Caved Zone Research Articles

Prediction of the height of caving zone above the undercut in block caving mining method

The block caving is one of the most commonly used methods in underground mining of massive, low/high grades, and deep deposits. As a result of undercutting, caving occurs and propagates toward the upper part of the ore block. It is essential to predict the height of caved ore and its volume based on shape of the caved zone to make an economic decision. This paper describes the mechanism of height of the caving zone development. Consequently, four new mathematical models (prismatic, parabolic, cubic, and hemispheric) are proposed to estimate the height of this caving zone. The results are compared with the existing approaches. The proposed methods have proven that the height of caving zone has a linear relationship with the uniaxial tensile and compressive strengths and the height of undercut as well. Also, the height of the caving zone has a negative exponential relationship with unit weight of undercut roof materials and its expansion factor. The Height of the caving zone of the prismatic model is 1.5 times higher than the parabolic model and 1.3 times higher than the hemispheric model. The cubic model estimates the lowest height of the caving zone (0.3 times the height estimated by the prismatic model). The average height of the caving zone, taking the average unit weight of 27 KN/m3 into account, is about 69.4 times the tensile strength, 5.8 times the uniaxial compressive strength of the rock mass, and 18.8 times the height of the undercut.

Distribution pattern and driving factors of mite communities in karst cave ecosystems.

Mites are among the most abundant invertebrates in subsurface ecosystems, and their community assemblages and distributions are often significantly influenced by the diversity of habitat resources. The cave ecosystem encompasses drastic changes in nonbiological factors, such as changes in lighting conditions from bright to extraordinarily dark and habitat gradients of surface plant resources from abundant to scarce or even disappearing, providing an ideal unique environment for evaluating the assembly mechanism of soil animal communities. Nevertheless, there still needs to be a sufficient understanding of the biodiversity patterns and drivers of mite communities across environmental gradients in karst caves. We conducted a comprehensive survey on the composition and diversity of soil mites in three photometric zones (dark, twilight, and light) of a typical karst cave and its adjoining extractive environments (forest scrub and farmland). Our research aimed to investigate the ecological relationships of mite communities between above- and below-ground habitats and the effects of abiotic factors on mite communities. We collected 49 families, 86 genera, and 1284 mites. In the external cave environment, we captured 1052 mites from 72 genera and 45 families; in the internal cave environment, we captured 232 mites from 46 genera and 29 families. The abundance, richness of genera, and diversity parameters of the mite community decreased from the cave entrance to the cave interior with decreasing light intensity. Oribatid mites dominated the mite community. Protoribates and Scheloribates were the dominant genera, along with Tectocepheus and 11 other genera, which primarily distinguished the mite communities among different habitats. Forty endemic taxa were found in the external cave environment, compared to 14 endemic taxa in the internal cave environment. The mite community showed a strong preference for the cave ecosystem habitat. Temperature, humidity, and soil nitrogen content significantly influenced the distribution pattern of mite communities (VIP > 0.8, p < 0.05).

The first emerging records of a dragonfly in the dark zone of subterranean ecosystems: Exuviae and newly emerged adults of Hyrcanian Goldenring, Cordulegaster vanbrinkae in Danial Cave, northern Iran.

Riverine caves are special habitats that are home to many aquatic and terrestrial species. Some Odonata species and their emerging are recorded at the entrance and in the twilight zones of subterranean habitats around the world. However, the emergence of any Odonata species has not been recorded in the dark zones of caves or other subterranean habitats. We report the first evidence of the emerging of the Hyrcanian Goldenring, Cordulegaster vanbrinkae Lohmann, 1993, as an endemic species of the Hyrcanian biogeographical region, in the dark zone of Danial Cave, in the World Heritage-listed Hyrcanian Forests, northern Iran. During 2020-2023, three newly emerged and three exuviae of the species were recorded in the entrance zone (25 m) and the dark zone of the cave (200-280 m). The main hypothesis of the study is the entry and exit of adults from the cave entrance. However, we still do not know if the newly emerged will leave the cave or not. We still need more study on the biology and ecology of the species inside and around the cave. Danial Cave, with its high biodiversity, is one of the most important caves in the Middle East, and is urgently in need of conservation as a national natural monument.

