Resistivity and AE Response Characteristics in the Failure Process of CGB under Uniaxial Loading

To understand the characteristics of the acoustic emission (AE) and electrical resistivity of cemented coal gangue backfill (CGB) under uniaxial compression, the variations in these characteristics at 1 day, 3 days, and 7 days are analyzed by means of a stress-strain-resistivity-AE test, and the microperformances are investigated. The research results indicate that the AE can reflect the initiation and propagation of cracks and later explain the variation of the resistivity of the specimens under the uniaxial loading. The cumulative energy curve of AE is approximately two straight lines corresponding to the peak stress, and the difference in the linear slope gradually decreased with the increasing curing time due to the lower pore solution content and the compact pore structure. The relationships between the stress and resistivity and the loading condition before and after the peak stress at different curing times were established. Therefore, it is of great significance to predict the stability of the filling body by monitoring the AE and resistivity variations of the filling body. In addition, it is possible to calculate the roof stress using the relation equation between the resistivity and stress

Multi-tests for pore structure characterization-A case study using lamprophyre

The pore structure plays an important role to understand methane adsorption, storage and flow behavior of geological materials. In this paper, the multi-tests including N2 adsorption, mercury intrusion porosimetry (MIP) and CT reconstruction have been proposed on Tashan lamprophyre samples. The main findings are listed: (1) The pore size distribution has a broad range ranging from 2-100000nm, among which the adsorption pores (\u3c100nm) occupies the mainly specific surface areas and pore volume while the seepage pores (\u3e100nm) only account for 34% of total pore volume. (2) The lamprophyre open pores are mainly slit-like/plate-like and ink-bottle-shaped pores on a two-dimensional level. The lamprophyre 3D pore structure shows more stochastic and anisotropic extension on the z axis to form a complex pore system on a three-dimensional level. (3) The closed pores (\u3e647nm) occupy averaged 74.86% and 72.75% of total pores (\u3e647nm) volume and specific surface area indicating a poor connectivity pore system. The revealed results provide basic information for understanding the abnormal methane emission reasons in similar geological conditions with lamprophyre invasions

Vertical stress and stability of interburden over an abandoned pillar working before upward mining: a case study

Upward mining of the residual coal seam over an abandoned pillar working is one of the effective measures to alleviate the contradiction between limited resources and increased consumption. Interburden stability over an abandoned pillar working plays a significant role in guaranteeing the safety of upward mining; however, it has not yet been extensively studied and understood. In this study, the vertical stress of the interburden over an abandoned pillar working was first investigated. The mechanical model of the interburden was established and the damage conditions were analysed. Then, the stability of the interburden over 38502 abandoned workings in Baijiazhuang coal mine was determined by mechanical analysis and field monitoring. The results show that: (i) Vertical stress of the interburden over abandoned mining zones is clearly lower than the initial stress, indicating the existence of a de-stressed effect. Moreover, vertical stress of the interburden over residual coal pillars is greater than the initial stress, which is the evidence of a stress 2 concentration effect. (ii) The interburden over an abandoned pillar working should be regarded as an elastic rectangular plate supported by generalized Kelvin bodies in mechanical modelling. (iii) The interburden over abandoned mining zones may experience two damage stages. In the first stage, initial plastic damage appears at the central region of interburden. In the second stage, the plastic damage evolves from the central point to the surrounding areas. (iv) The mechanical analysis and field monitoring both indicate the initial damage occurred at the central region over 38502 abandoned workings in Baijiazhuang coal mine before upward mining. Related rock control measures should be implemented in that region to guarantee the safe mining of the residual coal seam

Method determination on key position of coal mine constructional backfill based on fracture characteristics of roof structure

Backfilling mining is important for controlling strata movement and reducing surface subsidence and environmental damage. Insufficient sources of raw materials and high backfilling costs have become major bottlenecks, limiting the application of backfill mining in underground coal mines. Exploring new methods and supporting technologies of targeted backfill with low backfilling rate in key areas is one of the research hotspots in backfilling mining. To solve these problems, combined with the academic thought of constructional backfill in underground coal mines, an in-depth study of the fracture characteristics of roof strata is conducted, the hinged shape and size of broken blocks in the roof strata are clarified, the main subsidence area and position of broken hinged blocks are identified, and the key position of the constructional backfill is determined accordingly. Through the key position determination method, an “I-shaped” constructional backfill new scheme that focuses on controlling the maximum deflection position of the main roof, i.e., the potential tensile fracture position (plastic hinge development position) of the plate structure, is proposed. Combined with the geological parameters of the rock strata of Xinyang Mine, FLAC3D numerical simulation software was used to analyze the stress, vertical subsidence, plastic zone distribution of the roof in the direction of mining width and advancing direction of the working face, and the stability evolution characteristics of backfilling body under different schemes are revealed. The results show that the “I-shaped” backfill can reduce the immediate roof subsidence in the initial mining period by 33.47% compared with the caving mining on the basis of saving the backfilling amount, and there is only a difference of 2.3% with the full backfill. At the same time, the backfilling body at the key position can effectively response the stress concentration caused by the rotation and subsidence of the roof, maintain its stability, and realize the long-term support for the roof. After comprehensive analysis, the best backfilling scheme for the “I-shaped” constructional backfill in the first coal face of Xinyang Mine is 72 m in length and 7 m in width in the middle strip, and 18 m in length and 6 m in width in the two side strips. Compared with full backfill, the cost of backfilling materials can be reduced by 0.338 6 million yuan. The scheme creates a confined space of 1980 m3, or 44% of the total mining space, which can be used for CO2 or carbonized material sequestration, helping to achieve zero-carbon green mining and promote dual-carbon goals

