The variation in the width of the mining face significantly affects the stability of the face, leading to potential roof fracturing and collapse. Additionally, strong mining pressure can manifest, ...severely impeding the safe production of coal mines. This study uses the No. 16705 conventional working face of Jinda Coal Mine as its engineering background to investigate the characteristics of roof strata movement and instability under conditions of variable-width mining in shallow-buried thin coal seams. First, the dynamic load of the roof strata is estimated based on the key strata theory. Next, a mechanical model of the immediate roof strata movement in the working face is established based on the theory of elastic thin plates, which has been used to reveal the impact of different dimensions of the overhanging plate structure and residual overhanging structures in the corner on roof movement and its associated fracture mechanics. The findings indicated that the maximum bending deformation, deformation moment, and bending stress all have an exponential function relationship with the roof width. Similarly, these metrics have an exponential function relationship with the overhanging span of the roof. In addition, these parameters all have a linear functional relationship with the size of the residual overhanging structures in the corner. Finally, the effect of roof instability on overlying pressure is analyzed, and both the initial fracture step length and cyclic movement fracture step length of the roof are estimated. These insights offer valuable scientific guidance and a theoretical foundation for analyzing the adaptability of load-bearing pillars pressure in thin coal seam mining faces, bearing significant relevance to safety production.
Highlights
A mechanical model was constructed by theory of the thin plate and the key layer that cloud analyze the initial and periodic roof breakages of conventional working face.
During the periodic roof breakage phase, the mechanical response of the roof's bending under stress was more obvious than the initial fracture stage.
The study can provide scientific guidance and technical support for the stability control of the working face roof and the selection of load-bearing pillars.
The Cenozoic climatic evolution of the Tibetan Plateau (TP), together with its driving mechanism, have been a subject of interest for decades. This study presents detailed sedimentology, ...mineralogical (XRD), carbon, and oxygen isotope analyses of lacustrine deposits from the Eocene to the Miocene in the western Qaidam Basin, the northern TP. The petrological observation and XRD data of 109 samples reveal that the sediments are composed of mixed siliciclastic, carbonate, and evaporate minerals. And the carbonate isotopic results show negative δ
13
C (−7.49‰ to −3.41‰) and negative to slightly positive δ
18
values (−14.65‰ to 0.2‰). Both isotopes display a positive correlation with the contents of carbonates and evaporates, which suggests that evaporation is the major controlling factor of carbon and oxygen isotope. Therefore, the isotopes can be used as reliable indicators of the intensity of evaporation for paleoclimatic reconstruction. The reconstruction results reveal three distinct arid stages: top of the lower Xiaganchaigou Formation to the upper Xiaganchaigou Formation (ca.40-32 Ma), bottom of the Xiayoushashan Formation (ca.22-20 Ma), top of the Shangyoushashan Formation (ca.13-8.2 Ma). We suggest that the aridity during ∼40-32 Ma may have been related to the regression of the Paratethys Sea and uplift of the TP, while the aridity during 22-20 Ma may have been caused by the uplift and denudation of the mountains around the basin. The aridity after ∼13 Ma could be attributed to both global cooling and tectonic events in the northern TP. Furthermore, by comparing the climate records of the Qaidam Basin with those of other basins in Central Asia, a regional correlation can be established between different basins during the first and third drought stages. This study reveals that during the Eocene to Miocene, the climate change between different regions in the Qaidam Basin was synchronized and had a good connection with the surrounding basins, which responded to global climate change and regional tectonic activities.
This study investigates water-sand bursting disasters associated with fractured rock that affect safe mining in the mining areas of Western China. A broken rock water-sand seepage rheological test ...device was developed, and rheological tests were conducted on multiple groups of broken rock samples with single-stage axial loading and different load levels. When the rheology of each group of broken rock samples was stable, water-sand mixed fluid was injected into the samples at a certain pressure gradient to conduct water-sand seepage tests on broken rock masses. It was found that when the porosity of a fractured rock mass is within a certain range, the water-sand mixed fluid does not completely pass through the fractured rock mass and some sand particles are filtered by the fractured rock sample. There is an exponential relationship between the sand breaking ability and the sand filtration ability of fractured rock and its initial porosity, and the permeability of fractured rock decreases by a certain extent after sand filtration. However, for different load levels, when the flow through a fractured rock mass tends to be stable, the final porosity of the fractured rock mass decreases exponentially with axial compression. Based on the classical Kelvin rheological model and the basic theory of fractional calculus, a new fractional rheological model has been proposed and the rheological parameters under different load levels were fitted to the model. The new fractional rheological model is better able to describe the rheological characteristics of broken mudstone.
In comparison with tetragonal retaining structures, circular retaining structures have an advantage in terms of controlling the deformation caused by foundation excavation, and are a reasonable ...choice in engineering practice. Many results have been obtained regarding the effect of tetragonal excavation on the deformation of an adjacent tunnel. Nevertheless, a sufficient understanding of the circular excavation’s effect on the deformation of an adjacent tunnel is currently lacking. Therefore, this study focused on the problem of precise predicting tunnel deformation below a circular excavation. A numerical model was established to calculate the tunnel deformation caused by the circular excavation. An advanced nonlinear constitutive model, known as a hypoplasticity model, which can capture path-dependent and strain-dependent soil stiffness even at small strains, was adopted. The models and their associated parameters were calibrated by centrifuge test results reported in the literature. The deformation mechanism was revealed, and the calculated results were compared with those obtained with a square excavation and the same excavation amount. The differences between the deformations caused by these two types of excavation shapes were analyzed. It was found that under equal excavation area conditions, the excavation-induced deformations of the metro tunnel below a circular excavation were approximately 1.18–1.22 times greater than those below a square excavation. The maximum tunnel tensile bending strain caused by the circular excavation was 32% smaller than that caused by the square excavation. By comparing with the measured results, it is proved that the proposed numerical method can provide effective reference for engineers to analyze soil-structure problems.
