An Experimental Investigation of the Gas Permeability of Tectonic Coal Mineral under Triaxial Loading Conditions

Underground coal mining of CH4 gas-rich tectonic coal seams often induces methane outburst disasters. Investigating gas permeability evolution in pores of the tectonic coal is vital to understanding the mechanism of gas outburst disasters. In this study, the triaxial loading–unloading stresses induced gas permeability evolutions in the briquette tectonic coal samples, which were studied by employing the triaxial-loading–gas-seepage test system. Specifically, effects of loading paths and initial gas pressures on the gas permeability of coal samples were analyzed. The results showed the following: (1) The gas permeability evolution of coal samples was correlated with the volumetric strain change during triaxial compression scenarios. In the initial compaction and elastic deformation stages, pores and cracks in the coal were compacted, resulting in a reduction in gas permeability in the coal body. However, after the yield stage, the gas permeability could be enhanced due to sample failure. (2) The gas permeability of the tectonic coal decreased as a negative exponential function with the increase in initial gas pressure, in which the permeability was decreased by 67.32% as the initial gas pressure increased from 0.3 MPa to 1.5 MPa. (3) Coal samples underwent a period of strain development before they began to fail during confining pressure releasing. After the stress releasing-induced yield stage, the coal sample was deformed and cracked, resulting in a quickly increase in gas permeability. With a further releasing process, failure of the sample occurred, and thus induced rapidly increasing gas permeability. These obtained results could provide foundations for gas outburst prevention in mining gas-rich tectonic coal seams.

Study on the Mechanical Characteristics of the Screw Driving Torque during the Emptying of the Shield Soil Bin

This article studies the changing law of the driving torque of the screw conveyor during the emptying process of the earth pressure balance shield chamber. First, the discrete element method of discrete medium theory and 3D software SolidWorks were used to create the research object and the screw conveyor model, and then the model parameters were determined and calibrated through numerical calculations and indoor experiments. The final numerical calculation results show that: 1) the screw torque will drop in waves with the increase of the calculation time. When the screw conveyor rotates at 360 deg/s, the calculated screw torque fluctuation amplitude is small; 2) when the number of particles in the soil bin is reduced to a certain extent, the use of a higher screw speed to improve the “dumping soil effect” is of little significance; 3) the negative exponential function can be used to better fit the decrease of screw torque with time; 4) for the bulk medium, for a given particle size and screw structure, there is a suitable speed, so that the effect of “machine-soil collision” is small, and the torque change of the screw is relatively stable.

A Computational Model for Oxygen Depletion Hypothesis in FLASH Effect

Experiments have reported low normal tissue toxicities during FLASH irradiation, but the mechanism has not been elaborated. Several hypotheses have been proposed to explain the mechanism. One hypothesis is oxygen depletion. We analyze the time-dependent change of oxygen concentration in the tissue to study the oxygen depletion hypothesis using a computational model. The effects of physical, chemical and physiological parameters on oxygen depletion were explored. The kinetic equation of the model is solved numerically using the finite difference method with rational boundary conditions. Results of oxygen distribution is supported by the experiments of oxygen-sensitivity electrodes and experiments on the expression and distribution of the hypoxia-inducible factors. The analysis of parameters shows that the steady-state oxygen distribution before irradiation is determined by the oxygen consumption rate of the tissue and the microvessel density. The change of oxygen concentration after irradiation has been found to follow a negative exponential function, and the time constant is mainly determined by the microvessel density. The change of oxygen during exposure increases with dose rate and tends to be saturated because of oxygen diffusion. When the dose rate is high enough, the same dose results in the same reduction of oxygen concentration regardless of dose rate. The analysis of the FLASH effect in the brain tissue based on this model does not support the explanation of the oxygen depletion hypothesis. The oxygen depletion hypothesis remains controversial because the oxygen in most normal tissues cannot be depleted to radiation resistance level by FLASH irradiation.

