surrounding rock fracture stability region boundary of mining; use of field test data, analysis of Yu Yang coal mine N21110 mining fissure development scope and the degree of fracture development in mining area of roof stability.

Field and numerical investigation on roof failure and fracture control of thick coal seam roadway

Compared with rock roof roadways, coal roof roadways are more sensitive to changes in the stress field due to excavation and mining. For thick-coal-roof roadways, the roof damage range usually exceeds the length of roof bolts, which brings security risks to roadway support. Taking the thick-coal-roof roadways as the study subject, the failure evolution of a thick coal roof under inefficient support was studied for two important geological conditions, namely the gob effect and variation in the coal roof thickness. The field investigation and numerical simulation were performed. The results showed three severely damaged areas from shallow to deep in the thick coal roof, namely a lower area, upper middle area, and coal-rock interface area. The failure of lower coal roof directly affected the stability of the entire thick coal roof. The residual abutment stress of adjacent gob aggravated damage to the lower coal roof. Roof subsidence and damaged area obviously increase with the thickening of coal roof. A critical failure height in the thick coal roof is found. An improved support was proposed adopting a thick anchorage structure and roof surface reinforcement for thick-coal-roof roadways. A field test showed that the coal roof surface of the roadway remained smooth and intact during excavation and mining period. The deformation of the roadway in mining period was reduced by 50%. The damage height of the thick coal roof was also reduced.

Stability Analysis and Control Measures of Large-Span Open-Off Cut with Argillaceous Cemented Sandstone Layered Roof

This paper is based on the condition where layered argillaceous cemented sandstone as an engineering background is met by the No. 207 fully mechanized working face open-off cut (Wanli No. 1 Coal Mine). Through mechanical theory analysis and field practice, the engineering safety problem of the large-span argillaceous cemented sandstone layered open-off cut roof supporting structure was analyzed. The roof caving arch height of the open-off cut roadway in 207 working face was obtained based on the mechanical mechanism of instability and caving of the layered surrounding rock mass roof. The anchor cable suspension and bearing stability of the open-off cut roof were analyzed in terms of the layered beam structure model. Meanwhile, combining with conditions, reasonable and effective support countermeasures and key parameters are proposed for such open-off cut roadway and enhance the actual supporting engineering on-site. These research results could provide engineering reference for an open-off cut roadway with composite roof conditions featured to weak cementation and weak interlayer.

Stability Assessment of Deep Three-Soft Outburst Coal Seam Roof Based on Fuzzy Analytic Hierarchy Process

The stability of deep “three-soft” coal seam roof has always been a key issue in coal mining. There are a lot of factors affecting the stability of deep three-soft coal seam outburst roof. However, there is currently no definite method able to draw an accurate assessment conclusion on roof stability. In order to accurately determine the main influencing factors of the stability of deep three-soft coal seam outburst roof and reduce the loss of coal production, this paper performed three-soft coal seam risk identification on Lugou Mine based on the introduction of the fuzzy analytic hierarchy process theory. 23 main risk factors were identified. Then, it established a hierarchical structure model of coal seam roof stability in accordance with experts’ opinions. The analytic hierarchy process was used to calculate the weights of indicators at all levels. Next, the paper used the fuzzy comprehensive evaluation method and expert scoring to evaluate various risk factors in the indicator system, as well as the overall safety level. The results showed that the deep three-soft coal seam stability of Lugou Mine ranks the third hazard level. The main risk and harmful factors include safety awareness, safety monitoring system, roof weakness, ventilation system, fire-fighting system, and rock bolt quality. In response to the evaluation results, this paper formulated corresponding control measure in terms of ventilation risk, safety monitoring risks, construction personnel risks, and fire protection risk to reduce losses in the mining process, providing a new evaluation method for the stability assessment of deep outburst coal seam roof.

The velocity of roof deflection as an indicator of underground workings stability – Case study from polish deep copper mines

The paper presents the research results of a study concerning the recognition of roof instability mechanism in the conditions of the Legnica-Głogów Copper Belt (LGCB), in which the final mining phase encompasses roof collapse. Due to the high potential risk of this type of phenomenon to the health and safety of mine operators it was decided to launch a roof stability assessment study in order to identify timely preventive measures adequate for this hazard.The approach is based on taking measurements of the changes of roof angles of inclination near the sidewalls of the mine working using high-sensitivity inclinometers fixed to the end of rock anchors in the roof. In this paper, it is shown that this method works well as it captures the natural bending phenomena of the roof prior to its collapse. It is found that the observed roof bending deformation is predominantly the result of combined gravity force and in-situ horizontal stress, regardless of the presence of other factors.Field measurements indicate that the roof collapse is preceded by a characteristic process covering three specific phases of development: these are described in detail. A new criterion for the recognition of imminent roof destabilization was developed based on the analysis of collected field measurements.