When the mountain tunnel projects passing through the complex formation with coal, it happened along with disaster accidents such as gas outburst, gas combustion, and gas explosion. These disasters ...should seriously threaten the safety and life of the construction personnel and affect the normal operation of the tunnel construction. Ventilation is the most effective means to control gas, fire, dust, heat, and other disasters. To study the effects of different ventilation modes in highway gas tunnels, Fluent software was used to simulate forced ventilation, exhaust ventilation, and mixed ventilation in a high gas tunnel of a highway in Hunan. The distribution law of the airflow velocity and gas concentration of these three ventilation modes were obtained to determine the optimal ventilation system. It was shown that vortex zones of different ranges formed in the tunnel for all three ventilation modes, and the gas concentration was higher in the vortex zone than in other regions. Mixed ventilation of them is superior to the other two modes, showing the best ventilation effect with regard to airflow velocity and gas concentration.
(1 − x)(Pb
0.97
La
0.02
)(Zr
0.5
Sn
0.44
Ti
0.06
)–xBiYO
3
((1 − x)PLZST–xBYO), with x = 0.0–0.5, have been fabricated by the conventional solid-state reaction process. The phase evolution, ...microstructure, dielectric and energy storage properties have been investigated in detail. According to the XRD study, a secondary phase was found at room-temperature. The secondary phase was gradually increased with increasing x. The microstructure further revealed that the BYO doping leads to an increase in secondary phase. The EDS analysis revealed that Bi ions and Y ions are mainly substituted for A-site in the perovskite structure. As a result, the maximum value of energy storage density is 0.8 J/cm
3
, which is 2.66 times as high as that of pure PLZST ceramics.
•Landlord port is a kind of port financing model characterized by port lease.•Chinese model is a variation of the international mainstream landlord port financing.•Quasi-landlord port financing in ...China equals to a two-layer profit distribution.•The incentive effect of Chinese model is exactly the same as the international model.
Since the 1970s and 80s, landlord port has been the dominant port financing model in western large and medium-sized container ports. In China, many prospective port projects have also explored a landlord port financing model. However, some evidence suggests that landlord port financing in China is a variant of the international mainstream landlord port financing model. Based on an explanation of their unique features and practices, this paper analyzes the Chinese quasi-landlord port financing model from a contract theory perspective, in which it can be viewed as a double-level principal–agent relationship and two-layer profit distribution contract with three participants: the state-owned assets administration department, the port investment company and the operators. Furthermore, the results show that in the Chinese quasi-landlord port financing model, whether in the case of both joint venture and port land lease (fixed rent), or in the case of both joint venture and port land transfer, the optimal incentive scheme is the same as in the international landlord port financing model with profit sharing rent or mixed rent.
Nonlinear metasurfaces have recently been established as a new platform for generating photon pairs via spontaneous parametric down-conversion. While for classical harmonic generation in metasurfaces ...a high level of control over all degrees of freedom of light has been reached, this capability is yet to be developed for photon-pair generation. In this work, we theoretically and experimentally demonstrate for the first time precise control of the emission angle of photon pairs generated from a nonlinear metasurface. Our measurements show angularly tunable pair generation with high coincidence-to-accidental ratio for both co- and counterpropagating emission. The underlying principle is the transverse phase matching of guided-mode resonances with strong angular dispersion in a nonlinear metasurface consisting of a silicon dioxide grating on a nonlinear lithium niobate guiding layer. We provide a straightforward design strategy for photon-pair generation in such a device and find very good agreement between the calculations and experimental results. Here, we use all-optical emission angle tuning by means of the pump wavelength; however, the principle could be extended to modulation via the electro-optic effect in lithium niobate. In sum, this work provides an important addition to the toolset of subwavelength thickness photon-pair sources.
In the reuse stage of a gob-side entry retaining, failure of the structure and stability of the main roof have a significant effect on the safety of the advanced support and ventilation space at the ...working face. In this study, field investigation, theoretical analysis, and industrial experimentation were performed to analyse the fracture characteristics and formation process of the gob-side entry retaining roof during the reuse period. A dynamic-equilibrium mechanical model of the main roof structure is presented and the formation mechanisms of different types of short cantilever rock beam structures are clarified. The following major conclusions are drawn: (1) Three types of short cantilever rock beam structures occur in the main roof of a gob-side entry retaining during the reuse period, namely, the “short cantilever-articulated rock beam” structure, “short cantilever step rock beam (type I)” structure, and “short cantilever step rock beam (type II)” structure. (2) The stability criterion for these three short cantilever rock beam structures was also determined; that is, when the sliding instability coefficient K ≥ 1, the short cantilever-articulated rock beam structure will form, and when the sliding instability coefficient K < 1, the short cantilever step rock beam (type I or II) will form. (3) The governing law for the thicknesses of the main roof, immediate roof, and coal seam of the short cantilever rock beam structure was clarified; namely, the K-value gradually increases with increases in the thickness of the coal seam, drops sharply and then decreases gradually with increases in the thickness of the main roof, and decreases slowly with increases in the thickness of the immediate roof. The research results were validated at the gob-side entry retaining project in the Huainan mining area and have theoretical significance and reference value for roadway support projects with similar conditions.
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