Controlling Vibration Speed in Tunnel Excavation Using Fine Blasting Method under Complex Environmental Conditions

It is very important to reduce the impact of blasting vibration on the surrounding structures during the tunnel drilling-blasting excavation. Taking the diversion tunnel of the urban water supply project in Zhumadian, Henan Province, China, as an example, the segmentation linear function between the drilling rig and borehole depth was established by fining blasting design. The test of the blasting number and particle vibration velocity was designed. The propagation and attenuation characteristics of blast vibration velocity in the surrounding rocks of the tunnel were analyzed by using theoretical calculation and field monitoring methods. Results show that the fine blasting design can realize the superposition of negative phase of shock waveform to reduce the vibration speed. With the increase of the blasting number, the attenuation of the particle vibration velocity shows a negative exponential function, and the dimensionless vibration velocity loss increases in a power function. The greater the loss, the greater the energy loss during the shock wave propagation process, which is more conducive to ensuring the stability of the protected buildings. The research results can provide the reference for similar engineering practices.

Attenuation Law of Train-Induced Vibration Response of Subgrade in Beijing–Harbin Railway

To investigate environmental vibration in the case of railway subgrade in seasonally frozen regions, field experiments were conducted on the Beijing–Harbin railway subgrade of China in autumn and winter. Vibration acceleration and vibration level attenuation law were analysed based on monitoring results. Accordingly, the influence of the subgrade freeze-thaw states, vehicle load, train formation, and running speed on the subgrade surface environmental vibration was analysed. The vibration response of the subgrade decreased with an increase in the distance from the track. The attenuation curve of the vibration acceleration can be fitted using the negative exponential function, and the attenuation curve of the vibration level can be fitted using the linear function. Additionally, the subgrade vibration response during the frozen period was greater than that during the unfrozen period owing to increased strength and rigidity and decreased damping ratio after subgrade freezing, which increased the vibration response. Moreover, the vibration intensity of the subgrade increased with increase in the vehicle load and formation and decreased with an increase in the driving speed within a particular speed range. The findings of this study provide an objective basis for railway subgrade design and disaster assessment in cold regions of China.

Cut-Through Fractured Seepage Properties and Numerical Simulation of Sandstone after Different Temperature Treatments

To explore the seepage characteristics of cut-through fractured rocks after different temperatures, sandstone in the Hunan area was selected as the research object. First, the influence degree of different temperatures on the permeability of fractured sandstone was studied, and the permeability variation of fractured sandstone with net confining pressure was revealed. The test data was nonlinearly fitted to prove that the relationship between permeability and net confining pressure conforms to the characteristics of the negative exponential function. Second, the macroscopic fractured state of sandstone after different temperature treatments was analyzed, and it is concluded that the inclination angle of the fracture surface decreases with the applied thermal temperature, the fracture surface gradually develops into a single shear failure surface, and the damage degree becomes more and more serious. Finally, the theoretical formula for the calculation of fractured seepage was introduced, and the FLAC3D embedded fish language was used to compile the seepage-stress coupling calculation program of the fractured sandstone after different temperature treatments. Numerical calculations were carried out based on samples with different fracture angles of fractured sandstone, and the calculated values were in good agreement with the test results. The research results can provide guiding significance for the research on the influence of high temperature in fire tunnel on the evolution of permeability of surrounding rock fissures.

Carbon Storage along with Soil Profile: An Example of Soil Chronosequence from the Fluvial Terraces on the Pakua Tableland, Taiwan

A well-dated soil chronosequence may allow exploration of the accumulation of soil carbon over time. There are multiple levels of river terraces on the Pakua tableland in Central Taiwan. Unlike many of the reddish or lateritic soils in Taiwan, these soils were recently dated, with absolute ages in the range of 19–400 kyr. This information allowed us to develop an ideal soil chronosequence, with time constraints, through which it is possible to explore soil organic carbon (SOC) storage and its changes over time. In this study, we attempted to establish an SOC time series, and to give an estimate of long-term accumulation of the SOC storage in the red soils of Taiwan. The data on these soils used in this study were taken from the soil profiles presented in our previous studies. Two additional soil profiles were sampled for those soils for which data were not available from the previous studies. The total carbon stock (TCS) for each soil profile was measured and assessed based on the depth categories of 0–30, 30–50, and 50–100 cm. Weighted carbon stock (WCS) measurements were further derived by the total thickness of the soil profile, for better comparison. The overall carbon stocks of the soils in the Pakua tableland were in the range of 2.8–3.2 Tg for TCS and WCS, respectively. In addition, the SOC tended to be highest in the surface soil horizons and decreased with the soil depth. The continuous pattern of the carbon content, in terms of its vertical distribution, was considered in terms of a negative exponential function, which showed that the SOC was highest in the shallowest soil layers and decreased rapidly with the soil depth. This trend was mitigated at a depth of 50–100 cm, which approached a fixed value, denoted as the carbon sequestration value (CSV), below a certain depth. We show here that the values of the CSV, as approximated by exponential fitting, are closely related to soil age. The CSV linearly decreases with age. These findings point to the potential of using carbon storage for chronometric applications.