Roof fall hazard assessment with the use of artificial neural network

The paper presents the results of roof fall hazard analysis for the copper mines in Poland. At first, RMR and RFRI systems have been checked as a tool for roof fall hazard assessment. It was proved that rock mass deteriorates in time because of mining operation at the site, hence the roof fall phenomena analysis needs a new approach. In the next step of the analysis the key factors of roof deterioration were determined, and divided into four groups: geological, mining, technical and monitoring. It was demonstrated that only regular roof monitoring can ensure the proper rock fall hazard assessment. Subsequently, the artificial neural network was created and several dozen simulations were conducted. As a result, two dimensionless indices were developed. The first one shows predisposition of a part of the rock mass to destruction, dislocation and deformation - CRFP - Coefficient of Roof Fall – Predisposition and the second one represents predisposition and possibility of maintaining of the working - CRFM- Coefficient of Roof Fall - Maintenance. The research on roof fall details and the running workings’ roof observations allowed for categorization of the values of both indices, and providing that way the information about roof stability CRFP, and the information about necessary measures and solutions for the supervisory staff in regard of monitoring of the roof rocks CRFM. Both indices allow for a reliable roof fall hazard assessment by comprehensively combining information from monitoring, observations and investigations.

An integrated study on the geochemical, geophysical and geomechanical characteristics of the organic deposits (Coal and CBM) of eastern Sohagpur coalfield, India

Coal mining, especially the underground mines are often encountered with several types of geological constraints, where massive-hard roof and trapped gas are the most serious threats leading to catastrophic (fatal) effects in form of roof falls, mine explosion and gas contamination. Thus, for enhancing the safety of underground coal mines, identification of aforesaid constraints during detailed exploration is very essential. The minable seams of Sohagpur Coalfield are often associated with rich Coal Bed Methane (CBM) content, which enhances their economical importance, but the gas content along with the presence of stable sandstone roof parallely induces challenges during their extraction. CBM also acts as an environmental hazard having twenty-one times more greenhouse effect than carbon dioxide (CO2). Therefore, the present paper attempts to investigate the depositional environment of the high-quality methane-rich sub-bituminous reserves of eastern Sohagpur Coalfield through integration of geochemical, geophysical and geomechanical analysis of the drilled boreholes along with their respective gas content. The results showed that the six major Barakar coal seams possessed nearly consistent distribution in terms of reserve distribution and chemical properties. These seams consisted of good quality reserves (Grade: G5-G8) with high heat values and organic content, and had less impurity content. These coal seams are overlain by stable roof having lower cavablity in form of thick hard medium-grained sandstone beds with moderate elasticity and strength. The seam structures showed a stable profile without any abrupt discontinuity except Bahmni-Chilpa fault in southern part of block. Some of the higher rank coal seams also contained potential CBM content. Statistical analysis was also carried out for understanding the distribution of reserve properties and monitored their interdependence using suitable fitting tools. Thus, the present study helped in summarizing different aspects related to reserve quality, roof properties and gas content for understanding the geology and working environment of coal reserves of less explored but economically important Central Indian coalfields.

Assessment of instability initiation in exposed salt rock roofs

Stability of rocks is the main requirement for the safe operation of mines. For this purpose, certain measures are applied for the protection and support of underground openings, including roof support design and roof arrangement in the most stable rocks. Stability assessment of underground excavations is largely related to their roof stability. Determination of possible instability conditions in mine roofs governs the choice of support system design and parameters of mine excavations. The Upper Kama Potash Salt Deposit represents a stratified layer of solid salt rocks. Roof instability develops as stratification and roof collapse. The Mohr–Coulomb criterion of coherent rocks is currently used to estimate parameters of a possible damage zone in the exposed roof. This criterion allows evaluating shear plane angles in roof rocks and, as a result, finding parameters of the possible collapse zone. The experience of mining operations in the Upper Kama deposit shows different failure conditions as against the Mohr–Coulomb criterion as the stress state is scarcely included in the criterion used. This study is an attempt to assess parameters of rock exposures by solving a Lame problem in terms of a single mine excavation. The analytical results were compared with the parameters obtained from the plane elasticity solutions by the finite element method. Based on the implemented studies, an engineering approach is developed for the assessment of anticipated instability parameters in exposed roofs in horizontal excavations driven in salt rocks.