Mechanism, mode, and prediction of karst caves collapse in the deep marine carbonate fracture-cavity reservoir

Tahe oilfield in the Tarim Basin of China is a typical deep marine carbonate fracture-cavity type reservoir. Large paleokarst caves are the important storage space types in this reservoir. Drilling data reveals that most of the caves have varying degrees of collapsed breccia filling. A better understanding of cave collapse mechanism, modes, and distribution is crucial for highly efficient production. In this study, large karst caves in the Ordovician of Tahe oilfield are taken as research objects, and several types of typical cave conceptual models are established based on similarity theory. Physical tests and numerical simulation methods are used to reveal the collapse mechanism under different depths, shapes, filling types, and tectonic stresses of different periods. Collapse modes are established based on the revealed mechanism, morphology, and damage zone of simulated karst caves. Seismic prediction methods of different types of collapsing caves are explored according to their reflection characteristics. The results reveal that compaction and shearing damage are the main reasons for cave collapsing. The roof and surrounding rock of the cave sink in during the collapse process, with a shear-slip damage zone of 2–3 times larger than the cave diameter. The sand/mud-filled cave is shown to be more stable than the water-filled cave, and the unfilled cave is the least stable. Tectonic stresses acting in the early Hercynian period are the primary reason for the current collapsed caves. Two cave collapse modes are established, type Ⅰ being mainly affected by overlying gravity and type Ⅱ mainly affected by shear stress. There are local differences in seismic reflection characteristics between these two types. The prediction of the two types of collapsed karst caves can be realized by combining the seismic attribute of colored inversion, frequency division energy, GR inversion and coherent energy gradient, etc. The comprehensive prediction results of collapsed karst caves in Tahe oilfield show a good correspondence with oil well productivity.

Combined Petrosal Intertentorial Approach: A Cadaveric Study of Comparison With the Standard Combined Petrosectomy.

The combined petrosal intertentorial approach (CPIA) has been proposed as an alternative to standard combined petrosal approach (SCPA). CPIA has been designed to maintain integrity of the temporal dura with a view to reduce temporal lobe morbidity and venous complications. This study has been designed to perform a quantitative comparison between these approaches. Five human specimens were used for this study. CPIA was performed on one side and SCPA on the opposite side. The area of exposure (petroclival and brainstem), surgical freedom, and angles of attack to a predefined target were measured and compared. SCPA provided a significantly larger petroclival area of exposure (6.81 ± 0.60 cm2) over the CPIA (5.59 ± 0.59 cm2), P = .012. The area of brainstem exposed with SCPA was greater than with CPIA (7.17 ± 0.84 vs 5.63 ± 0.72, P = .014). The area of surgical freedom was greater in SCPA rather than in CPIA (8.59 ± 0.55 and 7.13 ± 0.96 cm2, respectively, P = .019). There was no significative difference between CPIA and SCPA in the vertical angles of attack for the Meckel cave, Dorello canal, and root entry zone of cranial nerve VII. Conversely, the horizontal angles of attack permitted by the CPIA were significantly smaller for the Meckel cave (52.36° ± 5.01° vs 64.4° ± 5.3°, P = .006) and root entry zone of cranial nerve VII (30.7° ± 4.4° vs 40.1° ± 6.2°, P = .025). CPIA is associated with a reduction in terms of the area of surgical freedom (22%), skull base (18%), brainstem exposure (17%), and horizontal angles of attack (18%-23%) when compared with SCPA. This loss in terms of exposure is counterbalanced by the advantage of keeping the temporal lobe covered by an extra layer of meningeal tissue, thus possibly reducing the risk of temporal lobe injury and venous infarction. These results need to be validated with adequate clinical experience.

