Roof Stability Research Articles

Prediction of coal seam details and mining safety using multicomponent seismic data: A case history from China

With the increasing mining depth of coal mines, geologic hazards have become the main source of accidents. For this reason, coal seam prediction requires more assessment parameters, such as the degree of anisotropy, [Formula: see text] value, and elastic modulus. These parameters are difficult to determine using only the PP-wave. We have inverted and interpreted multicomponent seismic data acquired from a [Formula: see text] area in the Guqiao mine located on the southern margin of the North China plate, under the constraints of drill, log, and rock-physics test data. The production coal seam 11-2 of the mine, located in the Permian Formation, was our focus of this study, and our objective was to determine the structure, fracture development, thickness, surrounding rock lithology, roof stability, and mining safety of the seam. To achieve this, we first performed an analysis of S-wave splitting and joint PP- and PS-wave inversions. The inversion results were then combined to derive additional parameters, such as the [Formula: see text] value and dynamic Young’s modulus. Finally, we established a safety coefficient for assessing the safety of coal mining. The coefficient was based on the degree of anisotropy, coal bed thickness, Young’s modulus, and [Formula: see text] value of the coal-bearing strata. The geologic data for two mining tunnels in the 11-2 coal bed, provided by Huainan Coal Mining Group, were used to verify the known structure and lithology predictions. In addition, the known structural interpretations based on the PP- and PS-wave sections were obviously superior to the results of a previous survey based on the PP-wave only. The predicted thickness of coal seam 11-2 was accurate, as confirmed by comparison with that determined from drill data. Our joint PP- and PS-wave inversion and interpretation provides more information for coal seam prediction, creating a new application for coal seismic survey.

Assessment of roof stability in a room and pillar coal mine in the U.S. using three-dimensional distinct element method

This paper examines the effect of different geological and mining factors on roof stability in underground coal mines by combining field observations, laboratory testing, and numerical modeling. An underground coal mine in western Pennsylvania is selected as a case study mine to investigate the underlying causes of roof falls in this mine. Three-dimensional distinct element analyses were performed to evaluate the effect of different parameters, such as the variation of immediate roof rock mass strength properties, variation of discontinuity mechanical properties, orientations and magnitudes of the horizontal in-situ stresses, and the size of pillars and excavations on stability of the immediate roof. The research conducted in this paper showed that the bedding planes play an important role on the geo-mechanical behavior of roofs in underground excavations. Therefore, an appropriate numerical modeling technique which incorporates the effect of discontinuities should be employed to simulate the realistic behavior of the discontinuous rock masses such as the layered materials in roof strata of the underground coal mines. The three-dimensional distinct element method used in this research showed the capability of this technique in capturing the important geo-mechanical behavior around underground excavations.

Bypassing and tightening of an underground water retention system in permeable karst: case study of the hydropower plant (HPP) Bribin, Indonesia

In the framework of the Integrated Water Resources Management (IWRM) joint research project in the karst area of Gunung Kidul, Province of Yogyakarta Special Region on the Java Island, Indonesia, an underground hydropower driven water extraction facility in the cave “Bribin” was developed using pump-as-turbine-driven systems for freshwater supply of the rural area. As numerous other caves in the Gunung Kidul area, Bribin is part of a ramified system of all-season water-bearing subterraneous rivers and natural caves in karstic limestone. The elliptic cross section of the cave was completely closed with a concrete barrage, thus creating a year-round underground retention volume with an operational storage level of approx. 15 m. This contribution highlights the geotechnical and geohydraulic challenges handled within the sub-project “Short-time and long-time behaviour of karst rock surrounding pressure-bearing underground water-retaining structures”. One key to the feasibility of an artificial water retention scheme in a natural cave is to ensure the mechanical stability of the cave roof and sidewalls. The necessary geotechnical investigations are described. Another key to the effectiveness of such a water retention concept is the control and minimization of “lost” seepage water bypassing the barrage structure through the karst rock mass. Measures to monitor and to explain the seepage phenomena are presented as well as grouting efforts to minimize them. The limitations of improving the overall tightness will be discussed. Interpretation includes the use of analytical and numerical methods.