Unit weight patterns of overlying rocks above commercial-value salt strata in the Upper Kama deposit

Stability of rocks is the main requirement for the safe operation of mines. For this purpose, certain measures are applied for the protection and support of underground openings, including roof support design and roof arrangement in the most stable rocks. Stability assessment of underground excavations is largely related to their roof stability. Determination of possible instability conditions in mine roofs governs the choice of support system design and parameters of mine excavations. The Upper Kama Potash Salt Deposit represents a stratified layer of solid salt rocks. Roof instability develops as stratification and roof collapse. The Mohr–Coulomb criterion of coherent rocks is currently used to estimate parameters of a possible damage zone in the exposed roof. This criterion allows evaluating shear plane angles in roof rocks and, as a result, finding parameters of the possible collapse zone. The experience of mining operations in the Upper Kama deposit shows different failure conditions as against the Mohr–Coulomb criterion as the stress state is scarcely included in the criterion used. This study is an attempt to assess parameters of rock exposures by solving a Lame problem in terms of a single mine excavation. The analytical results were compared with the parameters obtained from the plane elasticity solutions by the finite element method. Based on the implemented studies, an engineering approach is developed for the assessment of anticipated instability parameters in exposed roofs in horizontal excavations driven in salt rocks.

Determination and Engineering Practice of Unsupported Roof Distance for Rapid Excavation of Roadway with Thick and Hard Basic Roof

In order to determine the reasonable empty roof distance during roadway excavation, a mechanical model of the direct roof rock beam was established, and the relationship between the direct roof tensile stress and the empty roof distance was obtained. According to the criterion of roof stability, that is, the tensile stress of the roof does not exceed the tensile strength, the expression of the empty roof distance during the roadway excavation is determined. Taking the 150,802 roadway in Liuzhuang coal mine as the research background, the horizontal and vertical stress distribution characteristics of the direct roof under different supporting forces are analyzed, and combined with the direct roof surplus safety factor, the reasonable empty roof distance during roadway excavation is determined as 2.0 m. By using FLAC3D numerical simulation software to analyze the distribution law of vertical stress and plastic zone when the empty roof distance is 2.0 m, it is obtained that the overall stability of the roadway surrounding rock within the empty roof area is better when the empty roof distance is 2.0 m, and it is applied to Liuzhuang Coal mine 150,802 roadway. The field application results show that when the empty roof distance is selected as 2.0 m during roadway excavation, the deformation of the roof and bottom of the roadway and the two sides are small, the stability is good, and the roadway excavation speed is increased by about 30%.

Experimental Study on the Displacement Sensor of Coal Roadways Roof Settlement Based on Distributed Fiber Optic Sensing

Roof displacement monitoring is an important means to ensure roof stability and prevent roof accidents of Coal Roadways (CRs). In this paper a new sensor of roof settlement displacement (RSD) monitoring of CRs' roof by the pre-stretched optical fiber (PSOF) laid out horizontally at fixed points was proposed. The principle of the sensor is to qualitively and quantitively characterize the RSD by the Fiber Optic Strain (FOS) which is measured based on distributed fiber optic strain measurement. Through theoretical analysis, the mathematical expression for quantifying RSD is derived. The correctness of the sensor is verified by the indoors FOS experiments and the field monitoring of CRs' roof in Yinying Coal Mine. This new sensor can be applied not only to the roof of CRs, but also to the displacement monitoring of other structures with the similar settlement presentation.

Roof Stability of Rectangle Coal Roadway: In Light of Calculation of Compressive Bar Stability

The roadway roof is a key factor to the roadway stability. The analysis of roof stability is mainly based on numerical calculation and on-site observation, while the basic theory of the bearing mechanism is relatively weak. We have founded a critical pressure calculation model, on the theory of compressive bar, for the rectangle coal roadway stability. The model has been tested and verified on accuracy and feasibility while applied on a roadway case. The critical pressure for roof stability and roof bending moment and deflection under combined axial and lateral load was deduced using the theory of compressive bar stability. The numerical calculation verified the feasibility of numerical modeling of stability of compressive bar using FLAC3D, and the influence of the background ambient horizontal stress and the parameters of the contact surface to the roof stability were further studied. The result turns out that some factors lead to a higher instability tendency, including higher horizontal stress, higher cohesion force, and larger internal friction angle on the coal-rock interface and lower cohesion force and smaller friction angle on the rock-rock interface. The results contribute to bearing mechanisms of roadway roof stability, ground pressure and strata control theory and application, and design of bolting support.

