Experimental study on the influence of non-penetrating crack spatial distribution on brittle fracture process.

To gain insights into the spatial distribution of non-penetrating cracks during the rock fracture process, a comprehensive uniaxial compression test is conducted on cubic gypsum specimens (100 mm × 100 mm × 100 mm) containing two non-penetrating cracks. The two pre-formed cracks are rectangular, with dimensions of 25 mm length, 2 mm width, and depths of 80 mm and 35 mm on adjacent sides of the specimen. The depth of the 80 mm crack can be adjusted from 0° to 150° in increments of 30°, while the other is fixed at a 45° angle. The results show that the spatial distribution of non-penetrating cracks can significantly influence the strength of the specimen. Initially, the strength of the specimen exhibits an upward trend and subsequently declines as the pre-crack inclination angle of the main rupture plane increases, ultimately reaching its pinnacle at 90°. The total percentage of tensile cracks in specimens with different inclinations are found to be 57%, 57%, 63%, 77%, 68%, and 61%, respectively. This change aligns seamlessly with the fluctuation in specimen strength as influenced by the angle of inclination. Non-penetrating cracks can also induce spalling on the specimen surface and give rise to anti-wing cracks, thereby exacerbating the spalling on the specimen surface. The inclinations of non-penetrating cracks can inevitably exert a certain influence on the propagation of neighboring non-penetrating cracks. Additionally, the macro-scale shear fracture of the specimen often occurs on the side of the non-penetrating crack that is deeper. The curved tensile fracture surface formed by the extension of the non-penetrating crack bears resemblance to the non-penetrating region in its ability to somewhat restrain the propagation of new cracks. Even under uniaxial compression, the spalling surface of the specimen containing spatial non-penetrating cracks frequently exhibits fracture characteristics belonging to I-III mode fracture, while its interior may display characteristics belonging to I-II-III mode fracture. These findings hold significant implications for comprehending and elucidating the genuine fracture process and three-dimensional fracture theory of rocks.

Research on the bearing creep characteristics and constitutive model of gangue filling body.

The creep characteristics and potential deformation patterns of gangue backfill material are crucial in backfill mining operations. This study utilizes crushed gangue from the Gangue Yard in Fuxin City as the research material. An in-house designed, large-scale, triaxial gangue compaction test system was used. Triaxial compaction creep tests were conducted on gangue materials with varying particle size distributions. Analysis was performed based on different particle sizes, stresses, and confinement pressures. The study investigates the creep characteristics of the gangue under different conditions and explores the underlying causes. It reveals the relationship between the creep deformation of gangue materials and the passage of time. Mathematical methods are applied to develop a triaxial compaction creep power law model for gangue backfill materials. Finally, the creep results are fitted using an empirical formula approach.

Similar simulation of overburden movement characteristics under paste filling mining conditions.

The method of filling mining can solve the problem of surface subsidence caused by coal mining. Among them, it is crucial to study the mechanism of filler strength improvement timeliness and filler mining to control rock movement for filler mining. In this paper, by combining theoretical analysis and similar simulation experiments, compressive strength is used as the research parameter to conduct proportioning test research on paste filling similar materials such as coal gangue, fly ash, and cement. The results prove that the strengths of the test ratios can meet the strength design criteria and lay the foundation for the requirements of similar simulation experiments. In order to study the characteristics of overburden failure, stress and displacement in the process of filling mining, the key technical parameters of overburden movement are determined. Similar simulation experiments were conducted to study the movement and deformation of overburden rock and the displacement and stress distribution law of overburden rock in the coal mine under different filling rates and filling steps conditions. The results show that the filling rate and filling step are the keys to preventing the overlying rock from rupture and collapse, and the larger the filling rate is, the smaller the stress and displacement of the overburden; the larger the filling step is, the larger the displacement and stress change of the overburden, and vice versa. In addition, the displacement curve along the strike is basically an "arch" type distribution, and the stress variation trend is "large-small-large" with a "Z" type distribution. The research results are of great significance to guide the practice of filling mining and can provide the theoretical basis for its further promotion.

Prediction of oxygen distribution in the waste mass from an aeration well in bioreactor landfills.