Mechanical analysis of roof stability under nonlinear compaction of solid backfill body

Based on the compaction characteristic test and the nonlinear compaction deformation characteristics of backfill material, this paper applies the theory of nonlinear elastic foundation of thin plate to establish a mechanical model of backfill body and roof in solid dense backfill coal mining. This study critically analyses the deflection equation of the roof by the energy method, derives the conditions of roof breakage and combined with concrete engineering practice analyses, determines roof movement regularity and stability in solid dense backfill mining. Analysis of the engineering practice of the 13,120 backfill panel of Pingmei 12# mine shows the theoretical maximum of roof convergence in backfill mining to be 415mm which is in significant agreement with the measured value. During the advancing process of solid backfill mining at the panel, the maximum tensile stress on the roof is less than its tensile strength which does not satisfy the conditions for roof breakage. Drilling results on the roof and ground pressure monitoring show that the integrity of roof is strong, which is consistent with the theoretical calculations described in this study. The results presented in the study provide a basis for further investigation into strata movement theory in solid dense backfill mining.

A method for computing unsupported roof distance in roadway advancement and its in-situ application

A reasonable unsupported roof distance (URD) when advancing underground coal mine roadways can contribute greatly to safe and rapid roadway development. A mechanical model of the roof, using the relationship between the roof stress distribution and URD, obtained by the difference method, and roof stability according to the in-situ roof stress and rock mass strength was developed. We subsequently designed a proper range of URD, developed a testing method of URD with the function of mining protection, evaluated roof stability through analyzing the test data and then determined a reasonable URD. Considering the factors of the geological conditions, the immediate roof stability and the efficiency of the labor arrangement system, the URD of the advancing roadway of 9802 working face in Zhangshuanglou coal mine was determined to be 6m using the proposed method. The results show that, when a 2m length of roadway was reinforced by temporary support and high pre-stressed bolt support after the roadway advancement of 6m per cycle, the speed and the security of the roadway development can be achieved and the advance rate can reach more than 400m per month.

Research into stope roof control of compound roof by solid backfilling mining

Based on the analysis of the failure characteristics and backfilling effect of the compound roof at 1801 backfilling workface in Taiyuan coal mine, China, we propose a method of controlling the pre-subsidence of a compound roof by using pre-stressed bolts to improve the backfilling ratio of the workface so as to maintain the global stability of the stope roof. In addition, PHASE simulation software was employed to analyze the influence law of pre-stressing force, length, and interval on roof subsidence at the workface. On the basis of the numerical simulation results, a model for calculating the pre-stressing force and length of the bolts, the interval between the bolts, as well as roof subsidence at the workface, was established by using SPSS regression analysis software. Moreover, the research results were applied successfully to the 1801 filling workface. According to the monitoring data of roof closure, it was found that the final subsidence value for the goaf roof was 350mm and the filling ratio at the workface was 86%, which could fully meet the demand for safety production at the workface. The safe and effective control of the stope roof was therefore realized, which achieves the goal of safe and efficient backfilling mining under a compound roof.

Applying the ground reaction curve concept to the assessment of shield support performance in longwall faces

The longwall method is used in many countries around the world in the underground extraction of coal seams. This method enables significantly improved production results to be achieved when compared to the bord and pillar mining system. However, this mining method requires higher capital investment compared to bord and pillar mining. One of the essential elements required to achieve the anticipated level of production from longwall panels is good shield–strata interaction. This means that the shields used in the longwall faces should have an adequate capacity to ensure the maintenance of roof stability in the longwall working. The issue of determining shield capacity has been the goal of research in many countries resulting in a number of different methods for calculating the required capacity of shields. In recent years, numerical modeling and ground reaction curves (GRCs) have been used to determine adequate shield capacity. An important factor to be considered in analyses using the concept of GRC for shield support selection for ground and mining conditions is roof convergence. This paper presents an analysis of shield–roof strata interaction in two longwall panels with natural roof caving in the gob using the concept of GRC. The GRCs for the specific mining conditions in the two longwall faces were determined by means of numerical modeling using Phase2 software. Performance characteristics of two-leg shields were obtained from underground measurements conducted continuously during the retreat of the longwall panels. In a specially prepared measuring shield, the changes in the leg pressures were measured. In addition, the changes in shield geometry were assessed by means of inclinometers. For the two longwall panels studied, the selected variations of leg pressures and changes of shield height in time are presented for a single shield’s cycle during the longwall operations for shield advance, setting, loading, and lowering. An analysis of the interaction between the shield and the roof strata rock mass was performed based on a comparison of the GRC and the operating characteristics of the shield. The values of the roof convergence, which occurred in the longwall faces during the single shield’s cycle, are presented. It is strongly recommended that a system enabling the characterization and mining conditions appropriate for shield capacity determination and selection be developed.

