Roof Convergence Research Articles

Research on the Twin-Link Stepping Temporary Support Mechanism and the Repeatable Supportability of the Roof in the Underground Excavation Roadway of Coal Mine

The coal mine’s underground heading roadway suffers from a lack of effective temporary support, which contributes to low efficiency and inadequate safety in excavation operations. This paper presents a novel approach for roadway excavation, support, and anchoring, utilizing the twin-link stepping temporary support method. To address the stability requirements of the roadway roof in this support method, a numerical model of the excavation face was developed using the BBM (Bonded Block Model) method in 3DEC 7.0 software. Through numerical simulation, the repeatability of the twin-link stepping temporary support process was assessed for various roadway roof rock types, thicknesses, and burial depths. By considering aspects such as roof fragmentation, roof convergence, and relative displacement between the direct roof and the bedrock, the range of roadway roof repeatability was determined. The proposed twin-link stepping temporary support process, along with its applicable roof conditions, establishes a theoretical foundation for implementing the parallel operation mode in coal mine underground heading roadways, thereby significantly enhancing safety and efficiency.

Қарнақты бекітпелермен жоғары кернеулерден контурға жақын массивінің геомеханикасын бақылау арқылы қайта пайдаланылатын дайындау қазбаларының тұрақтылығын арттыру

An analysis of existing methods and means to improve the sustainability of reused development workings has been made. to protect the workings on the border with the goaf permanent artificial fences should be used: organ rows, bushes of racks, bonfires made of sleeper timber filled with rock, bonfires with an organ row, organ rows with a rubble strip, supports of high strength and limited compliance, etc. and portable protective supports made of metal friction struts, hydraulic struts, etc. The following factors are accepted as influencing factors: the presence of zones of increased rock pressure; strength characteristics of the rocks of the roof and soil of the formation and their thickness; structure and water saturation of rocks; landing step of the main roof; the presence of disjunctive geological disturbances; lava movement speed; power characteristics of the lining of the excavation. The output parameters are: the magnitude of the heaving of the formation soil, the deformation of the elements of the lining of the mine working; breaks in the interlocks of the support, displacement of the working contour in the vertical and horizontal plane, displacement of the side arched legs into the working cavity, etc. All these parameters were considered from the point of view of the need to invest in the development of material and labor costs to bring it into an acceptable operational condition. The nature of the pressure distribution depends on the extracted thickness of the reservoir, for example, the displacement and convergence of roof and soil rocks with an increase in reservoir thickness from 1.0 to 2.0 m increase, on average, by 10-20%. The results of the analysis indicate more "heavy" conditions for maintaining the workings, formed behind the lava by 1.3-2.5 times and reused 1.7-3.5 times compared to the undercut and workings made in the coal massif.

Study on the Stress Evolution and Strengthening Support Timing of the Retracement Channel under the Super-Thick Nappe

The superposition effect of the advanced support pressure of the working face in the final mining stage and the lateral support stress of the roadway is a key factor affecting the stability of the retracement channel. To study the stress evolution of the retracement channel under the super-thick nappe and the timing of strengthening support, this paper takes the mining of the 360808 working face in Xinji No. 1 Mine as the engineering background, analyzes the occurrence conditions of the working face and the measured rock pressure law, and constructs a roof structure model of the retreat area. The UDEC discrete element numerical simulation software was used to analyze the evolution characteristics of concentrated stress and the failure law of surrounding rock around the retracement channel under gradual excavation conditions. Based on the relationship between the position of the main roof fracture and the stability of the surrounding rock of the retracement channel, the instability mechanism of the surrounding rock of the retracement channel was revealed. A mechanical model of the surrounding rock of the retracement channel under the condition of a gradient coal pillar was established, and the energy criterion K for the instability of the surrounding rock was obtained. The method of adding anchor cables to strengthen the support of the surrounding rock of the retracement channel was proposed. The results indicate that the accumulation of energy in the surrounding rock of the retracement channel is greater than the internal consumption of energy, which is the direct reason for the instability of the surrounding rock of the retracement channel. The time to strengthen the support of the roof is when the working face is 15 m away from the retracement channel. According to the analysis of on-site monitoring results, the roof convergence and the two-sides convergence before and after strengthening the support were reduced by 90 mm and 140 mm, respectively. Under the strengthening of support, the slope of the retracement channel in the 360808 working face is slight, without roof fall, and the surrounding rock of the channel is effectively controlled, which is of great significance for ensuring the safe application of the retracement channel. It has reference value for the safety production of surrounding mines and is conducive to promoting the sustainable development of local resource-based society and economy.