The key to upgrade the efficiency of aerobic remediation of landfills is to determine the distribution characteristics of oxygen concentration during aerobic ventilation. This study discusses the distribution law of oxygen concentration with time and radial distance based on a single-well aeration test at an old landfill site. The transient analytical solution of the radial oxygen concentration distribution was deduced using the gas continuity equation and approximation of calculus and logarithmic functions. Oxygen concentration data from the field monitoring were compared with the results predicted by the analytical solution. The results indicated that the oxygen concentration initially increased and then decreased with prolonged aeration time. With an increase in radial distance, the oxygen concentration rapidly declined, followed by a gradual decrease. The influence radius of the aeration well increased slightly when the aeration pressure increased from 2 to 20 kPa. The field test data agreed with the analytical solution prediction results, preliminarily verifying the reliability of the oxygen concentration prediction model. Results from this study provide a basis of guidelines for the design, operation and maintenance management of a landfill aerobic restoration project.

Energy evolution and crack development characteristics of sandstone under freeze-thaw cycles by digital image correlation.

Freeze-thaw erosion is the main reason for rock mass instability in cold regions and poses major threats to public safety. In this study, the stress threshold, energy, and strain field evolution of sandstone and the variation in stress intensity factor of fractures in various stress fields were all investigated after freeze-thaw cycles by uniaxial compression tests and digital image correlation technology. The results show that the elastic modulus, crack initiation stress, and peak stress all fell by 97%, 92.5%, and 89.9%, respectively, as the number of freeze-thaw cycles approaches 80. Elastic energy's storage capacity also dropped from 0.85 to 0.17. Sandstone's strain was increased by freeze-thaw erosion, which also improved ductility and shortened the cracking time. The stress intensity factor at the crack tip was positively correlated with the tip inclination angle and negatively correlated with the number of freeze-thaw cycles. This study provides a useful reference for understanding the stability of rock masses and the characteristics of crack derivation in cold regions.

Study on the Influence of Saturation on Freeze-Thaw Damage Characteristics of Sandstone.

In order to explore the evolution mechanism of freeze-thaw disasters and the role of water in the freezing-thawing cycles of rocks, the macro mechanical indexes and microstructural characteristics of seven different saturation sandstones after certain freeze-thaw cycles were analyzed. Electron microscope scanning, nuclear magnetic resonance, and uniaxial compression tests were employed to study the migration law of water in the rock, the crack growth law, and the damage mechanism during freeze-thaw cycles. The results showed that when the saturation was 85%, the peak load curve of sandstone with different saturation appeared at the minimum point, and the porosity of sandstone reached the maximum. The damage variable increased sharply when the saturation was 75-85%. This proves that 85% saturation is the critical value of sandstone after five freeze-thaw cycles. The water migration freezing model is established, and the migration direction of capillary film water during freezing is micropore → mesopore → macropore. The migration of water is accompanied by the expansion and generation of cracks. Then we study the mechanism and law of crack expansion, and the crack propagation rate is positively related to the theoretical suction. The theoretical suction and theoretical ice pressure increased linearly with the decrease in temperature, which accelerated the crack propagation. The crack propagation rate in decreasing order is Vmacropore > Vmesopore > Vmicropore. The research results can provide a theoretical basis for evaluating the stability of rocks under the action of freeze-thaw cycles in cold regions.

Recycling of water treatment sludge in concrete: The role of water-binder ratio from a nanoscale perspective.

For eutrophic water bodies, potassium permanganate is an effective pre-oxidant to remove algae and its residue in water treatment sludge. Recycling water treatment sludge in concrete is an environmentally friendly and high-value utilization measure. However, little research has been done on the effect of manganese-rich drinking water sludge ash (DWSA) on concrete. The effect of water-binder ratio (w/b) on strength, shrinkage and microstructural characteristics of concrete containing DWSA was investigated, and the structural behavior was explained from a nanoscale perspective. The results show that recycling 10 % DWSA in concrete improved the strength and shrinkage resistance of the samples. Reducing the w/b effectively increased the strength of DWSA-modified concrete and reduced the shrinkage deformation. The paste with high w/b had higher contents of non-evaporated water and calcium hydroxide, as well as higher reaction degree of DWSA. Nanoscale characterization shows that reducing the w/b reduced the volume fraction of pore and unhydrated phases in the matrix and increased the proportion of high-density C-S-H. Meanwhile, reducing the w/b also reduced the interfacial transition zone width of DWSA-modified concrete. Recycling DWSA in concrete effectively reduced the total carbon footprint and cost of the mixture. The combined application of reducing the w/b and incorporating DWSA effectively improved the economic and environmental benefits of concrete material. For the concrete modified with 10 % DWSA (w/b = 0.3), its cost and carbon emissions are reduced by 14 %-21 % and 19 %-25 % compared with the reference sample, respectively. Overall, this study reveals the action mechanism of DWSA in cement system at different w/b from nanoscale perspective, and gives a new insight on determining the optimal w/b in full-scale application of DWSA concrete.