Control the drainage: the gospel accorded to sinkholes

Karst is a terrain distinguished by its underground drainage, natural cavities and sinkholes. New subsidence sinkholes (both dropout and suffosion) formed within the soil cover constitute the main karst geohazard. Nearly all are induced by increased drainage inputs or by water table decline, and control of the drainage is the primary means of reducing their hazard. Cave collapse and the development of collapse sinkholes in bedrock are less common, and the stability of a cave roof that is thicker than its width means that only those caves at shallow depth create any hazard. Predictions of the locations of caves or potential sinkhole sites are next to impossible, geophysical searches have severe limitations and borehole searches can incur significant costs. Consequently, controlling the drainage on construction projects is usually the most cost-effective means of minimizing the karst geohazard.

Sublevel filling method for a heavy orebody in deep stoping based on collaborative

To solve the problems of the precontrol roof sublevel filling method in deep mining, such as the complexity of the excavation process of precontrol roadway roofs and the suseeptibility of precontrol roofs to blasting vibration, this article introduces a collaborative stoping based sublevel filling method which utilizes horizontal and vertical holes at the same time. The sublevel filling method can effectively solve the roof stability problem of surrounding rock through changing the precontrol roof into a rear control[roof by altering the support order. Furthermore, optimization research is performed on the sublevel filling method based on collaborative stoping:firstly, the structural parameters are optimized by stability analysis using SURPAC and ANSYS; then, the partition drawing and supporting method is applied to solve the problems of the poor support quality of the two sides and the inefficient ore-drawing. A case study of Xincheng Gold Mine shows that the displacement of the roof significantly decreases and the economic indicators:improve, indicating safe and high-efficient mining of the thick and large orebody under high geostress.

Nonlinear instability analysis of long-span roofing structures: The case-study of Porta Susa railway-station

Instability problems are going to be more and more important for long-span structures, especially for those where structural shape and loading capacity are strictly correlated. In view of this, a complete buckling analysis seems to be essential for the correct prediction of structural behaviour. At the same time, recent disasters, occurred in the last few years, such as the collapse of the new pavilion of Charles de Gaulle Airport in Paris (2006), have brought the instability of shallow long-span roofs at the cutting edge of structural engineering research. In this paper, different studies devoted to the stability of the large span roof of the new railway station of Porta Susa in Torino (Italy) are proposed. In particular, 2D models were realized in order to evaluate in-plane linear and nonlinear instabilities for different loading and restraining conditions of the steel arches constituting the bearing framework of the roof. These arches have been subdivided into different groups according to the geometric characteristics. It has been found that nonlinear analyses are able to give not only an interpretation of the post-buckling behaviour, but also a more correct evaluation of the safety factor for this kind of structures. Moreover, a parametric evaluation, taking into account different cross sections, is presented. The results reveal that much slender arches would offer the same safety factor as the existing ones due to the activation of a different resisting mechanism, although with an evident reduction in the employed material. Finally, the outcomes of this case-study could be generalized in order to investigate the behaviour of other structural typologies and to suggest alternative design approaches.

Effect of discontinuities characteristics on coal mine stability and sustainability: A rock fall prediction approach

Rock fall related accidents continue to occur in coal mines, although artificial support mechanisms have been used extensively. Roof stability is primarily determined in many underground mines by a limited number of methods that often resort to subjective criteria. It is argued in this paper that stability conditions of mine roof strata, as a key factor in sustainability in coal mines, must be determined by a survey which proactively investigates fundamental aspects of said mine. Failure of rock around the opening happens as a result of both high rock stress conditions and the presence of structural discontinuities. The properties of such discontinuities affect the engineering behavior of rock masses causing wedges or blocks to fall from the roof or sliding out of the walls. A practical rule-based approach to assess the risk of a roof fall is proposed in the paper. The method is based on the analysis of structural data and the geometry and stability of wedges in underground coal mines. In this regard, an accident causing a huge collapse in a coal mine leading to 4 fatalities is illustrated by way of a case study. Horizontal and vertical profiles are prepared by geophysical methods to define the falling zone and its boundaries. The collapse is then modeled by the use of sophisticated computer programs in order to identify the causes of the accident.