From the front door to the basement: Invertebrate communities' structure as a proxy for determining cave zonation in Neotropics

AbstractIn caves, the absence of natural light in deeper regions starkly contrasts with the entrance areas, which still exhibit a light gradient extending into the cave interior. This interplay with the structural gradient of the environment potentially exerts distinct influences on invertebrate communities residing in different cave light zones. To investigate this, we formulated a hypothesis positing that communities within distinct cave light zones respond differently to habitat structure and microclimatic conditions. Our approach involved a spatial multi‐scale sampling of invertebrates and the application of statistical analyses to contrast the responses of communities inhabiting photic and aphotic zones. Photic zone richness is influenced by factors such as air moisture, resource availability, root presence, and shelter diversity. In contrast, the richness of communities in aphotic zones is shaped by resource availability, the presence of roots, branches, and distance from the cave entrance. As expected, the richness in the photic zone surpasses that of the deeper regions, highlighting the challenges faced by invertebrates attempting to establish themselves in the aphotic zone. The species composition of faunal communities varied predictably from the entrance to the aphotic zone, and the three most important factors driving this variation were geographic distance, humidity, and distance from the entrance. The composition between these cave zones differs significantly, primarily due to the high number of obligate cave species predominantly inhabiting the aphotic region. Indeed, communities associated with different cave zones exhibit distinct responses to resource, microclimatic, and structural variables.Abstract in Portuguese is available with online material.

Quantitative Assessment of Bed-Separation Dynamic Development Caused by Inclined Coal Seam Longwall Mining

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.

Steel Arch and Rock Bolt Support in Terms of the Gateroad Stability Maintaining behind the Longwall Face

The longwall system is an extraction system commonly used in coal mining in many countries, including Poland. One of the methods for reducing extraction costs is the dual use of the gateroad. In the first instance, the gateroad serves as the tailgate, and during the exploitation of the second coal panel, it functions as the headgate. Such a situation requires maintenance of the roadway behind the longwall face, which is typically challenging, due to significant stress-related loads on the support and its substantial deformation. The support design for this kind of roadway should take into consideration the dual impact of exploitation pressure and the caved zone influence behind the longwall face. This article presents the results of in-situ research conducted on two roadways behind the longwall face. In both roadways, the effectiveness of specially designed steel arch frames and rock bolt patterns were examined to minimize roadway deformations and maintain their functionality. The research project was comprised of several stages. Initially, mining and laboratory studies were conducted to determine the geomechanical parameters of the rocks. Subsequently, excavation stability and functionality forecasts were performed based on the authors’ empirical indicators. Then, numerical analyses were carried out to design support schemes (steel arches and rock bolt) in both roadways. A fully automated monitoring system with programmed data loggers was designed to check the behaviour of a specific rock mass and the support elements. The load on the steel arch support was measured with the help of load cells, while the load on the rock bolt support was carried out with the help of measurement bolts. Behind the longwall face, the loads on the wooden cribs set from the goaf side were also monitored. Additionally, the measurement station was equipped with extensometers to monitor the movement of roof layers and stress meters to determine changes in rock mass stress. Laser scanning or traditional surveying methods were also used to verify the support schemes through roadway convergence measurements. The obtained results allowed us to draw conclusions regarding the optimization of support schemes and to give recommendations for the practical application of specific reinforcements in excavations maintained behind the longwall face.

Sponge Communities of Submarine Caves and Tunnels on the Fernando de Noronha Archipelago, Northeast Brazil

Submarine caves are important biodiversity reservoirs, but there is little information about the biota of marine caves in the Southwestern Atlantic. Here, we describe three submarine cavities and their sponge communities on the Fernando de Noronha Archipelago, Northeast Brazil. The underwater cavities were explored and collections were made through scuba diving from 5 to 18 m depths. Sapata Cave has a wide semi-dark zone near the entrance, a narrow transition zone, and a dark chimney, which is closed at the top. Ilha do Meio Cave is narrower and shallower than Sapata Cave, but has a long passage that leads to two completely dark rooms. Pedras Secas Tunnel has only a semi-dark zone with high water movement. The sponge communities in the semi-dark zones of the three cavities are rich and dominated by the classes Demospongiae and Homoscleromorpha, but Calcarea are also common. The transition zones of both caves are dominated by a desma-bearing sponge, thinly encrusting spirastrellids, and small Homoscleromopha and Calcarea. The dark zone in Ilha do Meio Cave is almost azoic, with only three species. This study has increased the number of sponge species known in submarine cavities on Fernando de Noronha from 29 to 69, highlighting the great richness of the sponge communities in these cryptic environments.

