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With an increasing number of multi-seam mining projects, the problem of strata movement caused by multi-seam mining has attracted increasing attention. Previous studies have shown that the rock failure mechanism for multi-seam mining is obviously different from single-seam mining. When a lower coal seam is being mined, two issues are apparent: the activation that the upper goaf will suffer and the difference in the break mechanism of the stratum between single-seam and multi-seam mining. The research regarding these two issues is still inadequate. In this paper, a physical model experiment was used to simulate the full longwall mining process of multiple seams. Then, the failure mechanics of the stratum and the influence of mining the lower coal seam on the activation of the upper goaf are discussed under the condition that the upper coal seam has been fully mined. The results show that a part of the fracture zone that formed by mining the upper coal seam will be converted into a caving zone when the lower coal seam is mined. In multi-seam mining, the cracks in the overlying fractured rock mass will widen and propagate upward. During the process of mining the lower coal seam, the interburden rock mass presents a typical plate bending failure, and the break location is greatly affected by the distribution of the periodic weighting in the upper coal seam mining. Multi-seam mining will cause the surface subsidence trough produced by upper coal mining to move towards the end of the mining panel. The mechanism of rock stratum movement revealed by the test has a certain practical guidance for the control of roof pressure in multi-seam mining and the subsidence of surface.

Deep longwall mining of coal seams is made in the Upper Silesian Coal Basin (USCB) under complicated and mostly unfavourable geological and mining conditions. Usually, it is correlated with rockburst hazard mostly at a high level. One of the geological factors affecting the state of rockburst hazard is the presence of competent rocks in the roof of extracted coal seams, so rock falling behind the longwall face does not occur, and hanging-up of roof rocks remains. The long-lasting absence of caving may lead to an occurrence of high-energy tremor in the vicinity of the longwall face. Roof caving behind the longwall face may be forced by blasting. The column of explosives is then located in blastholes drilled in layers of roof rocks, e.g. sandstones behind the longwall face. In this article, a characterization of tremors initiated by blasts for roof caving during underground extraction of coal seam no. 507 in one of the collieries in the USCB has been made using three independent methods. By the basic seismic effect method, the effectiveness of blasting is evaluated according to the seismic energy of incited tremors and mass of explosives used. According to this method, selected blasts gave extremely good or excellent effect. An inversion of the seismic moment tensor enables determining the processes happening in the source of tremors. In the foci of provoked tremors the slip mechanism dominated or was clearly distinguished. The expected explosion had lesser significance or was not present. By the seismic source parameters analysis, among other things, an estimation of the stress drop in the focus or its size may be determined. The stress drop in the foci of provoked tremors was in the order of 105 Pa and the source radius, according to the Brune’s model, varied from 44.3 to 64.5 m. The results of the three mentioned methods were compared with each other and observations in situ. In all cases the roof falling was forced.

Water inrush at floor area is a natural hazard during coal mining. Especially in the northern coalfield of China, more than 55% of coal mines are threatened by water inrush at floor area. The hazard of water inrush is becoming more serious with increasing mining depth. In this paper, the deformation and failure behavior at floor area in deep longwall mining site were analyzed. The correlation between the floor failure and main roof breakage was studied using kinematics theory. Meanwhile, the impact loads of cantilever beam breakage on the floor area at both longwall face and gob area were calculated. Combining with “Pressure arch hypothesis”, the compression and unloading mechanics processes at floor area were analyzed, and the unloading deformation model of floor structure after the breakage of main roof was established as well. In addition, the correlation between unloading deformation at floor area and the main roof breakage, mining depth and unloading stresses were also obtained. Finally, these studies have been verified by using micro-seismic monitoring data in deep longwall mining site. The results show that the impact loads are proportional to the span and loads of cantilever beam. After the breakage of cantilever beam, the impact loads were transferred to the floor area at longwall face side and gob side, and the rock masses at both sides were failed in compression. Consequently, the position of back arch foot of the pressure arch was rapidly transformed into the contact gangue zone at gob area from the last breakage position of main roof. While as the unloading stress of rock masses inside of the floor pressure arch is increasing, the depth of unloading fracture and heave below longwall face are greater than those before cantilever beam breakage. In addition, the unloading deformation at surface floor increases nonlinearly with the increase in mining depth and unloading stresses.