Effect of foundation rigidity on stratified roadway roof stability in underground coal mines

The stability of roadway roof has been a long-standing issue in underground coal mines and thus has received a great deal of attention by both researchers and practitioners over the past few decades. This paper presents a simple analytical model for stratified roadway roof stability analysis. The model treats the immediate roof stratum of the roadway as an elastic beam resting on Winkler-type foundation and subjected to uniform roof loading that is determined from the lithology and depth of overlying strata. The developed model calculates bending stresses and deflections of the roof beam. Two case studies from an underground coal mine are used to demonstrate the model. A model parametric study reveals that the coal foundation elasticity plays a key factor in roof deformation and potential failure. Field monitoring data is used to validate analytical model results, and the comparison shows a good agreement.

Seismic Resonance Modes for Mine Roof Stability Monitoring

SUMMARY This work aims at the detection of instabilities in underground mine roadway roof, with the goal of predicting and preventing roof failure and collapse. Openings in the rock have their own resonances, due to the propagation of seismic waves in the rock around the opening. If the surrounding rock is damaged or fractured, this would result in the resonant frequencies decreasing. An experiment was set up in an underground mine to detect these resonances and see how they change in the process of rock degradation leading up to collapse. Accelerometers were grouted into a mine roadway roof, along with displacement and stress sensors. Waveforms from mining-induced microseismic events were recorded. The spectra of the coda of these events were used to search for resonances. Strong resonance modes were indeed seen, which were stable over time. The resonance frequencies did decrease in the days prior to roof collapse, in parallel with measured stress changes. At the time when significant movement was detected in the roof rocks, the resonance modes changed completely, probably due to delamination of the rock causing seismic decoupling. This means that resonance modes could be used for roof stability monitoring.

Condition evaluation of a unique mining site

The primary objective of this study was to evaluate the existing conditions and the stability of a mining site in which the unique features of seismicity, mining activity, hydrological conditions, geological conditions, environmental conditions, and future development plans were considered. In particular, the potential subsidence locations near the proposed construction site, the effects of mining boundary profile, and the influence scope of the mining activity on the neighboring areas were investigated using the finite element method. The study results indicate: (1) the overlying sandstone layer to the coal layer is the key to the stability of the mining roof; (2) the broken boundary has the most effect, followed by the arc boundary and linear boundary; (3) the safe distance from the mining boundary should be at least 400m if the proposed structure is to be built near an active mining site. Other relevant engineering recommendations are also proposed. The concluded results from this study may serve as a guide to other similar sites in the world.

Analytical approaches for studying the stability of laminated roof strata

This paper presents two analytical approaches for studying the stability of voussoir beams by taking into account the horizontal loading condition of the beams: with and without horizontal in-situ stresses. For beams without being subjected to horizontal stresses, a bilinear truss represents the compression arch formed in the voussoir beam. The stability of the compression arch is analysed by the energy method. An iteration algorithm was proposed for the solution. The results of the approach are verified with UDEC simulations as well as experimental results. For beams subjected to horizontal in-situ stresses, the classic beam theory was employed to study their behaviour. The assumptions for the analytical approaches are verified by numerical simulations. The study shows that an increase in the Young's modulus of the beam rock improves the stability of the beams against buckling. However, it elevates the stresses in the horizontal direction within the compression arch, which may result in crushing failure in the beam abutments and the midspan. Low horizontal in-situ stress reduces the risk of beam buckling but increases the risk of sliding at the abutments. However, high horizontal in-situ stress may cause either beam buckling or crushing failure in the abutments. The developed analytical approaches would be applicable for roof stability analysis in stratified rocks.