Research on the coupled support technology of a composite rock beam-retained roadway roof under close coal seams

IntroductionThe technology of gob-side entry retaining without coal pillars in close (distance) coal seams is still immature, and the roof control and support technology in this case is not perfect.MethodsIn this paper, the coupled support technology of a composite rock beam roof under close coal seams is systematically studied by using theoretical analysis, numerical simulation and field test.ResultsBoth the floor slip calculation results and numerical simulations indicate that the vertical failure depth in the plastic zone of the #8 coal seam has not penetrated the roof of the #9 coal seam after mining, which is consistent with the field electronic imaging results. A theoretical formula for a composite rock beam model anchored by high-prestressed anchor cables was derived, and a formula for the optimal spacing of anchor cables under noncompressive shear failure of the roof was obtained. Identification of the internal stress hazard region of the rock beam provides a basis for determining the locations of vertical support. Through numerical simulation of different support schemes, including roof cutting, arrangement of high-prestressed anchor cables, and setting up of vertical supports, roof cutting was found to effectively reduce the stress of supporting structure and roof pressure. Setting up of vertical supports can reduce the roof convergence by 25.2%, and coupling with anchor cables can reduce the convergence by more than 49.1%. The feasibility of this support scheme was verified through field tests, with a maximum convergence of 99 mm between the roof and floor.DiscussionThis two-way verification approach, in which the damage degree of the roof of a close coal seam is analyzed through multiple means, targeted support plans are proposed, the support mechanisms are explored, and feedback is conducted through field tests, plays a certain guiding role in solving roof control of the gob-side entry retaining under similar geological conditions.

Research on gob‐side entry‐retaining technology with coal rib and corner strengthened support in soft rock strata

AbstractThe gob‐side entry‐retaining by roof cutting and pressure relief (GERRC) method is an advanced no‐pillar mining method that can significantly improve coal recovery and reduce roadway accidents. However, according to case studies, there is relatively little research on the application of the GERRC method in geological conditions of soft rock formations (especially when the roof and floor are both mudstone), and such research is needed. Therefore, based on the typical soft strata of the Xinyi Coal Mine, this paper analyzes the failure mechanism of the roadways and proposes the GERRC method with coal rib and corner strengthened support (CRCSS). The evaluation indexes of the roof convergence rate and cable safety margin are established, and the effects of roof cutting and CRCSS on roadway deformation control are explored by a numerical comparison test. The results show that roof cutting has a limited effect on roadway deformation. The deformation can be reduced by 75.77% by implementing CRCSS on the basis of GERRC. Then, the evolution characteristics of roadway stress and deformation in the process of mining are further discussed through field contrast tests. The field application results show that compared with that of GERRC with conventional support, the surrounding rock deformation of GERRC with CRCSS is reduced by more than 55%, the safety margin of the cable is improved. The overpressure of the hydraulic prop is diminished, and the stability distance of the roadway is reduced by approximately 30%. Finally, this paper discusses the mechanism to control roadway deformation by CRCSS. This research expounds the rationality of CRCSS, and the results can provide a reference for other situations with similar geological conditions.