The present research primarily focuses on the investigation of gate-entry stability of longwall trial panel under weak geological condition in Indonesia coal mine by means of numerical analysis. This work aims at identifying appropriate roof support at 100 m and 150 m of depth during gate development. Due to depth depending competency of dominant rock, the stability of gate-entry at 100 m of depth can be optimized by leaving at least 1 m of remaining coal thickness (RCT) above and below the gate-entry. The appropriate support for the trial panel gate-entry is steel arch SS540 with 1 m and 0.5 m spacing for 100 m and 150 m of depth, respectively. The influence of panel excavation on gate-entry is also discussed. Regarding the aforementioned influence, the utilization of additional gate mobile support is recommended at least 10 m from the longwall face.

The increment of stress level and fracture depth in deep longwall mining can cause groundwater inrush accidents frequently, and it is essential to study the characteristics of fracture development around laminated rock layers at the floor. This can help to understand the mechanism of groundwater inrush events and to reduce the potential risk. According to the rock mass stress–strain curve and the stress redistribution around the floor area in a deep longwall face, this study focused on the failure mechanisms in this area. A physical longwall model was established to study fracture development at the unloading zone around the floor area. The results showed that the plastic fracture at the floor was generated by high compressive stress around the floor and coal rib area after the breakage of main roof. The unloading starting point of stress changed nonlinearly, and the unloading stress increased nonlinearly. Therefore, the unloading fracture depth and the floor heave deformation can be much larger than those in shallow mining. In addition, the horizontal bedding plane can accelerate fracture development. When the unloading stresses increase, the branch fractures develop downward and connect the original joints and bedding planes at deeper floors. The dominant angle of main cracks varies in the range of 50°–85°, and the number of branch fractures can reduce when the dip angle of the main cracks increases (close to 85°). In addition, the direction of main fractures was nearly vertical, and it was hard to connect them to the horizontal joints and bedding planes.

Total extraction mining, such as longwall mining, results in subsidence at the surface above the mined area. It was assumed that subsidence occurs within a relatively short time after mining. Based on theoretical considerations and conventional levelling, it was generally assumed that subsidence stops 3–5 years after mining. Since satellite data images became freely available for research at the end of the last century, surface movements above mined areas can be studied over very long periods of time. This results in new insights. The case study focuses on the coal longwall mines in the Belgian Campine coal district. Mining stopped completely in 1992, when the last mine was closed. The analysis is based on successive remote sensing datasets since 1992. The main conclusion is that surface movements are still recorded more than 30 years after the mines were closed, i.e. much longer than the usual 3–5 years that were assumed. The variation in surface movements in the post-mining phase is also more complex. After a period of further subsidence, the direction of surface movements reverses and an uplift of the surface above the mined areas is observed (relative to the end of the subsidence phase). This uplift is still ongoing in 2025. These new findings have practical implications, as these long-term movements further affect the loading of buildings and other infrastructure, leading to possible new damage. It also has a negative impact on the natural environment, e.g., on surface water management. Article highlights In the post-mining phase, additional surface movements still occur above coal longwall panels. The long-term behaviour has consequence for the stability of buildings, the integrity of infrastructure and the natural environment. Remote sensing satellite data are extremely useful to study surface movements over large areas and during long time periods.

The top coal of extra-thick coal seams is susceptible to irreversible failure under cyclic loading and stress rotation caused by longwall top coal mining. To ensure efficient mining and avoid wastage of coal resources, the failure evolution process of coal under cyclic loading and stress rotation must be investigated. Hence, true triaxial conventional loading, true triaxial graded cyclic loading and unloading, and true triaxial main stress rotation path tests were conducted to reveal the mechanical behaviors and failure characteristics. First, the evolution of deformation, elastic modulus, Poisson’s ratio, and peak strength were analyzed. Then, the fracture volumetric strain, dissipated energy, and failure modes were characterized. Finally, the relationship between the fracture evolution of top coal under cyclic loading and stress rotation was studied. The results showed that the mechanical characteristics of top coal are complex. The deformation and expansion characteristics under three paths were different. Additionally, the elastic modulus and Poisson’s ratio showed negative correlation. Moreover, the internal coal damage was more severe and the failure was more violent under the true triaxial graded cyclic loading and unloading path. The failure mode of coal was dominated by shear failure and supplemented by tensile failure. The failure mechanism of top coal under true triaxial cyclic loading and unloading and stress rotation was revealed. Compared with static loading failure under conventional conditions, the longitudinal fractures caused by cyclic loading and the transverse fractures caused by stress rotation penetrated each other, affording the crushing of the top coal.