Cyanobacteria and Algae in the Karlamanskaya Cave (Bashkortostan Republic, Russia)

The article presents data on the composition and structure of cyanobacterial-algal cenoses of various habitats of the Karlamanskaya Cave. The study identified 46 species and intraspecific taxa of cyanobacteria and algae belonging to 5 divisions (Cyanobacteria – 17 species and intraspecific taxa, Bacillariophyta – 12 species and intraspecific taxa, Chlorophyta – 14 species, Charophyta – 1 species, Ochrophyta – 2 species), 7 classes, 16 order, 28 families and 36 genera. Leptolyngbya boryana (Gom.) Anagn. et Kom., Oscillatoria rupicola (Hansgirg) Hansgirg ex Forti, Luticola mutica (Kütz.) Mann, Mychonastes homosphaera (Skuja) Kalina et Punс., Chlorella vulgaris Beijerink present the dominant complex of cyanobacteria and algae in all studied habitats. Cyanobacteria dominate in all biotopes of the cave except aquatic ones where diatoms predominated. The maximum number of species was found in the soil at the cave entrance and in the soil of the illuminated cave zone: 31 and 29 species respectively. The lowest number was found in the soil and on the walls (16 and 11 species respectively). Ecological analysis of cyanobacterium and algae revealed 12 life forms: Сh9B9P5X5CF5hydr4amph2PF1NF1C1M1H1. Most of the life forms were found in the illuminated cave zone: in the soil (10 forms), on the wall (10 forms) and at the cave entrance (11 forms). The ecobiomorphs composition of the dark zone is the poorest (5 forms).

Phylogenetic analysis, complete mitochondrial genome, and redescription of the East Asian genus Aphaenomurus (Collembola: Tomoceridae)

The genus Aphaenomurus Yosii, 1956 includes one species and one subspecies, with a limited distribution in Korea and Japan. The phylogenetic relationships between Aphaenomurus and other genera of the subfamily Tomocerinae have not been resolved. In Korea, Aphaenomurus is generally observed in caves. In this study, specimens collected from the twilight zones of caves were used for an expanded phylogenetic analysis, complete mitochondrial genome analysis, and morphological redescription of Aphaenomurus interpositus Yosii, 1956. Maximum likelihood, Bayesian, and parsimony analyses based on five regions (COI, 16S rDNA, 18S rDNA, 28S rDNA D1–2, and 28S rDNA D7–10) indicated that A. interpositus is a sister group to the genus Plutomurus Yosii, 1956 with strong support.

The geomicrobiology of limestone, sulfuric acid speleogenetic, and volcanic caves: basic concepts and future perspectives.

Caves are ubiquitous subterranean voids, accounting for a still largely unexplored surface of the Earth underground. Due to the absence of sunlight and physical segregation, caves are naturally colonized by microorganisms that have developed distinctive capabilities to thrive under extreme conditions of darkness and oligotrophy. Here, the microbiomes colonizing three frequently studied cave types, i.e., limestone, sulfuric acid speleogenetic (SAS), and lava tubes among volcanic caves, have comparatively been reviewed. Geological configurations, nutrient availability, and energy flows in caves are key ecological drivers shaping cave microbiomes through photic, twilight, transient, and deep cave zones. Chemoheterotrophic microbial communities, whose sustenance depends on nutrients supplied from outside, are prevalent in limestone and volcanic caves, while elevated inorganic chemical energy is available in SAS caves, enabling primary production through chemolithoautotrophy. The 16S rRNA-based metataxonomic profiles of cave microbiomes were retrieved from previous studies employing the Illumina platform for sequencing the prokaryotic V3-V4 hypervariable region to compare the microbial community structures from different cave systems and environmental samples. Limestone caves and lava tubes are colonized by largely overlapping bacterial phyla, with the prevalence of Pseudomonadota and Actinomycetota, whereas the co-dominance of Pseudomonadota and Campylobacterota members characterizes SAS caves. Most of the metataxonomic profiling data have so far been collected from the twilight and transient zones, while deep cave zones remain elusive, deserving further exploration. Integrative approaches for future geomicrobiology studies are suggested to gain comprehensive insights into the different cave types and zones. This review also poses novel research questions for unveiling the metabolic and genomic capabilities of cave microorganisms, paving the way for their potential biotechnological applications.