Evaluation of Rockburst Hazard Under Abandoned Mine Workings

Recalling the body of experience gathered in the collieries of the Upper Silesian Coal Basin, the increased risk of seismicity and rockburst occurrences in confined conditions including the exploitation of remnants were identified. This study investigates geomechanical aspects of longwall mining in the areas affected by old excavations aimed at relaxation of a multi-bed deposits within a thick coal seam or a group of seams. It is assumed that high-energy seismicity is another factor determining the rockburst hazard alongside the state of stress. A case study is recalled, describing a colliery where mining-induced seismic activity of a de-stressed coal seam remained at the level comparable to or higher than it was experienced in the de-stressed seam operations. An analytical model was used to study the stress state and potential loss of structural continuity of an undisturbed rock body surrounding the longwall panel being mined beneath or over the abandoned workings. Recalling the developed model of the system involving nonlinear functions demonstrating the existence of abandoned mine workings within the rock strata, computer simulations were performed to evaluate the rockburst hazards along the face area. Discussions of results are based on observations of immediate roof convergence and the vertical stress concentration factor at the longwall face zone. Computational data of the modelled mining situations demonstrates that despite using the de-stressing method of mining, the occurrence of events impacting on mine working beneath and over abandoned workings cannot be precluded. Here the scale of rockburst hazards is determined by local mining and geological conditions, such as the type and extent of abandoned workings, their age and vertical distance between them and the coal seam currently mined.

IOT Powered Smart Strata Monitoring System for early and precise detection of Convergence in Underground Coal Mines

Roof fall, bed separation and convergence are one of the serious life-threatening problems faced by the miners in all the underground coal mines since early mining age to current mining situations. Most of the statistics regarding fatal accidents and trapped miners in underground coal mines are the results of sudden roof fall due to bed separation of overlying strata in the underground mine galleries or roadways. In Indian underground coal mines, many of fatalities were seen over the years although this rate has been gradually decreasing since past decade only by implementing some of physical and mechanical methods to detect or monitor strata convergence. With the advancement of technology, sensors become an extensive part of everyday life. Various researchers are trying to monitor, detect and prevent fatalities due to roof fall in real time scenario in underground coal mines with the use of sensors, IOT and other technology. The main aim is to Install a non-invasive laser proximity sensor in the hole drilled into the roof where the necessity arises to remotely monitor the convergence in the strata under bad roof conditions in underground coal mines and monitor them remotely with an automated display showing the details of the bed separation, roof convergence and detailed information about the roof condition remotely. Key Word: Internet of Things, Strata Monitoring, Roof Fall, Convergence, Real time, Display device, Proximity sensors, Remote Monitoring.

Critical Height of Unfilled Zone in Underlying Gob Piles

To address problems associated with mining above gob piles in the Chinese midwest mining area, a mechanic model for a coal pillar was established based on the displacement variation method. Additionally, the effect of critical unfilled zone height in the underlying gob (UZHUG) on the coal pillar was investigated through a 3DEC numerical simulation; fieldwork was conducted with a borehole televiewer to measure the UZHUG in the Yuanbaowan Coal Mine. The results indicated that the shear stress value and distribution layout in the coal pillar were affected by the UZHUG. The larger the UZHUG, the greater the shear stress and the wider the distribution scope, moving from two sides to the middle zone. Moreover, the larger the UZHUG, the smaller the maximum bearing capacity of the coal pillar, and subsequently, the greater the horizontal deformation of the coal pillar and roof convergence. The critical UZHUG is 2 m considering coal pillar deformation and stress transfer characteristics. Field measurements have confirmed that the UZHUG of 2 m ensured safe mining in the No. 6 coal seam. However, a second round of filling is needed when achieving a UZHUG of 4.4 m. This study serves as a reference for safe mining above gob piles.