Investigation of the failure mode of shale rocks in biaxial and triaxial compression tests

High horizontal in-situ stress and weak sedimentary laminated roof rock can severely affect underground coal mine roof stability. These sedimentary rocks possess planes of weakness along the horizontal direction in a mine roof and delaminate easily when acted upon by high lateral stresses. Empirical studies have shown that the magnitude of these stresses can be as much as two to three times the local overburden stress. The resulting ground control problems (buckling, cutters, etc.) in such conditions are quite challenging for mining engineers. This paper describes the failure observed in coal mine shale rocks under biaxial and triaxial stress conditions. To carry out the investigation, special platens were fabricated that are capable of applying biaxial compressive stress on a cubic rock specimen when the entire arrangement is used inside a uniaxial compressive loading device. This experimental set-up was further modified to apply a pseudo-triaxial compressive stress. Laminated shale specimens tested under biaxial stress condition showed tensile failure along the laminations at macrolevel. Black shale specimens showed extreme brittle failure. Limestone specimens tested under similar conditions failed violently and it was concluded that failure observed in each rock type tested was unique and not an artifact of the experimental design. The pseudo-triaxial conditions reduced the influence of laminations on the failure mode. Laminated shale specimens were found to fail along multiple shear planes.

Roof collapse of shallow tunnels with limit analysis method

A new failure mechanism is proposed to analyze the roof collapse based on nonlinear failure criterion. Limit analysis approach and variational principle are used to obtain analytical findings concerning the stability of potential roof. Then, parametric study is carried out to derive the change rule of corresponding parameters on the influence of collapsing shape, which is of paramount engineering significance to instruct the tunnel excavations. In comparison with existing results, the findings show agreement and validity of the proposed method. The actual collapse in certain shallow tunnels is well in accordance with the proposed failure mechanism.

Upper bound analysis of progressive failure mechanism of tunnel roofs in partly weathered stratified Hoek–Brown rock masses

A reliable prediction of roof stability in stratiform or deteriorated rock banks remains a difficult problem in tunnel engineering, while it is still one of the most classical engineering issues needed to be investigated so as to provide relevant reference to practical projects. In previous research, homogeneous geomaterials were adopted to determine the analytic solutions of collapse profile in Hoek–Brown rock masses. However, when rock heterogeneities are considered in different rock layers, the results gained from existing collapse mechanisms would underestimate the dimensions of falling blocks. Thus in this paper, a new curved failure mechanism constructed with two continuous lines intersecting at the interface, namely progressive failure mechanism, is employed to derive the analytic solutions of the shape and region of roof collapse on the basis of upper bound theorem and variational approach. The corresponding results calculated are in well accordance with the existing findings, which shows sound agreement and validity of the proposed mechanism, and from which one can notice that additional detaching blocks would occur apart from the initial collapsing block when there exist various rock layers with different mechanical parameters, due to physical erosion or other factors, such as weathering process. To investigate the influence of different parameters on the range of roof failure, special attention is given to some examples of square and circular tunnels, and the results computed can give some instruction for practical engineering projects.

A new attempt of a CBM tree-like horizontal well: A pilot case of Well ZS 1P-5H in the Qinshui Basin

A multi-lateral horizontal well is one of the main well types employed in the coal bed methane (CBM) development. In a traditional CBM multi-lateral horizontal well, either a major hole or laterals will go crossing through coal beds to the largest extent, which may eventually result in bed collapse, footage failing to meet the design requirement, and the major hole unable to be monitored, re-entered and flushed. Therefore, this paper puts forward a design idea of a tree-like horizontal well composed of a major hole (for dredging passage), laterals (for controlling drainage area) and sub-laterals (for enhancing production). The major hole is usually drilled on the stable roof or floor of a coal bed, laterals are drilled from the major hole into coal beds, and several sub-laterals are drilled from laterals. The major hole, laterals and sub-laterals constitute a tree-like drainage and recovery system, whose major hole remains stable for a long period and can be monitored and maintained. The technique has been successfully applied in Well ZS 1P-5H in the Qinshui Basin, Shanxi. The well included one major hole, 13 laterals, and 26 sub-laterals, with a total drilling footage of 12288 m, a total coal bed footage of 9512 m, and a total pure coal footage of 9408 m. Its major hole was fully placed in the mudstone of the coal bed roof. This well records a successful case of incident-free single well drilling with a footage of more than 10,000 m and serves as a new well type for efficient CBM development in China.

The Roof Technology Research for LNG Cryogenic Storage Tank Based on ABAQUS

Based on the structural strength and stability theory, the paper introduce the basic principle and key considerations about the roof structural analysis, especially it should pay more attention to the concrete segment pouring effects. Taking the Tianjin LNG cryogenic storage tank as an example, which is independently finished by CNOOC, The finite element method is used for the roof strength and instability mode analysis, and study the failure sequence of strength damage and instability about the roof shell structure.