Unstable Roof Research Articles

Research on roof caving mechanism and influencing factors in inclined ore body caving mining

In the process of caving mining, the overlying strata often fall down to form a concealed goaf. The goaf of caving method is characterized by strong concealment, complex shape and unstable roof. The mathematical model of energy dissipation structure of goaf instability and caving is constructed, and the mechanism of goaf instability and caving is revealed from the perspective of energy. By establishing the joint coupling model of inclined ore bodies in metal mines, the development mechanism and influencing factors of roof caving arch along dip and strike are studied. The research shows that the roof surrounding rock of goaf has the characteristics of double arch of “stress - caving” when mining with inclined ore body caving method; The falling arch is basically symmetrical arch along the strike and asymmetrical arch along the dip, which is higher in the middle and upper part of the dip and lower in the lower part; The relationship between the maximum displacement of caving and the thickness of ore body is quadratic polynomial; The lower the dip angle of the ore body is, the larger the roof caving range is, but the smaller the maximum caving displacement is; The existence of hanging wall fault will affect the expansion path of roof caving, but make roof caving more sufficient.

Experimental research on the instability characteristics of the overlying strata structure that characterizes shallow interval goaf mining

Abstract Here, we analyze the instability characteristics of the overlying strata structure that characterize shallow-depth seams under insufficient goaf in Yushenfu mining area. To simulate the No. 20107 longwall interval working face situated in Nanliang Coal Mine, physical simulation, an acoustic emission (AE) monitoring system, a stress acquisition system and a total station were used. The results indicate that during interval goaf formation, which is correlated with mining, the immediate roof collapses, and the main roof strata remains stable. Gradually, the stress that acts on the temporary coal pillar (TCP) gradually exhibits the ‘uniform increase–accelerated increase catastrophe instability’ change characteristics. Due to the concentrated load of the overlying strata, the bearing capacity of the TCP gradually deteriorates until the catastrophic instability occurs, and the unstable roof strata forms a ‘W-shaped voussoir beam’ structure. The research results provide evidence for the strata control that is associated with shallow-seam mining.

Determination of roof rock weakening zones in extraction columns based on seismic exploration

This article highlights the difficulties that arise in coal mining when roof areas have fractures with an opening of up to 0.2 - 0.5 m. Such situations may cause roof collapse. The seismic exploration method used by the authors to assess the geomechanical characteristics of extraction columns is described. The results of work on identification and registration of unstable roof areas are presented.

Neural Network of Roof Cutting Blasting Parameters Based on Mines with Different Roof Conditions

The design and construction of roof cutting and blasting is a key part of the roof cutting pressure releasing gob-side entry retaining (RCPRGER) technology. In the existing research, the blasting parameters of roof cutting have been primarily determined by field tests. However, the disadvantages of field tests include a complicated process, which hinders the succession of related procedures, and an unstable roof cutting effect. Therefore, in this work, the authors attempt to use a mathematical analysis method to simplify the design process of the key parameters of roof cutting blasting. First, the mechanics process mechanism of surrounding rocks with roof cutting and pressure releasing is investigated, and the stress evolution process of the surrounding rock is divided into the following six stages: original rock stress state, excavation stress state, supporting stress state, roof cutting stress state, premining stress state, and postmining stress state. Furthermore, the relationship between roof cutting and entry retaining from the perspective of Mohr’s stress circle is discussed. Next, using four typical mines, including the Tashan, Yuanlin, Jinfeng, and Hengyuan coal mines, as examples, the existing design methods of roof cutting and blasting, geological data characteristics of each mine, distribution rule of roof cutting connectivity rate, and explosive charge structure of roof cutting blasting are summarized. Based on these results, the logic of roof cutting blasting design is obtained, the key indices affecting blasting design are determined, and the blasting design is defined as a complex fuzzy problem with multiple factors. Finally, based on the study of the above mechanics mechanism and blasting rule, a three-layer back propagation (BP) neural network, including six input units, nine hidden units, and three output units, is developed with the four typical mines as the sample space. This neural network realizes the rapid determination of the three key parameters pertaining to sealing length, blasthole spacing, and the explosive charge weight of a single hole. Through training, the calculation error is less than 0.48%, which considerably simplifies the design process of the blasting parameters. The charge structure parameters can also be determined according to this method. At present, the construction of this neural network has the shortcomings of limited sample space. This problem can be overcome by supplementing the sample size in the subsequent research and practice, which will improve the efficiency and accuracy of this design method and promote the application and development of the RCPRGER technology. The interdisciplinary research reported in this paper is an attempt that uses an intelligent algorithm to simplify the design process of roof cutting blasting in RCPRGER, and it represents not only an application development of the intelligent algorithm, but also a new step regarding the intelligent design of RCPRGER technology.