Study on Safety Control of Large-Section Roadway with High Stress and Broken Surrounding Rock

In order to solve the problem of difficult support of the roadway with high stress and large-section broken surrounding rock, this paper takes the subinclined shaft in Gaokeng mine of Jiangxi Province as the engineering background, analyzes the deformation mechanism and support mode of the roadway under the influence of mining through field investigation and mechanical derivation, and concludes that the stress concentration point of the roadway is in the middle point of roof and floor and the middle point of left and right sides. Through the modeling analysis of FLAC3D numerical software and the comparison of various support schemes, it is concluded that, after the combined support method of “anchor, net, and spray + grouting + full-section anchor cable + floor anchor cable“ is adopted, the convergence of roof and floor is reduced by 508 mm, and the convergence of two sides is reduced by 663 mm. And, it is applied in engineering practice. The results show that the combined support scheme can effectively control the stability of the surrounding rock.

The forecast of a possibility to re-use preparatory roadway with anchor-frame supports in mines of Western Donbass

The authors present a solution for an live scientific applied problem on predicting a possibility to re-use a preparatory roadway - a mother entry, which is reinforced with the anchor-frame supports - in the conditions of the mines in Western Donbass. The purpose of the research is to develop a complex of practical recommendations, implementation of which will create the conditions for re-use of the roadway in the conditions of the Western Donbass mines. The solution of this problem is based on the analysis of changes in geomechanical state of the enclosing rocks in the vicinity of the roadway. The obtained analytical data on changes of the rock geomechanical state in the vicinity of the mother entry under typical operating conditions outside zone of longwall influence, in the bearing zone of advancing face of the longwall and behind the face of the longwall show that it is possible to predict a possibility for re-using a roadway during the implementation of routine measures on its maintenance and protection. Identification of the calculated values of the convergence of the roadway roof and soil showed good convergence with the corresponding factual values measured during the mine research. As a result of the research, a set of measures is proposed, implementation of which will create conditions for the roadway reuse. It is also proposed to use a combined timbering of strengthening which will allow to support the roadway edge and to change direction of load on the roadway frame support for providing more yielding and, besides, to reduce roof rock lowering up to 400 mm, preserve geometry of the roadway and prevent deformation of the frame support elements. The proposed recommendations will reduce operating costs of the roadway maintenance, improve safety of its operation and increase economic efficiency of mining district. Keywords: reuse of production, convergence of roofing and soil working, prefab stripping, operational safety.

Digital modeling of geomechanical processes in a structurally heterogeneous geomassif for predicting the parameters of safe underground mining of coal seams

A digital model of geomechanical processes in a structurally heterogeneous geo-massif with consideration for the influence of natural and technogenic forces was developed. Based on the results of numerical modeling, the regularities of stress change, convergence of roof and soil rocks, stability of coal pillars between production and development workings were revealed with an increase in the depth of development and the thickness of the interbed rocks within the excavation column of a coal mine.

Design of panel and support system for extraction of inclined thick coal seam using numerical modelling

The depillaring operation develops high stress concentration in the workings and poses many strata control issues such as roof convergence, floor heaving, side spalling and pillar/ribs crushing. A detailed stability analysis need to be conducted to control these strata issues and estimate the optimum coal panel size and support system. In this paper, the stability analysis of the panel and design of the support system are presented for safe winning of inclined coal seam using 2D and 3D numerical modelling approach. The panel is developed by rhombus pillars of 25 m × 25 m size with 4.2 m and 4.8 m wide galleries and being proposed for depillaring. 3D and 2D numerical models are used to analyse the development of stresses during depillaring operation and determine the working conditions respectively. This study suggests the suitable pillar size, gallery width, mining height and the required support resistance.

Design of an Optimum Longwall Face for Improved Ground Control: a Review

This paper presents a critical review of the various approaches for the selection of the optimum length of longwall face considering ground control and cost parameters. Various factors affecting the efficacy of roof control and their impact on the productivity of a mechanized longwall face have been identified. The outcome of the study has been used to assess the knowledge gap in existing know-how and exploring the possibility of developing a hybrid methodology for the design of techno-economical optimum length of face combining the experiences of ground control and production economics of such operations. The critical ground control parameters identified for the purpose include massiveness of the strata formation and abutment loading that creates severe roof convergence and cavity formation in the face area apart from uncontrolled goaf settlement. Such roof control difficulties not only lead to frequent production delays and stoppages of the face but also contribute to the increased cost of production and reduced face productivity. Establishing a field representative relationship between the face length and severity of roof weighting and its integration with the existing cost economics is proposed for the optimum selection of face length to ensure smoother face operation with consistent face productivity.