Research on short wall continuous mining technology with full roof falling and roof control measures

Short wall mining technology is an important method to coal resource mining in the United States, Australia and other countries, and also one of important methods to boundary coal resource recovery in China. Short wall mining technology through leaving a large number of coal pillars to support and control roof, induces a series of accidents, such as large area roof suspended, collapse and shock, which restricts the further popularization and application of short wall mining technology seriously. By improving the existing short wall mining technology, the article proposed short wall continuous mining technology with full roof falling, and designed measures to control roof. The main research work is as follows: (1) By reducing and eliminating coal pillars in mining section as much as possible, including entry protecting pillar and pillar between caverns, also ensured stability of roof during mining period at the same time; after mining, roof can be collapse, to prevent large area roof suspended and collapsed inducing shock accidents; mining process and equipment of short wall continuous mining technology with full roof falling, were designed comprehensively. (2) Measures to control roof of short wall continuous mining technology with full roof falling are designed, using self-moving hydraulic support to control roof of mining cavern, anchor bolt and cable to ensure branch and cross entries stable during mining period, and forced caving technology to control large area roof suspended; (3) Short wall continuous mining technology with full roof falling and roof control measures were successfully industrialized in 42,210 boundary coal face of Yujialiang coal mine, which belong to Shenhua energy group in Inner Mongolia. The research result is significant to enrich and develop short wall mining technology.

Analytical solutions and 3D numerical analyses of a shallow tunnel excavated in weak ground: a case from Turkey

Due to the increasing population and resulting transportation needs, the number of subway and high-speed railway projects has also increased. The geometric constraints of such projects have caused many tunnels to be built in weak ground. Thus, weak ground tunnelling has attracted the attention of tunnel engineers and researchers. The main purposes of this study are to analyse the T4 tunnel excavated in weak ground and to compare the results obtained from the analytical solutions and 3D numerical analyses. This study specifically considers the T4 tunnel support system used in the Ankara İzmir High Speed Railway Project (Afyonkarahisar-Banaz Section). The T4 tunnel route encounters weak ground composed of layers of extremely weak mudstone, clayey sand, weakly cemented sandstone, and silty–clay matrix with pebbles. The tunnel overburden ranges from 10 to 35 m, which is shallow. After the excavation work of the T4 tunnel, severe deformation and critical stability problems in the shallow part (where the overburden is approximately 10 m) were encountered inside the tunnel, leading to a halt in construction. This was followed by revisions to the tunnel support system, leading to successful completion of the tunnel excavation. Numerical simulations of the low overburden section are performed using the commercially available FLAC3D program that uses the finite difference method. The characteristics of insufficient/ineffective support systems and adequate support systems for shallow tunnels excavated through weak ground are discussed in this study. Additionally, problems that pertain to the tunnel itself and its support system are discussed. The results of the 3D numerical analyses and analytical solutions are compared, and the advantages of 3D numerical analyses are discussed. The importance and necessity of tunnel face stability and roof stability for tunnel stability in weak ground is illustrated. Consequently, solutions based on analytical and numerical analyses are presented, and the analysis methodology and solutions proposed in the study can help guide weak ground tunnelling design and evaluation.

Algorithm for Vertical Bearing Capacity Calculation of Rock-Socketed Piles in Karst Area Based on Load Transfer Method

Due to the complex geological conditions and many influencing factors in karst areas, this paper conducted theoretical, experimental research and calculation analysis on the design and construction of engineering piles of bridge engineering in karst cave areas. An algorithm for vertical capacity calculation of rock-socketed piles in karst area based on load transfer method was developed. The results using this new method were analyzed and compared with the results from the static load test of three engineering piles in karst area, and the differences are relatively small. Through static load tests of pile foundation in karst area and study of the side resistance of rock-socketed pile in karst area, the friction resistance of rock-socketed pile and the effect of pile-end resistance, the bearing capacity characteristics of rock-socketed pile in karst area were obtained, achieving the load-sharing effect of cave roof in a rather scientific way. In view of the failure mechanism of the karst cave roof under the pile tip and the large proportion of lateral resistance of the rock-socketed pile, a calculation model for the stability of the karst cave roof under the combined action of the side resistance of the rock-socketed pile and the pile tip resistance is proposed.