Coal pillar design when considered a reinforcement problem rather than a suspension problem

Current coal pillar design is the epitome of suspension design. A defined weight of unstable overburden material is estimated, and the dimensions of the pillars left behind are based on holding up that material to a prescribed factor of safety. In principle, this is no different to early roadway roof support design. However, for the most part, roadway roof stabilisation has progressed to reinforcement, whereby the roof strata is assisted in supporting itself. This is now the mainstay of efficient and effective underground coal production. Suspension and reinforcement are fundamentally different in roadway roof stabilisation and lead to substantially different requirements in terms of support hardware characteristics and their application. In suspension, the primary focus is the total load-bearing capacity of the installed support and ensuring that it is securely anchored outside of the unstable roof mass. In contrast, reinforcement recognises that roof de-stabilisation is a gradational process with ever-increasing roof displacement magnitude leading to ever-reducing stability. Key roof support characteristics relate to such issues as system stiffness, the location and pattern of support elements and mobilising a defined thickness of the immediate roof to create (or build) a stabilising strata beam. The objective is to ensure that horizontal stress is maintained at a level that prevents mass roof collapse. This paper presents a prototype coal pillar and overburden system representation where reinforcement, rather than suspension, of the overburden is the stabilising mechanism via the action of in situ horizontal stresses. Established roadway roof reinforcement principles can potentially be applied to coal pillar design under this representation. The merit of this is evaluated according to failed pillar cases as found in a series of published databases. Based on the findings, a series of coal pillar system design considerations for bord and pillar type mine workings are provided. This potentially allows a more flexible approach to coal pillar sizing within workable mining layouts, as compared to common industry practice of a single design factor of safety (FoS) under defined overburden dead-loading to the exclusion of other relevant overburden stabilising influences.

Mechanism of mining-induced slope movement for gullies overlaying shallow coal seams

This paper provides an improved understanding of the movement mechanisms of both bed-rock gully and sandy soil gully when underground mining occurs underneath, followed by systematic analysis of the contributing factors such as mining advance direction, gully slope angle, gully erosion coefficient and mining height. This paper presents the results from monitoring, theoretical analyses and up to date modeling based on the geological features in the gully affected area, and the implications of these results to the success of roof support trial. It was observed that when mining occurred towards the gully, sliding of slope block along the fracture surface occurred, which resulted in unstable roof condition; when mining progressed away from the gully, polygon blocks developed in the gully slope and rotated in reversed direction forming hinged structure; within the bed-rock slope, the hinged structure was unstable due to shear failure of the polygon block; however, within the sandy soil slope, the structure was relatively stable due to the gradual rotating and subsiding of the polygon block. The increase of the value of slope angle and mining height lead to a faster and more intensive fracture development within the gully slope, which had a pronounced effect on gully slope stability and underground pressure. Various remediation approaches are hence proposed in this paper including introducing more powerful support and reasonable mining height, setting up working face along or away from gullies, using room and pillar, strip mining and backfill instead of longwall mining.