Effects of particle sizes on compressive deformation and particle breakage of gangue used for coal mine goaf backfill

Broken gangue from the collapsing roof of a longwall coal panel can be used as the backfilling material for goaf backfilling. The particle size gradation has important influence on the compressive deformation of the gangue backfilling material and particle breakage. The compressive deformation of the gangue backfilling materials at 4 different particle size grades is simulated by PFC3D in this paper. The law of the compressive deformation of the gangue backfilling material, the particle cluster distribution and the variation of the gangue block shape are analyzed. The microscopic mechanism of the compressive deformation of the gangue backfilling material is investigated based on the force-chain distribution. The results indicate that when the gangue backfilling material has a small particle size, the specimen has low porosity and a few amounts of the broken particles can fill the pores of the material. When the particles have relatively large sizes, the specimen has relatively high porosity and the broken particles are unable to completely fill the pores. The loading conditions of the particles can only be changed by the frame structure formed by the large particles. If the proportion of the particle size grade is reasonable, a frame structure forms due to the large particles and the small particles with various particle sizes can fill the pores of the backfilling material. This enhances the deformability of the gangue and decreases the compressive deformation of the backfilling material. After backfilled into the goaf, the backfilling material bears the overburden pressure, which means the process of roof convergence is the process of compressive deformation of the backfill body. That is to say, the stiffer the backfilling material, the better the control effect. The importance of the reasonable proportion of the particle size grade is explained from the aspect of microscopic breakage of the particles, which provides a scientific basis for backfill mining.

Assessing longwall shield–strata interaction using a physical model

The stability of the longwall face and shields in thick coal seams requires a thorough understanding of shield–strata interaction, but the realistic hydraulic operations of longwall shields have been widely ignored to date. This paper reproduces the behaviour of the longwall face and shield in response to roof loading through a physical modelling study with a consideration of the basic operating principles of the shield leg cylinder. The extension and retraction of the leg cylinder and important characteristics of longwall shields are introduced. The results show that: (1) the shield vertical stiffness has a positive correlation with shield capacity, and it decreases with the extension of the hydraulic liquid inside the leg cylinder; (2) higher capacity shields develop more load in response to roof convergence, but retain an adequate reserve capacity to protect the shield from damage; (3) yielding of a modern shield represents a 10% drop in yielding pressure and produces 5–10 mm of reduction in shield height and 100–250 t of loss in support load; (4) the physical modelling study of shield–strata interaction with a consideration of leg hydraulic operation shows a step function of roof deflection. Face and roof displace at a slow rate during the initial shield loading stage but increase rapidly during the yielding events.

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.

Assessment of roof convergence during driving roadways in underground coal mines by continuous miner

In India, a number of coal seams are being extracted by using the continuous miner (CM). There is a need to standardise some of the major parameters like cut-out distance (COD) and the convergence of the roof strata for safe operation of the CM. The production and productivity of this system mainly depend on the COD, i.e. the length of the stable drivage made in the coal seam by the CM at a time without installing any reinforcement/support. This factor is determined by assessing the convergence of the roof strata. Therefore, prior to driving of the roadways by the CM, the estimation of the convergence of the roof strata is necessary for safe and stable driving of the roadways. No suitable empirical relationship is found between the convergence and the COD. This paper deals with the assessment of the roof convergence during development of the coal seams by the CM. Variations of the convergence with the major influencing parameters, i.e. the COD, the rock mass rating and the width of the gallery, are obtained by the parametric study through elasto-plastic numerical modelling. The trend of the convergence data with respect to the input parameters is found non-linear. Thus, a non-linear multivariate model is framed by putting the constant and exponents in the input parameters. These constant and exponents are determined through regression analysis to develop a predictive model. The coefficient of determination of the model is around 0.98. The model is validated with the monitoring data of different mines. This model can be applied for the assessment of the convergence of the roof in underground coal mines for safe driving of the roadways by the CM. Alternatively, optimum COD of the CM can be designed by fixing the threshold limit of the convergence value for a particular geomining condition.