Experimental Study on Compaction Deformation and Gas Permeability Properties for Crushed Limestone.

The mining goaf is enriched in coalbed methane (CBM) resources, and it is imperative to realize its efficient extraction. The gas permeability properties of the crushed coal and rock in the caved zone of mining goaf are the basis for the study of its internal CBM migration and enrichment law. In this study, the compaction deformation and gas permeability properties of crushed limestone with different particle sizes were revealed. The results show that (1) the deformation resistance capacity of the crushed limestone increased with increasing stress. The decreasing trend of porosity of samples with different particle sizes in the early and later compression periods is significantly different. Particle RR of the lower layer is smaller than that of the other layers. (2) The permeability of the sample decreases with decreasing porosity and nitrogen pressure, and it is between 10-12 and 10-10 m2. Nitrogen migration within the crushed limestone requires the pseudo-threshold pressure gradient, which ranges from 64.86 to 311.42 Pa/m. (3) The average permeability growth amplitude of the sample shows a logarithmic decreasing trend with the decrease of porosity. The average permeability growth amplitude of the 5-10 mm sample at the same porosity was 15.9-22.3 times that of the 0.315-0.63 mm sample. (4) The permeability of crushed limestone on both sides of the lower layer in the caved zone is much larger than that of other locations. The results are of great practical significance for accurately predicting the CBM enrichment area of mining goaf and then selecting the final position of the extraction drilling hole.

First cavernicolous record of the holoparasitic Balanophora abbreviata Blume (Balanophoraceae), from Tayabas, Quezon, Philippines

A new record of a holoparasitic plant is hereby reported to thrive in the twilight zone of Dry Cave I, Tayabas, Quezon Province, Philippines. This is the first record of a Balanophora species thriving within cave ecosystems. Based on the observed production of white sap from the roots inside the cave and a preliminary survey of trees above the cave, the probable host of the Balanophora abbreviata is Ficus sp. (Moraceae). Fig trees are capable of producing deeply penetrating and lithophytic roots. Based on the existence of various arthropods in and within the vicinity of the plant clumps, the possible role of B. abbreviata as part of the food web within Dry Cave I is also discussed.

Research and engineering practice on space characteristics of gangue slurry filling

The macroscopic characteristics of the fractured space and the microscopic pore features are critical factors determining the effectiveness of gangue slurry backfilling. To identify the key areas for slurry backfilling, a combination of theoretical analysis, simulation experiments, and on-site industrial trials was used to reveal the movement laws of backfilling space overlying the fractured zone. The distribution characteristics of voids available for slurry backfilling within the fractured space were explored, and the interaction between gangue slurry and goaf voids was clarified. A formula for calculating the void ratio in the caved zone was derived, and a model for the distribution of voids in the slurry backfilling space was established. This model identified the main areas where slurry backfilling should be focused, namely the overlapping space between the free accumulation zone and the load-influenced zone. Experimental results demonstrated that the porosity of the collapsed rock mass within the goaf follows a negative logarithmic function along both the dip and strike directions, which was consistent with the theoretical calculations. Through in-situ grouting backfilling experiments on the ground, the feasibility of gangue slurry backfilling in the goaf was verified, and the process of interaction between gangue slurry and goaf voids was elaborated. This process generally involves three stages: initial flow, vertical upwelling, and horizontal diffusion, all of which are closely interconnected. Practical engineering applications of gangue slurry backfilling were carried out in the free accumulation zone and the load-influenced zone at the Huangling No. 2 coal mine. The successful validation of underground gangue slurry backfilling technology demonstrated its feasibility and the correctness of the theoretical approach. This research provides new evidence for environmentally friendly disposal of gangue materials.