Sub-surface structures and collapse mechanisms of summit pit craters

Summit pit craters are found in many types of volcanoes and are generally thought to be the product of collapse into an underpressured reservoir caused by magma withdrawal. We investigate the mechanisms and structures associated with summit pit crater formation by scaled analogue experiments and make comparisons with natural examples. Models use a sand plaster mixture as analogue rock over a cylinder of silicone simulating an underpressured magma reservoir. Experiments are carried out using different roof aspect ratios (roof thickness/roof width) of 0.2–2. They reveal two basic collapse mechanisms, dependant on the roof aspect ratio. One occurs at low aspect ratios (≤1), as illustrated by aspect ratios of 0.2 and 1. Outward dipping reverse faults initiated at the silicone margins propagates through the entire roof thickness and cause subsidence of a coherent block. Collapse along the reverse faults is accommodated by marginal flexure of the block and tension fractures at the surface (aspect ratio of 0.2) or by the creation of inward dipping normal faults delimiting a terrace (aspect ratio of 1). At an aspect ratio of 1, overhanging pit walls are the surface expressions of the reverse faults. Experiments at high aspect ratio (>1.2) reveal a second mechanism. In this case, collapse occurs by stopping, which propagates upwards by a complex pattern of both reverse faults and tension fractures. The initial underground collapse is restricted to a zone above the reservoir and creates a cavity with a stable roof above it. An intermediate mechanism occurs at aspect ratios of 1.1–1.2. In this case, stopping leads to the formation of a cavity with a thin and unstable roof, which collapses suddenly. The newly formed depression then exhibits overhanging walls. Surface morphology and structure of natural examples, such as the summit pit craters at Masaya Volcano, Nicaragua, have many of the features created in the models, indicating that the internal structural geometry of experiments can be applied to real examples. In particular, the surface area and depth of the underpressured reservoir can be roughly estimated. We present a morphological analysis of summit pit craters at volcanoes such as Kilimanjaro (Tanzania), San Cristobal, Telica and Masaya (Nicaragua), and Ubinas (Peru), and indicate a likely type of subsidence and possible position of the former magma reservoir responsible for collapse in each case.

Imaging coal seam structure using 3-D seismic methods

Coal mining and production is a very large industry within Australia with over 100 active collieries. However, to run at a profit, the coal mines must have continuous production and fulfil large annual quotas. This is generally achieved through the use of the capital intensive, underground, longwall mining technique. This technique requires that the coal seam structure be accurately known ahead of the extraction coal face. Any unknown faulting, unstable roof conditions or gas accumulations encountered while mining can lead to equipment damage, down-time and injury to workers. Thus, the responsibility is on the mine geologist to ensure that the coal seam structure is accurately mapped ahead of mining, to provide a safe mine plan. Before the introduction of the seismic method in coal mining, all coal seam mapping was done using surface boreholes. This is an expensive method to use and provides only a low level of lateral resolution.Seismic methods are gradually replacing boreholes and becoming more common place in coal industry as an accurate and cost effective alternative method for imaging the seam structure. Burton Downs colliery in central Queensland, was one such mine to take advantage of this method as apart of an Underground Feasibility Study for their northern, Kerlong mine area. With no previous seismic work done in the area, reverse vertical seismic profiling (RVSP) and two-dimensional (2-D) surveys were initially shot. RVSP results showed that the seismic method was viable, imaging the coal seam of interest and provided acquisition parameters for the 2-D surveys. The 2-D lines gave high-resolution images of the coal seams within the mine area, and showed a complex faulting system which included low-angle reverse faulting and normal faulting. The success of the 2-D profiles prompted the acquisition of a three-dimensional (3-D) survey. After processing the 3-D depth volume, a much more complete structural interpretation and hence, mine plan could be made.

Estimation of critical convergence and rock load in coal mine roadways ? an approach based on rock mass rating

Based on field instrumentation in eight different coal mines representing varying depths and strata conditions, a relation for obtaining the critical convergence value has been established. In development heading for bord and pillar workings this relation can be used successfully to control the premature collapse of the roof. An empirical relation for rock load has been established. This can be utilized for optimum design of support system. The roofs have been categorized as stable, short-term stable and unstable. Proper attention should be provided for an unstable roof and the support design is to be changed before the convergence reaches the critical value.

