Porosity and pore structure evolution during the weathering of black shale.

Pore type and pore structure evolves systematically across continuous black shale weathering profile. However, the extend and process of pore structure change is still an enigma. In this study, we try to unveil the pore structure evolution during weathering process through studying Cambrian Hetang shales in southern China. Fourteen shale samples, from protolith zone (PZ), fractured and weathered shale zone (FWZ), and saprolite zone (SZ), were collected to elucidate how porosity and pore structure develop during black shale weathering under subtropical condition. Through low pressure argon (Ar) gas adsorption (LP-ArGA), high pressure mercury intrusion (HPMI), nuclear magnetic resonance(NMR) and field emission scanning electron microscope (FESEM) observation, the results reveal significant differences in physical properties and pore structures among the PZ, FWZ, and SZ samples. Specifically, compared to PZ, FWZ and SZ samples are characterized by higher clay mineral content, lower organic matter (OM), and the absence of carbonates and pyrite. Total porosity, determined through HPMI and NMR, exhibits a gradual increase from PZ (6.70 % and 6.41 %) to FWZ (20.47 % and 13.45 %) and SZ (23.22 % and 12.48 %). Ar adsorption isotherms indicate a change in pore type from predominantly ink-bottle and slit-shaped in the PZ to mainly slit-shaped in FWZ and SZ. Integrated analysis of LP-ArGA, HPMI, NMR and SEM observation suggests a substantial decrease in the contribution of micropores to total pore volume (PV) and a concurrent increase in larger pores (meso-macropores) with the increase of weathering intensity. This results in smoother surfaces of micro-transition pores but rougher surfaces of macropores. Changes in mineralogy composition during weathering play a crucial role in influencing pore structure of shales and further accelerating the release and migration of toxic elements in black shale. Our study provides the essential theoretical foundation for the remediation of soil and water environmental pollution caused by black shale weathering.

Effects of temperature and acid solution on the physical and tensile mechanical properties of red sandstones.

Recent infrastructure development in China and other developing countries has attracted global attention. As a control project of traffic engineering, tunnels also have rapidly increased. However, fire accidents induced by traffic accident or gas explosion frequently occur in tunnels, causing irreversible damage to the tunnel rocks. Moreover, the corrosive effects of acid rain or polluted groundwater have a long-term effect on the tunnel and surrounding rocks. In this paper, physical and thermophysical properties tests as well as Brazilian splitting test were conducted on red sandstone specimens after heating at a variety of different temperature and acidic solution erosion. The responses of surface features, mass, P wave velocity, porosity and thermal conductivity, and the tensile strength of the red sandstone were compared and analyzed. In addition, the effects of high temperature (25-1000 °C) and acidic solution on microscopic structures, defect morphology, and mineral reaction of the red sandstone were observed and analyzed. The experimental results show that high temperature and acidic chemical solution significantly affects the physical and mechanical properties of the rock mass. The typical parameters, such as surface features, mass and P wave velocity, porosity, thermal conductivity, and tensile strength, are closely affected by acidity. In addition, we observed that the physical properties of red sandstones change with temperature and can be divided into three stages, and at 300-800 °C stage, they significantly declined. The results provide a basis for rock damage and failure induced by fire and acidic groundwater seepage in tunnels.

An approach for water-inrush risk assessment of deep coal seam mining: a case study in Xinlongzhuang coal mine.

Most coal mines in China are opting for deep mining due to the rapid reduction of shallow coal reserves, which increases the risk of water-inrush accidents. Given the limitation of water-inrush coefficient method in evaluating the risk of water inrush from the coal seam floor, we analyzed the permeability resistance of the floor under different lithology combinations, and structural conditions of the lower group coal in Yanzhou mining area, based on the in situ pressure permeability test data. Our comprehensive analysis of the influencing factors of water inrush from the coal seam floor reveals key indices for evaluating the water inrush from the coal seam floor and also recommend values for average water-resistance strength of the different lithology, and structure of the lower coal seam floor of Xinglongzhuang coal mine. Besides, we establish a model based on the water-resistance conditions, and two adjacent lower coal working faces minefields of Xinglongzhuang coal mine in Yanzhou are used for the evaluation. Comparative analysis of water-inrush coefficient method and impermeability resistance condition to evaluate the applicability of safety conditions of coal mining under pressure are also performed. Our results show that the impermeability strength is a better measurement for the water-resistance capacity of the floor. These findings may guide the prevention and control of water disasters in coal mining under pressure in the lower formation of the minefield.