A review of cavability evaluation in longwall top coal caving Le, Tien Dung; Mitra, Rudrajit; Oh, Joung ...
International Journal of Mining Science and Technology/International journal of mining science and technology,
11/2017, Letnik:
27, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Longwall Top Coal Caving has been considered as one of the most effective technologies for the extraction of underground thick coal seams. A large number of studies on the applicability of Longwall ...Top Coal Caving into new mine sites have linked the success of its application to the geomechanical understanding of the cavability of the top coal. The paper aims to improve the knowledge of the top coal cavability evaluation. A range of parameters that affect the top coal cavability were first identified. Afterward, a number of cavability assessment methods and classifications were reviewed. The result is important in that it assists researchers in developing an advanced and reliable tool for the top coal cavability evaluation.
This paper presents a discontinuum modelling approach to investigate Longwall Top Coal Caving (LTCC) behaviour including stress distribution, coal and rock failures, top coal caving and roof strata ...rupture, and to analyse the impact of overburden movement on top coal caving. The current model ia successful in using plastic material in a discontinuum code for intact rocks. The model scale is large enough to capture the critical features of LTCC, including steady-state caving of top coal and repeatable periodic weighting of roof strata. The applicability of the numerical model was assessed by calibration with field measurements obtained from a longwall mine site. The numerical study found that the stress distribution caused by LTCC mining is in general similar to that caused by conventional longwall mining; top coal predominantly fails in shear whereas roof rock mostly fails in tension; top coal starts to cave in stress caving while main roof strata first rupture in crushing mode; and roof strata weightings periodically increase and decrease top coal cavability. The findings of this study should assist engineers in better understanding fundamental rock mechanics associated with LTCC, identifying key geotechnical parameters dominating caving behaviour, and managing top coal productivity and mine safety involved in LTCC operation.
•Plastic material is used in UDEC for modelling Longwall Top Coal Caving (LTCC).•Stress distribution in LTCC is similar to that in conventional longwall method.•Top coal and roof rock predominantly fail in shear and tension, respectively.•The changes of top coal cavability are consistent with periodic roof strata weightings.
Mechanisms responsible for the caving of the strata overlying Chengchao Iron Mine were investigated by analyzing in situ deformation monitoring results and the distinctive engineering geological ...conditions. The results indicated that waste rocks caved from the overlying host strata and the large-scale sliding rocks at the footwall near the goaf continuously flow towards the caved zone when surrounding rocks of goaf are relatively broken. As a result, a large air gap does not generally form above the caved zone. In this case, the strata overlying the goaf do not globally cave upwards to the surface on a large scale. Instead, they cave in a direction in which the rock is weak and soft and thus form locally caved pipelines within the overlying strata. That is, pipe caving occurs in the overlying strata which results in collapse sinkholes in the ground surface that have a punctate distribution. The continuous flow of large-scale sliding rock masses towards the caved zone can effectively slow down the occurrence of pipe caving. More importantly, rocks surrounding a caved pipeline will cave continuously with excavation of orebodies. However, this caving tends to decrease or completely stop after a certain period of time. This is mainly due to a change in the location of the underground orebody being excavated or because the flow in the caving channel has become blocked. Pipe caving has become a caving mode that is typically encountered in strata overlying the metal ore in mines excavated through sublevel caving with no sill pillars.
An innovative gateroad layout in highly gassy longwall top coal caving in a Chinese coal mine based on gob-side entry retaining (GER) was introduced. The numerical model was used to design the ...rational roadside backfill body (RBB) with high-water quick-setting materials. In order to improve the reliability of the numerical modeling, the double-yield model for gob modeling and the strain-softening model for RBB modeling are validated in detail. The results of the validated model indicate that when the RBB width is 1.5m, the RBB peak vertical stress is minimal and surrounding rock convergence variation of GER becomes gentle. Consequently, the rational RBB width can be estimated as 1.5m. Field test and field monitoring indicate that the GER in N2105 panel with a width of 1.5m roadside backfill body and high-strength cable-rockbolt support could meet the requirement for gas drainage and ventilation during panel N2105 was retreating. It also proved that the new gateroad layout met the requirement for the highly gassy thick seam operation, and provided a new way to the coal mine safety production in similar conditions.