Survey of southern Missouri Plethodontidae in Ozark caves

Plethodontidae is a large family of salamanders with a distribution ranging from eastern North Americato northern South America. Three genera and nine species occur in Missouri, most of which are typically found in cavesfor at least part of their lives. These species have been recorded to emerge from hibernation in early April and throughoutMay. This study shares the species abundance and diversity in ten Missouri Ozark caves: seven near St. Louis andthree near Springfield. Each cave was surveyed at least once between November 2021 and September 2022 using visualencounter surveys. Observational data were recorded without handling any individuals to minimize disturbance. Thethree caves near St. Louis had few to no salamanders present in early April, despite optimal weather conditions, buttended to have larger populations throughout the summer. Two caves near Springfield held at least a dozen individualsof multiple species in mid-April. Few individuals were recorded in the most southwestern cave sampled, potentiallydue to pollution or recent flooding. Our findings could aid others studying plethodontid salamanders to determine theprime conditions for emergence from hibernation, optimal habitats for different species in each cave zone, and potentialbiological or chemical impacts that could affect salamander abundance in caves.

A Comparative Case Study on Stress Redistribution due to Extraction of Conventional and Split-Level Longwall Panels in Deep Inclined Coal Seams

Through field observations, theoretical analysis, and a calibrated numerical model, a study of stress redistribution due to the extraction of longwall panels at depths ranging from 580 to 660 m with a 30° dip angle at Tangshan coal mine is presented in this paper. Conventional and new split-level longwall layouts are compared regarding their stress redistributions. The height of the caved zone is 21.7 m; angles of break of 55.6° on the left and 54.2° on the right side of the gob are observed using cross-measure boreholes. Structural models as well as numerical models are constructed based on the above field data to make the geometry of the gobs closer to the in situ situation and more realistic. Compared with the conventional layout, the theoretical analysis shows that the overall influence of the elevated split-level longwall gob on the lowest intact stratum increases by more than 5.07%, meaning that the split-level longwall layout is more likely to maintain the stability of the overlying strata. This is also corroborated by numerical modeling. Conventional longwall panels and split-level longwall panels with and without considering the gob are all simulated using FLAC3D. Instead of only backfilling the height of the coal seam or the height of the coal seam and the immediate roof, as in many numerical modeling studies in the past, in this study, the whole caved zone is backfilled with “double-yield” material. It is found that along the floor, the split-level longwall gob assumes 23.4% more load than the conventional longwall gob, and the split-level longwall abutment bears 6.2% less load than the conventional longwall abutment; stress arches are developed within the gob; concave-down stress beddings are more evident at higher locations of the gob; a self-supporting structure develops within the gob and surrounding rock mass around the lower end of the gob, forming a protective localized intact destressed zone around the location where the split-level tailgate is situated; the yield zone in the floor of the curved section tends to extends toward the center of the curved part, where the curvature is the maximum; the upper stress concentration zone is within the coal seam, while the lower one is above the coal seam; the upper one is more concentrated.

Study on Overburden Structure Characteristics and Fracture Evolution Law in Upward Mining at Kuangou Coal Mine