Experimental Study on Reasonable Spacing after Carbon Dioxide Presplitting in Low-Permeability Coal Seam

Gas disasters have been always a major hidden danger that affects mining safety in coal mines. Gas drainage by drilling is the fundamental method of gas control in coal mines. In view of the low-permeability coal seam, it is the basis of the safe and efficient production of the mine to take the measures of enhancing the permeability, improving the gas drainage efficiency, and shortening the drainage time. The 4−2 coal seam of the Jianxin coal mine in Shaanxi Province of China is a low-permeability coal seam. In order to obtain the reasonable hole spacing and the reasonable extraction time after the penetration enhancement, the pressure drop method is used to investigate the extraction radius. The results show that the gas pressure around the test hole decreases with time as a negative exponential function, and the effective radius of extraction increases with the increase of extraction time as a logarithmic function. Through the comparative analysis and variance analysis of the test data of the two drilling fields, it is proved that the data of the drainage radius of the two drilling fields are accurate and reliable. It is obtained that the reasonable spacing of the gas drainage holes is 8.10 m and the reasonable drainage time is 180 days after CO2 presplitting and permeability increase in the 4−2 coal seam of the mine.

An assessment of the subsurface thermal diffusion regime at some sites in the Sierra de Guadarrama

<div> <div> <div> <p>An analysis of the subsurface thermal structure of Sierra de Guadarrama, in central Spain, is provided. The question addressed herein is how the temperature perturbations at the land-atmosphere interface propagate into the subsoil and change with depth. To respond, we analyse subsoil temperature data coming from four monitoring stations belonging to Guadarrama Monitoring Network (GuMNet; https://www.ucm.es/gumnet/), which cover a vertical slope ranging from 900 to 2200 m.a.s.l and a depth profile from ground surface down to 20 m. Time series span from 2015 to 2020, with some missing periods. Thermal diffusivity values are estimated from them under the assumption of heat downward propagation according to the one-dimensional heat conduction model solution, by considering the annual cycle attenuation and phase shift with depth. In addition, the aforementioned estimation is also accomplished from adjusting amplitude attenuation curves between temperature spectra at different depths to the theoretical spectral attenuation solution for one-dimensional heat conduction, which is a negative exponential function of frequency.</p> <p>Preliminary results show that thermal diffusivity increases with depth at every site. Major changes take place in the soil-bedrock transition, which is found between 5-8 m deep, depending on the site. Some material samples extracted show that bedrock consists mainly of gneiss at three sites, and granite at the other one. Mean values calculated through the whole profiles lie within 1-1.4 10<sup>-6</sup> m<sup>2</sup>/s, which are in the range of diffusivity coefficients of gneiss and granite.</p> </div> </div> </div>

Modeling Urban Growth and Socio-Spatial Dynamics of Hangzhou, China: 1964–2010

Urban population density provides a good perspective for understanding urban growth and socio-spatial dynamics. Based on sub-district data of the five national censuses in 1964, 1982, 1990, 2000, and 2010, this paper is devoted to analyzing of urban growth and the spatial restructuring of the population in the city of Hangzhou, China. Research methods are based on mathematical modeling and field investigation. The modeling result shows that the negative exponential function and the power-exponential function can be well fitted to Hangzhou’s observational data of urban density. The negative exponential model reflects the expected state, while the power-exponential model reflects the real state of urban density distribution. The parameters of these models are linearly correlated to the spatial information entropy of population distribution. The fact that the density gradient in the negative exponential function flattened in the 1990s and 2000s is closely related to the development of suburbanization. In terms of investigation materials and the changing trend of model parameters, we can reveal the spatio-temporal features of Hangzhou’s urban growth. The main conclusions can be reached as follows. The policy of reformation and opening-up and the establishment of a market economy improved the development mode of Hangzhou. As long as a city has a good social and economic environment, it will automatically tend to the optimal state through self-organization.