•We proposed an innovative gateroad layout in highly gassy longwall top coal caving in a Chinese coal mine based on gob-side entry retaining.•We analyzed the rational roadside backfill body width based on the double-yield model for gob modeling and the strain-softening model for RBB modeling.•Field practice and observations show that the innovative gateroad layout based on gob-side entry retaining with the roadside backfill body width of 1.5m has achieved an expected effects for surrounding rock control.
•The asymmetric failure mechanism of coal pillar roadway was revealed.•The limit caving thickness of studied mine was determined.•A special surrounding rock support technology was proposed.
...Considering serious asymmetric deformations and failures of the gob side roadway in fully mechanized top coal caving face and taking coal pillar roadway as engineering background, we have discussed the surrounding stress condition and the deformation failure patterns of coal pillar roadway by the theoretical analysis, numerical calculation, field tests. The results show that the deformations and failures of gob side coal roadway with fully mechanized caving mining are characterized by “two-way” asymmetric failure patterns along vertical and horizontal directions. The reason for the asymmetric deformation was uneven stress conditions of surrounding rocks under the effect of strong mining. The asymmetric deformations and failures of coal pillar roadway surrounding rocks were closely related to caving thickness in former panel. The fracture position of main roof varied with top coal thickness, and limit caving thickness was 9.4 m. Based on the asymmetric deformation characteristics and mechanisms of roadway, this paper has proposed a control strategy which mainly adopts asymmetric high prestressed truss anchor cable. Field tests were carried out in the repair section of roadway. The results show that roadway asymmetric deformation was well controlled and long-term stability of roadway was realized, which provides a reference for similar asymmetric deformation roadway.
Ore recovery and dilution in caving mines are strongly determined by the flow characteristics of broken rock, especially under inclined boundary conditions. A physical draw model was designed and ...used to investigate the gravity flow characterization of granular materials during longitudinal sublevel caving. The three-dimensional (3D) shapes of flow axis, isolated extraction zone (IEZ) and ore remnants under inclined walls were obtained using the discharged quantity method and 3Dmine software, which indicated that stope boundary conditions and orebody dip have an impact on the characteristics of gravity flow. The experimental results also showed that the gravity flow characteristics of 3D draw experiments under the inclined walls were significantly different from those of the two-dimensional (2D) draw experiments. The flow axis is a line segment and lies in the same plane and only the width of the IEZ can be displayed in 2D experiment. However, the flow axis is a space curve segment in 3D draw experiment. Since being cut off by the front and inclined upper walls simultaneously, the 3D IEZ is more complicated than the 2D IEZ and both the width and thickness of the draw body are displayed simultaneously in 3D state. The shape and location of ore remnants in 3D state are also different from that in 2D state. Therefore, the 3D IEZ is more consistent with the actual situation than 2D IEZ for sublevel caving stope. The 3D gravity flow presented may be used to determine the design of the extraction layout and improve ore recovery during longitudinal sublevel caving.
John Volanthen is a legendary British cave-diver and one of the two-man team who discovered and rescued the Wild Boars soccer team from the sunken cave in Tham Luang, Thailand. Thirteen Lessons that ...Saved Thirteen Lives is the deeply revealing and nail-biting account of that incredible rescue. Each chapter tells one part of the story from Tham Luang but also imparts a life lesson, gleaned from John's exploration and rescue efforts, that can be applied to everyday obstacles and challenges. We are in the cave with John and his story of how an average guy can do the unimaginable will inspire the superhero in us all.