The development of vertical fractures in the overburden rock below after upward mining of a coal seam can influence the safe mining of both this seam and the seams above. This paper employs the method of physical similarity simulation to experimentally study the overburden structure and fracture evolution law during upward mining of the No. 4 coal seam in Kuangou Coal Mine. The study reveals that the overburden rock of the lower No. 4 coal seam after mining exhibits a typical "three-zone" characteristic, with the caved zone overburden showing a distinct cantilever beam-hinged structure, and the overburden structure of the bending and sinking zone is relatively intact. In the experiment, no significant vertical through-layer fractures were observed during the first five periodic pressures. However, during the 6th to 11th periodic pressure periods, when mining pressure became intense, a total of six vertical through-layer fractures appeared near the coal wall of the working face. These fractures, aligned with the caving angle direction, extended upwards through the No. 3 coal seam located 60m above the No. 4 seam. The overall fracture evolution showed opening during intense mining pressure and gradual closing after the pressure subsided. Two of these vertical through-layer fractures had a larger extension range, eventually reaching the surface. During the upward mining process, the appearance of vertical through-layer fractures did not lead to sliding instability or rapid sinking of the overburden roof. The working face could thus continuously advance. This research has implications for the safe upward mining in similar mines.

Quantitative analysis of layered re-crushing of crushed coal particles during compression based on CT scanning

Due to the compaction of the overburden strata, the crushed coal in caved zone will be re-crushed. The compaction and re-crushing degree of crushed coal differ at different heights, which is called layered re-crushing. Quantitatively revealing the layered re-crushing characteristics of crushed coal particles is the key to elucidating pore structure evolution of caved zone and coalbed methane enrichment pattern in the gob. In this paper, quantitative characterization of layered re-crushing of coal particles during compression is achieved using self-designed CT visual compacting apparatus. The results showed that: (1) the degree of particle re-crushing decreased from the loaded side (the upper layer) to the fixed side (the lower layer). In the early stage of compression, the overall skeletal structure is looser, which is not conducive to the transfer of force chains. Thus, the stress has the greatest effect on the particles in the upper layer, with the proportion of particle increase reaching 49.3% from 0 Mpa to 2.14 Mpa. There was a pattern of becoming larger first and then smaller in the difference of average diameter among different layers, and the final average diameter decreased by about 60% in three layers. (2) The particle diameter distribution transferred to a smaller diameter fastest in the upper layer and slowest in the lower layer, which is due to the gradual transfer of external load from the top to the bottom. After compression, the number of particles smaller than 4 mm predominated in the upper layer (40.3%) and the number of particles larger than 5 mm predominated in the lower layer (47.1%). (3) With the increase of stress, the void volume gradually decreases regardless of the whole sample or the three layers, while the void number increases instead. And at both 2.14 MPa and 23.24 MPa, the void volume shows the law of upper layer < middle layer < lower layer.

Ground Movement in the Hanging Wall of Underground Iron Mines with Steeply Dipping Discontinuities: A Case Study.

To ensure the safety of underground mining activities and effectively protect the surface production facilities and houses of the nearby residents, the ground movement caused by the sublevel caving method needs to be studied. In this work, the failure behaviors of the surface and drift of the surrounding rock were investigated based on the results of in situ failure investigations, monitoring data, and engineering geological conditions. The results were then combined with theoretical analysis to reveal the mechanism responsible for the movement of the hanging wall. Driven by the in situ horizontal ground stress, horizontal displacement plays an imperative role in both the movement of the ground surface and underground drifts. Accelerated movement is found to occur in the ground surface which coincides with the occurrence of drift failure. Failure occurs in the deep rock masses and then gradually propagates to the surface. The steeply dipping discontinuities are the main reason for the unique ground movement mechanism in the hanging wall. As steeply dipping joints cut through the rock mass, the rock surrounding the hanging wall can be modeled as cantilever beams subjected to in situ horizontal ground stress and lateral stress due to caved rock. This model can be used to obtain a modified formula for toppling failure. Also, a mechanism of fault slipping was proposed, and the condition required for fault slipping was obtained. Based on the failure mechanism of steeply dipping discontinuities, the ground movement mechanism was proposed considering the horizontal in situ ground stress and caved rock mass: slippage of fault F3, slippage of fault F4, and toppling of rock columns. Based on the unique ground movement mechanism, the goaf surrounding rock mass could be divided into six zones: a caved zone, a failure zone, a toppling-slipping zone, a toppling-deformation zone, a fault-slipping zone, and a movement-deformation zone.