Shale facies and mine roof stability: A case study from the Illinois basin

The Hornsby area in south-central Illinois contains one of the largest deposits of low-sulfur coal in the Illinois basin coal field. Mining of the resource has not proceeded apace with the demand for low-sulfur coal, mainly because of fear of unstable roof conditions beneath the Energy Shale (Desmoinesian). Three different facies of the Energy Shale overlie the Herrin Coal in the Hornsby area. All three facies are fine-grained, argillaceous rock, but their response to mining is different. The laminated shale facies has a pervasive lamination and in many places is cut by small faults and joints which make the rock prone to slabbing and potting prior to bolting. The gray shale facies that is interbedded with the laminated shale facies lacks pervasive lamination, and fractures are not well developed in it. The gray shale facies generally provides good roof. The carbonaceous shale facies had not yet been encountered in mine workings in the Hornsby area when this study was completed. Lamination in the carbonaceous shale facies is better developed than that in the gray shale facies, but it is not as well developed as that in the laminated shale facies. When the carbonaceous shale facies is the roof rock, the extensive roof control measures that were implemented for mining beneath the laminated shale facies will probably not be required.

Instability forecasts aid in reducing roof fall rates at the beckley mine : Min Engng, V34, N1, Jan 1982, P63–67

At the Beckley mine in West Virginia, forecasting unstable roof conditions in advance of mining is accomplished by using a rating system that combines historical information on mine roof failures with known mine geology. Failure frequency rates are derived for each category of roof rock composition, unit thickness, structure, and extraction ratio. In this manner, known geological environments are rated according to instability based on actual experience. Drill hole control and geological interpretation provide the basis for forecasting unstable conditions beyond working faces. The forecasting technique has assisted mine management in reducing fall frequencies by 80%. This reduction in fall frequency was accomplished by modifying the plan of advancing sections to minimize mining exposure to unstable roof conditions.

Prediction of Geologic Anomalies in Advance of Mining: Three Case Studies: ABSTRACT

Studies at three underground mines in Illinois show great differences in the predictability of geologic anomalies that interfere with mining. Exploratory drilling can be used to detect major features of the depositional environment such as areas of thin or no coal, ancient channels, and unstable roof lithologies. Local features such as faults, jointing, clay dikes, and rolls can best be delineated and predicted by in-mine mapping. Some anomalies, however, such as unexpected stresses, some clay dikes and rolls, and faults encountered for the first time, defy successful prediction by either method. Mine A, a small drift mine in the Herrin (No. 6) Coal, was opened with very little test drilling. It soon became apparent that the single set of main entries was confined to an 800-ft wide (240 m) trough of minable coal bounded by areas of nondeposition and erosion. When the roof rock became so unstable that further advance was impossible, the mine was abandoned. With an additional investment in exploratory drilling, the company could have been forewarned about the confining nature of the trough and the presence of unstable roof, and perhaps could have relocated to a more minable body of coal. At Mine B, also in the Herrin Coal, the density of drill holes was well above average for Illinois. Nevertheless, abnormally hard, thin coal was encountered at several faces. While exploratory drilling failed to detect these thin coal areas, in-mine mapping delineated them as narrow, sinuous, branching features, and offered a means of short-range projection. A thorough exploration program preceded shaft sinking at Mine C. This program and subsequent in-mine mapping failed to predict severe roof stability problems in the north-south headings. The in-mine mapping did indicate that the north-south weakness was not due to lithologic or structural changes, but to high horizontal stresses. Ultimately, the mine headings were turned 45°, with an overall improvement in roof stability. End_of_Article - Last_Page 1167------------

Anticipating Facies-Related Coal Mining Problems in Hartshorne Formation, Arkoma Basin: GEOLOGIC NOTES