Coal mining under the Quaternary thick loose layer affects key strata breakage, Bed-separations development, ground subsidence, and other studies. This paper presents a method for solving the deflection of a large-deflection inclined thin plate under a thick loose-layer cover with additional lateral loads and midplane forces. The methods presented are based on the principle of large-deflection of thin-plate, energy method, and fracture mechanics theory. The 7225 work face in Anhui Province, China, was studied. Combined with the large-deflection inclined thin plate model, the initial breakage distance within the main roof plate was calculated to be 33 m with the initial breakage angle of 61.2°, and the period breakage distance was calculated to be 21 m with the period breakage angle of 55.4°. The distribution range of “Vertical Three Zones” from 7225 working face to the ground, including the height of the caved zone is 38.07 m, the height of the fractured zone is 41.13 m, and the height of the curved zone with the thick loose layer removed is 187.56 m. During the dynamic development of the principal key strata (PKS), the deflection value develops from 0 mm to 2714 mm with 7225 working face mining, and the maximum value of the spatial volume is 56,485 m3, which is verified by Three-dimensional Discrete Element Code (3DEC) numerical simulation. The dynamic development of Bed-separation within the overlying strata, with a maximum development height of 545.2 mm and a maximum volume of 11,228.1 m3 of the Bed-separation cavity. The dynamic development of the Bed-separation height and the cavity under different mining length and width conditions of the working face are also discussed. The large-deflection inclined thin plate model proposed in this paper effectively explores the dynamic deflection and fragmentation law of the overlying strata induced by the inclined working face of Longwall mining and provides a theoretical basis and computational model for quantitatively evaluating the dynamic development of the Bed-separation cavity.

This paper presents a new index to assess caving potential of roof strata above longwall panels. Nine inherent parameters were chosen as significant affecting factors in three categories, including roof strata characteristics, roof discontinuities properties and local features. Fuzzy hybrid multi criteria decision making was used by combining fuzzy analytic network process technique and fuzzy decision making trial and evaluation laboratory method to develop a new rating system. Subsequently, roof strata cavability index (RSCi) was defined as a simple summation of ratings for all parameters, indicating the cavability level qualitatively. The RSCi was applied to determine the cavability of various actual cases of worldwide longwall panels. In addition, the correlation between RSCi and span of main caving was examined where reliable performance of the new index was noted. It was concluded that RSCi is a simple and efficient tool to assess the cavability of immediate roof and evaluation of caving intervals in longwall mining.

Many water inrush disasters have occurred in mines that were attempting to recover extra-thick coal seams located under aquifers. This paper presents a technology known as upward slicing longwall-roadway cemented backfilling (USLCB) which can provide a safe and effective solution to extract these seams. The fundamentals of the technology are to divide a coal seam into several slices and extract each slice sequentially, from bottom to top, using longwall-roadway mining technology. Gobs are filled with cemented backfill which then act as a new floor allowing the overlying slices to be mined, similar to the overhand cut and fill method. This paper also presents the compressive properties of cemented backfill material with various mixing ratios and curing times. Moreover, a physical simulation was conducted to verify the effects of USLCB. The results of the simulation indicate that USLCB can be used to control strata movement as well as the height of the water-conducting zone (HWCZ). The USLCB was applied at Gonggeyingzi Coal Mine in Inner Mongolia, China. The results indicate that the recovery ratio has increased from 39.2 to 95%, the backfill ratio reached 95%, water inflow decreased from 245 to 120 m 3 /h and the maximum HWCZ was 39.2 m after extracting the entire 21 m of the coal seam. The conclusions of this research show that implementing USLCB can significantly improve productivity and safety when extra-thick coal seams are mined which are located under aquifers.