Optimizing the layout of communication base stations and positioning substations for mobile communication, Internet of Things (IoT) communication, personnel and vehicle positioning in mines requires calculating electromagnetic wave propagation losses in mines. Therefore, it is necessary to study the calculation method of electromagnetic wave propagation loss in mines. However, existing statistical calculation methods for electromagnetic wave propagation loss in general, outdoor open-space, indoor, tunnel, and mine environments primarily consider the effects of distance and frequency but do not account for underground-specific factors such as tunnel cross-sectional geometry. As a result, these methods yield significant errors when calculating electromagnetic wave propagation loss in the complex underground environment of coal mines. A relative wavelength calculation method for electromagnetic wave propagation loss in mines has been proposed, which depends not only on frequency and distance but also on factors such as the cross-sectional dimensions of the tunnel. The electromagnetic wave propagation characteristics of mines are revealed as follows: in the mine line-of-sight (LOS) scenarios: ① In auxiliary transportation tunnels, electromagnetic wave propagation exhibits a turning point. When the relative wavelength is less than 10, the relative spatial scale of propagation is small, making the electromagnetic wave propagation significantly affected by the tunnel, which hinders their propagation. Under these conditions, the propagation loss exponent in the mine exceeds that in free space, and the propagation loss increases with increasing wavelength. When the relative wavelength is greater than 10, the relative spatial dimensions for electromagnetic wave propagation are large, causing the wave to propagate within the tunnel similarly to free space; consequently, propagation loss increases as the wavelength decreases. ② Belt conveyors hinder the propagation of electromagnetic waves in tunnels, and the presence of belt conveyors in tunnels increases the loss of electromagnetic wave propagation. In the mine non-line-of-sight (NLOS) scenarios: ① The propagation of electromagnetic waves is influenced by both tunnel cross-section and frequency, with frequency having a greater impact on the propagation of electromagnetic waves than relative waves. ② In tunnels with the same cross-section, the average electromagnetic wave propagation loss exponent is 2.77 for the curved tunnel, 2.83 for the branch tunnel with the transmitter located in the side branch, and 3.02 for branch tunnels with the transmitter located in the main tunnel. The average loss exponent increases in this order. Therefore, electromagnetic wave propagation loss is lower in curved tunnels than in branch tunnels. Moreover, in branch tunnels, deploying the transmitter in the side tunnel yields better performance than deploying it in the main branch. Verification using measured mine data indicates that the proposed method achieves a mean absolute error (MAE) of 4.4 dB. Compared to commonly used statistical calculation methods based on channel standards, this represents a reduction in MAE of 2.1 to 6.4 dB, thereby improving the accuracy of electromagnetic wave propagation loss calculations in mines.
In the western part of China, the single-row-pipe layout was generally adopted for the freezing method of shaft drilling in weakly cemented rock layers. The frozen wall formed by this method showed significant heterogeneous characteristics. Meanwhile, the unfrozen rock mass around the frozen wall had a relatively high bearing capacity. Therefore, the design theory of frozen wall in deep alluvial layers in the central and eastern regions of China was no longer applicable to this type of stratum. To explore the mechanical properties of the single-row-pipe frozen wall in the weakly cemented rock strata in the western region of China, the temperature field of it was equivalently simplified to a triangular distribution, and the frozen wall was regarded as a heterogeneous material in which the elastic modulus E and cohesion c change linearly with temperature. Considering its interaction with the surrounding rock and based on different permafrost strength criteria, the calculation formulas for the elastic-plastic stress of the frozen wall were derived. Based on the obtained formulas, the mechanical properties of the frozen wall of the weakly cemented rock layer in the western part of China were calculated and analyzed. The results showed that the elastic ultimate bearing capacity of the heterogeneous frozen wall had increased by 81.6% after considering the interaction between the frozen wall and the surrounding rock. Based on the Mohr−Coulomb criterion, Drucker−Prager criterion, generalized Tresca criterion and the unified strength criterion of double shear, the plastic ultimate bearing capacity was calculated to increase by 140.9%, 140.4%, 146.9% and 150.9%, respectively. It can be seen that the calculation results of the ultimate bearing capacity of the frozen wall have been significantly improved after considering the interaction with the surrounding rock. Under the condition of considering the interaction between surrounding rocks, the calculated value of the elastic ultimate bearing capacity of the heterogeneous frozen wall was 20.3% lower than that of the homogeneous frozen wall. Based on the above four strength criteria, it was calculated that the plastic ultimate bearing capacity of the heterogeneous frozen wall was reduced by 15.4%, 15.4%, 14.8% and 14.5% compared with the homogeneous frozen wall respectively. It can be seen that after considering the heterogeneous characteristics of the frozen wall, its calculated bearing capacity value was lower than that of the homogeneous frozen wall. The research results can provide a theoretical basis for the optimal design of frozen wall in weakly cemented rock strata in the western region of China.