The close relationship observed between sedimentary facies and potential coal mining problems within the Hartshorne Formation of the Arkoma basin provides the basis for anticipating where mining problems may jeopardize safe and economical coal production in undeveloped parts of the basin. Deltaic facies within the Hartshorne include distributary-channel sandstones and interdistributary-bay shales, siltstones, and sandstones. A thick, linear, distributary-channel sandstone is closely related to several potential mining problems: (1) both underlying and overlying coalbeds are locally thin and discontinuous along the trend of the sandstone; (2) coal quality decreases significantly adjacent to the sandstone (decrease in free swelling index and increase in sulfur); (3) the sandstone is a natural gas reservoir which may emit gas into adjacent coal mines; and (4) geologic factors favoring unstable roof conditions and coalbed rolls are locally associated with the sandstone. In contrast, interdistributary-bay deposits, because of relative homogeneity and lateral persistence, do not present potential facies-related mining problems. Basin-wide availability of geophysical logs from gas wells makes facies mapping and delineation of potential problem areas a straightforward procedure which must be integrated into formulation of exploration strategy, evaluation of reserves, and mine planning if Hartshorne coal resources are to be safely and economically exploited.

Anticipating Coal Mining Problems in Hartshorne Formation, East-Central Oklahoma, Using Sedimentary Facies Analysis: ABSTRACT

A study of sedimentary facies associated with the Hartshorne coal beds of east-central Oklahoma provides an opportunity for preventive planning to minimize mining problems. Sedimentary facies within the Hartshorne formation End_Page 724------------------------------ include delta distributary sandstones and interdistributary bar-fill shales, siltstones, and sandstones. One distributary channel sandstone body displays a shoestring geometry, over 12 mi (19.2 km) long and 1 mi (1.6 km) wide, with an average maximum thickness of 200 ft (61 m). This sandstone rests directly on and locally replaces the Lower Hartshorne coal bed. Three major mining problems are related to this sandstone body: (1) the Hartshorne sandstone in this area is a natural gas reservoir which might emit gas into adjacent coal mines; (2) the sandstone body is directly related to local discontinuities and rolls in the Lower Hartshorne coal bed; and (3) an unstable roof may be locally associated with trough cross-bedding and jointing near the base of the sandstone, and with facies changes and slickensides along the lateral margins of the sandstone body. In contrast to this sandstone body, interdistributary bay deposits, because of their relative homogeneity and lateral persistence, do not present potential facies-related mining problems. Potential mining problems associated with these facies are local and are directly related to structural and stress-release features which are difficult to predict in advance of mining. To insure safe and economic coal production from the Hartshorne formation, the distribution of major sandstone bodies overlying the coal beds must be considered when planning degasification programs and shaft, slope, and main entry locations. End_of_Article - Last_Page 725------------

Geological approach for predicting unstable roof and floor conditions in advance of mining : Ealy, D L; Mazurak, R E; Langrand, E L Min Congr J, V65, N3, March 1979, P17–22

Geological approach for predicting unstable roof and floor conditions in advance of mining : Ealy, D L; Mazurak, R E; Langrand, E L Min Congr J, V65, N3, March 1979, P17–22

Use of a tunnel boring machine on a coal mine decline : White, S B Paper to 3rd Australian Tunnelling Conference, Sydney, 12–15 Sept 1978, 5P

A Calweld 5.1 metre diameter full face tunnel boring machine was used by the Allied Codelfa Joint Venture to excavate a 1600 metre long decline for Coal Cliff Collieries Pty. Ltd. at their new West Cliff Mine on the southern coalfields of New South Wales. This was the first time in Australia that such equipment had been used on a 1 in 3 gradient. The tunnel boring machine owned by Codelfa Construction Pty. Limited had to be modified firstly to accommodate the 1 in 3 slope and secondly to comply with the electrical requirements of the Coal Mines Regulation Act. The decline traverses horizontally bedded sedimentary strata over a vertical distance of 530 metres from the surface to the Bulli coal seam. Ingress of ground water from the overlying Hawkesbury sandstone caused problems during the initial drivage and necessitated alterations to construction methods. Adverse geological conditions in the interbedded shale and sandstone below the Hawkesbury sandstone gave rise to unstable roof conditions which considerably reduced the rate of advance. Notwithstanding the problems encountered on this project, experience has shown that a tunnel boring machine could be successfully used on a 1 in 3 decline. However the high initial cost of providing and installing the equipment may well limit its application to coal mine declines.