Using the Relaxation Test to Study Variation in the Time-Dependent Property of Rock and the Consequent Effect on Time-Dependent Roof Failure

Field observations have demonstrated that roof failure occurs spatially in a mine from the time of excavation. It is suspected that time-dependent deformation propagates failure in the rock mass. In this paper, the relaxation test is used to study variation in the time-dependent property of rock and the consequent effect on time-dependent roof failure. This investigation uses a numerical simulation in 3DEC. The relaxation equation is developed from Burgers model. Variations in the time-dependent property in the post-failure region show negligible variation and, therefore, are averaged to represent the time-dependent property of the failed rock. Finally, these parameters are used in the numerical simulation of underground excavations. Two groups of parameters are used to represent the time-dependent property for pre- and post-failure conditions. FISH functions within 3DEC are used to monitor the state of each zone. Once failure is detected, the parameters are changed to the values corresponding to failed rock. The results show that the new relaxation model accurately predicts the time-dependent propagation of the failure zone. The variation of the time-dependent parameters significantly affects the rock mass behavior and roof convergence.

Tunnel Portal Construction using Sequential Excavation Method: A Case Study

Portal excavation in soft rock is one of the most challenging tasks in the construction of underground facilities. Significant convergence, collapses and surface settlement are usually associated with portal construction. Alluvium is loose unconsolidated material and most often tunnels constructed through tend to destabilize. In this study, portal excavation design has been analyzed using the finite element based computer program known as Phase2. The method of top heading and benching was not a suitable approach, keeping in view the previous experience. Subsequently, a different design of sequential excavation method (SEM) was proposed. Minimum convergence, minimum surface settlement, and machinery constraints were considered to be the vital importance for selection of final design. The results of finite element method (FEM) analysis showed that the finally selected SEM design has a roof convergence of 4 mm for the heading and full face excavation has 25 mm. These values were comparable with the ones obtained from a 5-pin convergence station, installed during the portal excavation (0.5 m inside from the portal). No or very little surface settlement was shown by the numerical model and actual field observation. Consequently, a proper SEM design based on numerical modeling allowed a successful construction of a large portal in alluvial deposits.

Quantification of ventilation enhancement using the Eye CAN roof support

Convergence of roof and floor in underground mine openings is a common occurrence. This convergence not only adversely affects the ability of workers, equipment and supplies to travel through the mine, it also reduces the effectiveness of the mine ventilation system, which is essential for the dilution of methane gas and airborne respirable dust. While installing secondary standing supports to control floor and roof convergence, such supports, by nature, partially obstruct a portion of the airway. These added obstructions inhibit the ability of the ventilation system to operate as efficiently as it could by increasing the resistance in and reducing the cross-sectional area of the airway. This study introduces and demonstrates the benefits of The Eye CAN™ standing roof support, which controls floor and roof convergence and is less obstructive to air flow than conventional wooden cribs. Laboratory findings show that the normal resistance of a supported lined airway is reduced by using this new product from Burrell Mining Products, Inc., while providing the same roof support characteristics of an established product—The CAN®. Load vs. displacement curves generated from laboratory tests demonstrated that this new product behaves with the same roof support characteristics as others in The CAN product family. Ventilation data gathered from a simulated mine entry was then used for computational fluid dynamics (CFD) modeling. The CFD analysis showed an improvement with The Eye CAN vs. other accepted forms of standing roof support. This proof-of-concept study suggests that, when using this new product made by Burrell Mining Products, Inc., not only will the convergence from the roof and floor be controlled, but airway resistance will also be reduced.