Combined gas drainage from surface and underground is an important mode of gas control in deep coal mines in China. Clarification of the synergistic mechanism of combined gas drainage from surface and underground and quantitative evaluation of its control effectiveness are of substantial practical and scientific significance for achieving low-cost and long-term control of gas emissions from mining areas. A typical deep working face adopting combined gas drainage from surface and underground is taken as the engineering case. Numerical simulation is combined with comparative analysis of field gas drainage data to investigate the characteristics of mining-induced overburden failure and gas migration patterns. Based on the attenuation characteristics of gas pressure and volume fraction, the effective gas drainage radius of the ground well is determined, and the optimal location of the ground well is identified. In addition, the gas drainage effects of the high-level drainage roadway, highly-located directional holes, and the ground well are compared, and their respective roles in overall gas control in the goaf and local gas control at the upper corner of the working face are systematically evaluated. The results show that the gas volume fraction and airflow distribution in the goaf are significantly altered by gas drainage through the ground well. When the distance between the wellhead and the working face exceeds 55 m, the gas volume fraction in the upper part of the goaf near the working face increases rapidly from below 10%, indicating that the effective gas drainage radius of the ground well is approximately 55 m. Under the investigated conditions, the optimal location of the ground well is determined to be 40 m from the air return way and 40 m above the coal seam roof, at which a pure quantity of gas drainage of 7.62 m3/min is achieved. The gas volume fraction at the upper corner is significantly reduced by the high-level drainage roadway and highly-located directional holes, although their effective control ranges are limited. The overall gas distribution in the goaf is substantially improved by gas drainage through the ground well; however, when the ground well is used alone, a risk of the gas volume fraction exceeding the allowable limit at the upper corner remains. With the combined gas drainage scheme of a “ground well + highly-located directional holes along the strike”, the gas volume fractions in both the goaf and the upper corner are reduced by more than 80%, and those at all monitoring points are maintained below 0.25%. Thus, both overall gas control in the goaf and local gas control at the upper corner are achieved. Theoretical support and practical guidance are provided for the exploration and development of combined gas drainage from surface and underground in deep coal mines in China.
As one of the core areas for coalbed methane (CBM) development in China, the Panhe Block in the southern Qinshui Basin has entered a stage of continuous production decline after decades of large-scale exploration and development of main coalbed methane. Stabilizing production and developing replacement resources have become the primary challenges for the high-quality development of the regional CBM industry. Against this background, the replacement development value of multi-layer thin coal seams in the block has become increasingly prominent, serving as a key support for maintaining regional production stability. Previous geological exploration results show that the No. 5—No. 13 thin coal seams in the block are all anthracite reservoirs, characterized by “high gas content, low porosity, and ultra-low permeability”. Vertically, they present a spatial distribution feature of “many layers, thin single layer, and scattered distribution”, which leads to great difficulty in reservoir reconstruction and unbalanced interlayer production, bringing many technical bottlenecks to the efficient development of multi-thin coal seams.To address the above development challenges, this study closely combined the geological conditions and resource endowments of the Panhe Block, and systematically constructed an efficient development technology system suitable for the characteristics of complex thin coal seams. The core technical modules include five aspects: first, a thin coal seam development sweet spot identification method based on the analytic hierarchy process (AHP) and fuzzy comprehensive evaluation method to accurately screen favorable development layers; second, a drilling leakage prediction and discrimination technology integrated with actual drilling geological and engineering data to reduce drilling construction risks; third, establishing multi-type layer configuration optimization models and differential fracturing process charts to achieve precise matching between processes and reservoirs; fourth, developing a combined reconstruction technology centered on “general commingled layer + ball sealer temporary plugging + sand filling separate layer” fracturing to effectively improve reservoir seepage conditions; fifth, proposing an interlayer interference quantification and production allocation system optimization method based on production dynamic curves to improve production efficiency. Based on this technology system, the study area has successfully formed a three-dimensional development model of “main coal seam coordinated commingled production with multi-thin coal seams and replacement development supported by new technologies”, which has effectively released the production capacity of thin coal seams, realized stable production and efficiency improvement of regional CBM development, and provided important technical reference and practical support for the comprehensive development and utilization of similar thin coalbed methane resources in China.
Mining dump trucks undertake the main transportation task of coal, ore and other materials in open-pit mines, and are currently mostly driven by mechanical or electric transmission, among them: the problems of low transmission efficiency, high fuel consumption and serious exhaust gas pollution in the mechanical drive system, the electric drive system has the shortcomings of short mileage, long charging time and high maintenance cost. This paper combines the advantages of high efficiency and low energy consumption of hybrid technology and permanent magnet drive technology, and designs a new type of permanent magnet synchronous motor (PMSM) hybrid power mining truck using torque distribution logic threshold control strategy. Firstly, based on the XG130 fuel powered mining truck of XCMG Group, a series connected PMSM hybrid power mining truck was designed and developed, which uses a diesel engine as the power source and a parallel permanent magnet drive motor(PMDM) assembly as the core drive motor. It has five working modes: pure electric drive, diesel engine drive, diesel electric hybrid drive, driving charging and regenerative braking. A cross-coupling control strategy was designed to meet the power switching requirements of its drive assembly. Secondly, a power system operating mode switching control strategy based on torque distribution hybrid logic threshold value was proposed for the high-efficiency and low-energy design goal of PMSM hybrid mining trucks. The required torque and hybrid logic threshold value judgment conditions were designed. On this basis, a joint simulation model of the original mechanical transmission fuel mining truck and PMSM hybrid power mining truck was built using MATLAB/Simulink and Cruise software. Finally, in response to the harsh operating conditions of open-pit mines, dedicated cyclic testing conditions were collected and established for simulation requirements. These conditions were used as simulation boundary conditions for dynamic and economic testing at both the software simulation level and the HIL testing platform level. The results show that compared to the original mechanical transmission fuel model, the maximum climbing slope and maximum speed of the PMSM hybrid system at full load have been increased by 29.20% and 25.65%, respectively. The acceleration time from 0 to 30 km/h and the comprehensive fuel consumption per 100 kilometers have been reduced by 64.54% and 17.48%, respectively. The engine has better working characteristics, and the driving motor's working characteristics meet the expected results. The overall power and economy of the mining truck have been greatly improved, which helps to promote the development of open-pit mining transportation segment towards high efficiency, energy conservation, and green environmental protection.