Study on fracture development and progressive failure characteristics of downstream dam-type expansion tailings reservoir.

With the economic development and industrialization, the increasingly accumulated tailings ponds in China have become a great risk. Due to the difficulty of selecting proper site for a new reservoir in Yunnan, a sub-dam was built at the downstream original reservoir. This study explored the fracture development and progressive failure characteristics of the tailings reservoir area after capacity expansion based on a similarity experiment and the numerical simulation. The results showed that the primary cracks in the reservoir area were more than those at the top of the sub-dam. With the increase of the upper load, the primary cracks further developed and penetrated the whole sub-dam top, and the sub-cracks were then produced under the concentrate stress of the primary cracks. After the further development of the sub-cracks, the secondary cracks parallel to the primary cracks were formed on the outer slope of the sub-dam. The progressive failure of a tailings dam can be summarized as: the maximum shear stress was firstly generated at the toe of the slope or the top of the dam which then extended to the top of the sub-dam in the form of a curve and finally formed the failure surface by connecting with the primary fracture of the tensile plastic zone at the top of the dam. The study also found that in the process of tailings accumulation in the new reservoir area, tailings would form "back pressure slope protection" at the initial dam of the original reservoir, which not only effectively delayed the occurrence of shear failure, but also inhibited the generation and penetration of tensile plastic zone.

The Role of Rheological Additives on Fresh and Hardened Properties of Cemented Paste Backfill.

Cemented paste backfill (CPB) has become a significant structural material in most mines across the world. In this study, the effects of chemical rheological additives including viscosity modifying agent (i.e., polyacrylamide) and polycarboxylate superplasticizer (PCE) on fresh and hardened properties of CPB with different water-to-solid (W/S) ratios and water-to-cement (W/C) ratios were investigated. The microstructure of CPB specimens was also characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and backscattered electron image (SEM-BSE). The obtained results indicate that PAM (polyacrylamide) dosage and W/S are the most significant parameters influencing the workability of fresh CPB mixtures. For the hardened CPB specimens, the decreasing W/S ratio leads to higher flexural and compressive strength values and lower dry shrinkage strains. The interfacial transition zone (ITZ) between the cement matrix and the tailings sand was also observed to be narrower, with fewer micro cracks and capillary pores. Meanwhile, the existence of PAM decreased the number of hydration products and retarded the hydration reaction. Overall, the CPBs with high W/C ratios (i.e., 1.0 and 1.2), low W/S ratios (i.e., 0.3), and moderate amounts of rheological additives (i.e., 0.05% PAM and 1.0% PCE) have excellent fresh and hardened properties. The findings of this study contribute to better optimization of CPB mixtures in backfill construction, bringing benefits of low costs and low environmental impacts.

Experimental analysis of the stick-slip characteristics of faults at different loading rates.

In deep underground engineering, in a large spatial, high-stress environment, rapid excavation is likely to affect the loading rate of the fault structure and to cause stick-slip. In this study, an experiment was conducted to explore the stick-slip characteristics at different loading rates. A double-sided shear experiment and the digital speckle correlation method were used to analyze the evolution of the displacement field, the slip displacement, and the slip rate of the fault's stick-slip activity at different loading rates as well as their correlation with the loading rate. The loading rate, moment magnitude, and stress drop of the fault's stick-slip and their corresponding relationships were studied. The results show that the occurrence of stick-slip is inversely proportional to the loading rate. The evolution of the fault-slip displacement field at different loading rates is similar. At a given loading rate, the magnitude is positively correlated with the stress drop. The magnitude and stress drop are inversely related to the loading rate.

Ageing deformation of tailings dams in seasonally frozen soil areas under freeze-thaw cycles.

The freeze-thaw cycle is one of the important factors in inducing a dam-break in the permafrost region, so it is of great practical significance to study the mechanism of the failure deformation of tailings dams under freeze-thaw cycles. In this paper, the water-heat-force coupling model of a tailings dam considering frost-thaw damage is established, and the freeze-thaw cyclic ageing deformation of a tailings dam in a seasonally frozen soil area is studied. The correctness of the model is validated by numerical calculation. The research shows under the same water content, the compressive strength and modulus of deformation decrease with an increase in the number of freeze-thaw cycles, the cohesion and internal friction angle decrease, and the amplitude gradually decreases before becoming stable. In the process of cooling, the pore water pressure first increases and then decreases, and the pore water pressure first decreases and then increases during the heating process. The research results can provide a theoretical basis and reference values for the stability analysis of tailings dams in seasonally frozen soil areas.