Because of the problem of gangue discharge and surface subsidence during coal mining, the current research on underground filling mainly focuses on the paste filling, solid filling, and grouting ...filling of the overburden separation layer after scaffolding. We proposed the technology of fluidization gangue grouting for filling the collapse area based on our previous research. The prediction method of residual space in the collapse area and the diffusion law of gangue fluidization filling slurry are two essential points for successfully implementing the technology and maximizing the use of goaf for gangue backfilling and reducing overburden settlement. To further explore the remaining space distribution law of the collapsed goaf in thick seam fully mechanized top coal caving mining and the effect of coal gangue fluidization filling, the caving goaf of the 3307 fully mechanized top coal caving face in Sangshuping No. 2 coal mine in the Weibei mining area was detected by the transient electromagnetic method. We studied the distribution law of the measured abnormal area in the caving goaf, which reflects the distribution law of the remaining space from one aspect. The coefficient of the remaining space was calculated to be 19.5%. Then, we applied COMSOL simulation software. The diffusion law of coal gangue fluidized slurry in the caving goaf was simulated and analyzed. It shows that the most obvious diffusion direction of the coal gangue slurry is the trend of the gradual expansion of the “cavity pore” multi-type residual space, indicating that with the increase in the diffusion distance, the diffusion resistance gradually increases, and the slurry morphology gradually presents the “claw” form. According to the space theory and fractal dimension theory, the prediction method of the remaining space in the caving goaf is given, and the design basis of the filling drilling parameters is determined. Finally, the field-filling test was carried out. The results show that the high- and low-level fluidized filling in the caving goaf can safely and efficiently handle the gangue in the mine, and the residual space characteristics and slurry diffusion law in the caving goaf are consistent with the above. The research results provide theoretical support for the fluidization filling technology of coal gangue in thick seam fully mechanized top coal caving areas.
Stress analysis of longwall top coal caving Alehossein, Habib; Poulsen, Brett A.
International journal of rock mechanics and mining sciences (Oxford, England : 1997),
2010, 2010-1-00, 20100101, Letnik:
47, Številka:
1
Journal Article
Recenzirano
Longwall top coal caving (LTCC) is a relatively new method of mining thick coal seams that is currently achieving high productivity and efficiency in application, particularly in China. The technique ...is similar to traditional longwall mining in that a cutting head slices coal from the lower section of the coal seam onto a conveyor belt installed in front of the hydraulic support near the cutting face. In modern LTCC an additional rear conveyor belt is located behind the support, to which the flow of the caved coal from the upper part of the seam can be controlled by a moveable flipper attached to the canopy of the support. The mining method relies on the fracturing of the top coal by the front abutment pressure to achieve satisfactory caving into the rear conveyor.
This paper develops a yield and caveability criterion based on in situ conditions in the top coal in advance of the mining face (yield) and behind the supports (caveability). Yielding and caving effects are combined into one single number called caving number (
CN), which is the multiplication result of caving factor (
CF) and yield factor (
YF). Analytical derivations are based on in situ stress conditions, Mohr–Coulomb and/or Hoek–Brown rock failure criteria and a non-associated elastoplastic strain softening material behaviour. The yield and caveability criteria are in agreement with results from both numerical studies and mine data.
The caving number is normalised to mining conditions of a reference Chinese mine (LMX mine) and is used to assess LTCC performance at fourteen other Chinese working longwalls that have had varying success with the LTCC technology. The caving number is found to be in good agreement with observations from working LTCC mines. As a predictive model, results of this analytical/numerical study are useful to assess the potential success of caving in new LTCC operations and in different mining conditions.
A self-designed scaled physical draw model was used to mimic the extraction of a moderately stable and inclined medium-thick ore body. In terms of the ore dip and width, the gravity flow of the ...blasted ore and waste rock was investigated using tracing with labeled markers. The laboratory results indicate that the ore dip and its width have an impact on the flow axis and shape of the draw body. The flow axis was divided into two parts with lower and upper flow axes, respectively. The upper flow axis was parallel to the upper inclined wall, and the lower flow axis coincided with the axis of the draw point. The length of the upper flow axis and the area of draw body, influenced by the upper inclined wall, increased as the ore width or its dip decreased. The shape of the draw body in the lower part had longitudinal asymmetry and was similar to an inverted water drop; however, the shape of the draw body, which was influenced by the upper inclined wall, produced a variation and rotation to a certain extent, and no longer had the revolution symmetry about the central axis of the draw point. To this end, the gravity flow presented can provide a theoretical basis for ore recovery optimization.
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