Analysis of the Impacts of Mining Sequence and Overburden Depth on Stability at a Dipping Limestone Mine

Ground falls represent a significant hazard at underground mines in the stone, sand, and gravel (SSG) sector in the USA. Researchers from the National Institute for Occupational Safety and Health (NIOSH) are currently conducting detailed investigations into the complex loading conditions at underground stone mines operating in challenging conditions. This paper presents the application of numerical modeling to analyze pillar and roof stability at a dipping underground limestone mine. A validated numerical model was used to explore the potential behavior of the pillars and roof as loading conditions change. The validated model was used to compare changes in mining sequence, overburden depth, and the in situ stress field. This will allow mine operators and engineers to have a better idea of the conditions that could be encountered as mining progresses. Results from the numerical modeling indicate that roof displacement more than doubles as the vertical stress increases from 10 MPa (1450 psi) to 19 MPa (2750 psi) when the maximum and minimum horizontal stresses were 41 MPa (5950 psi) and 22 MPa (3190 psi), respectively. Consequently, as the pillar load increases, the safety factor of the pillars is projected to decrease by about 25%. The impact of the practical application of numerical models can result in a reduction of ground-fall accidents and injuries as well as generally safer working conditions.

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.

A Method for Determining the Thicknesses of the Reserved Protective Layers in Complex Goafs Using a Joint-Modeling Method of GTS and Flac3D

In this study, a C-ALS underground cavity scanner was used to detect the shapes of mining goafs. In addition, GTS software was adopted to establish a three-dimensional geological model based on the status of the stopes, geological data, and mechanical parameters of each rock mass and to analyze the roof areas of the goafs. In regard to the morphology of the study area, based on a thin plate theory and the obtained field sampling data, a formula was established for the thicknesses of the reserved protective layers in the goafs. In addition, a formula for the thicknesses of the protective layers in the curved gobs was obtained. The thickness formula of the protective layers was then successfully verified. The detection results showed that the roof shapes of the goafs in the Yuanjiacun Iron Mine were mainly arc-shaped, and the spans of the goafs were generally less than 20 m. The stability of the arc-shaped roofs was found to be greater than that of the plate-shaped roofs. Therefore, by reducing the thicknesses of the protective layers in mining goafs, the ore recovery rates can be increased on the basis of safe production conditions. The formula of the thickness of the security layers obtained through the thin plate theory was revised based on the statistical results of the roof shapes of the goafs and then combined using GTS and FLAC3D. The modeling method successfully verified the stability of the mined-out areas. It was found that the verification results were good, and the revised formula was able to improve the recovery rate of the ore under the conditions of meeting safe production standards. Also, it was found that the revised formula could be used in the present situation. At the same time, it was also determined that the complexity of the rock masses obstructed the full identification of the joints and fissures in the present orebodies. Therefore, it is necessary to incorporate C-ALS underground cavity scanners to regularly observe the shapes of the goafs in order to ensure that stability and safety standards are maintained.

On the existence of optimal shapes in architecture

We consider shape optimization problems for elasticity systems in architecture. A typical objective in this context is to identify a structure of maximal stability that is close to an initially proposed one. For structures without external forces on varying parts, classical methods allow proving the existence of optimal shapes within well-known classes of bounded uniformly Lipschitz domains. We discuss this for maximally stable roof structures. We then introduce a more general framework that includes external forces on varying parts (for instance, caused by loads of snow on roofs) and prove the existence of optimal shapes, now in a subclass of bounded uniformly Lipschitz domains, endowed with generalized surface measures on their boundaries. These optimal shapes realize the infimum of the corresponding energy of the system. Generalizing further to yet another, very new framework, now involving classes of bounded uniform domains with fractal measures on their boundaries, we finally prove the existence of optimal architectural shapes that actually realize the minimum of the energy. As a by-product we establish the well-posedness of the elasticity system on such domains. In an auxiliary result we show the convergence of energy functionals along a sequence of suitably converging domains. This result is helpful for an efficient approximation of an optimal shape by shapes that can be constructed in practice.

A robust approach to identify roof bolts in 3D point cloud data captured from a mobile laser scanner

Roof bolts such as rock bolts and cable bolts provide structural support in underground mines. Frequent assessment of these support structures is critical to maintain roof stability and minimise safety risks in underground environments. This study proposes a robust workflow to classify roof bolts in 3D point cloud data and to generate maps of roof bolt density and spacing. The workflow was evaluated for identifying roof bolts in an underground coal mine with suboptimal lighting and global navigation satellite system (GNSS) signals not available. The approach is based on supervised classification using the multi-scale Canupo classifier coupled with a random sample consensus (RANSAC) shape detection algorithm to provide robust roof bolt identification. The issue of sparseness in point cloud data has been addressed through upsampling by using a moving least squares method. The accuracy of roof bolt identification was measured by correct identification of roof bolts (true positives), unidentified roof bolts (false negatives), and falsely identified roof bolts (false positives) using correctness, completeness, and quality metrics. The proposed workflow achieved correct identification of 89.27% of the roof bolts present in the test area. However, considering the false positives and false negatives, the overall quality metric was reduced to 78.54%.