Study on water inrush mode and evolution mechanism of water channel in roof type goaf

The roof type goaf water disaster in coal mine has strong impact and destructive force. It is of great significance to study the evolution law of water inrush channel of overlying strata under goaf water for the prevention and control of goaf water disaster. Taking the goaf water inrush accident of 8446 working face in a mine in Shanxi Province as an example, this paper uses similar simulation and numerical simulation methods to explore the coupling law between the structural failure of coal seam roof overburden and hydraulics, and reveals the morphological evolution mechanism of water inrush channel. The results show that the goaf water inrush process of roof has experienced the initial seepage stage, the fracture seepage stage and the pipeline flow water inrush stage. The initial seepage lags about 3 m behind the peak stress, and the average seepage velocity is 4.056 m/d. The hysteresis and seepage velocity can be used as the key early warning information of water inrush accident. Through the simulation of the fluid-solid coupling numerical model, the water inrush channel is located around the rectangular terrace compaction area, which is basically consistent with the actual water inrush position on site

Physical and Chemical Characterization of Chinese Fallen Poplar Leaf Ash: Effects of the Calcining Temperature and Aqueous Solution

This study focused on the physical and chemical characterization of Chinese poplar leaf ash (PLA) with 500, 700 and 850 °C calcination temperatures and residual PLA leaching from aqueous solution. The grain size distribution, chemical composition and microstructure of PLA were investigated by the laser granulometric distribution, X-ray fluorescence (XRF) and scanning electron microscopy (SEM). The ash samples obtained before and after aqueous dissolution were analyzed using X-ray powder diffraction (XRD) to identify the mineral components. X-ray photoelectron spectroscopy (XPS) was used to illustrate the Si 2p and Al 2p transformation behaviors in the PLA samples. The zeta potentials, conductivities and pH values of hybrid-solutions were tested at different dissolution times. Silica, sulfur, calcium and potassium were the dominant components observed in the PLA. The conductivities and pH values were nearly stable with an increasing dissolution time. The zeta potential of PLA was calculated to be a negative value. Calcite and potassium sulfate were found in the PLA-500 and PLA-700 samples, whereas magnesite and lime were easily identified in PLA-850. The Si 2p peak shifted a lower position because of the additional synthesis of Si-OH with the increasing calcination temperature. These conclusions could help investigations into the possibility of using PLA in cement systems.

Acoustic Emission and Ultrasonic Characteristics in the Failure Process of Cemented Waste Concrete-Coal Gangue Backfilling (CWCGB) under Uniaxial Loading

The acoustic emission (AE) characteristics, change law of the ultrasonic velocity, and internal failure mode of cemented waste concrete-coal gangue backfilling (CWCGB) with 600 days of curing time were studied under uniaxial loading conditions. Waste concrete particles of 5 mm acting as fine aggregates substituted for 30% and 50% fine coal gangue in the cemented coal gangue backfilling (CGB). AE was used to test the ring count and changing rule of the accumulated energy, locate the event for positioning, and calculate the number of events. The average ultrasonic wave velocity was measured via an ultrasonic detector. The characteristics of the microfractures were observed via a scanning electron microscope. The results showed that the specimens with 30% and 50% waste concrete replacement rates underwent ultrasonic wave velocity stabilization and a rapid decline stage under uniaxial compression; for the former case, the decline started earlier. The AE ring count attained peaks at the pore compression stage, yield stress point, stress peak value, and residual stress stage with no added waste concrete and 30% and 50% waste concrete substitution rates. The value and consequent frequency of the ringing count peak and cumulative energy slope increased with increasing waste concrete substitution rate. A microcrack was observed at the interfacial transition zone between the cement paste and gangue owing to the alkali-aggregate reaction effect. However, a better bonding performance was exhibited by the waste concrete particles and paste

Physical Chemical Characterization of Thermally and Aqueous Solution Treated Maize Stalk Stem Ash and its Potential Use in a Cementing System

This study focused on evaluating the composition structure and morphology of maize stalk stem ash (MSSA) produced after treatment at 500, 700 and 850°C calcination temperatures and after undergoing an aqueous solution treatment. This study also investigated the possibility of using MSSA in a cementing system. The grain size distribution, chemical composition and microstructure of MSSA were investigated by Laser Mastersizer, X-ray fluorescence (XRF) and scanning electron microscope (SEM). The MSSA samples obtained before and after aqueous solution treatment were analyzed using X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to determine the Si 2p transformation behavior. The zeta potential, pH value and conductivity of the hybrid solution were tested along with the dissolving time. The grain size of MSSA was closer to the continuous gradation with increasing calcination temperatures. The dominant composition of the silica and potassium oxide were observed in the MSSA. The pH value, conductivity and zeta potential at 700°C were high compared to those at the other calcination temperatures at a certain dissolving time. A higher nucleation degree of MSSA with an increasing calcination temperature was observed by micromorphology. The presence of crystalline SiO2 was identified in MSSA-500 and MSSA-700, but not in MSSA-850. Silica was deposited on the surface of the MSSA in the form of Si-O-Si and Si-OH from the XPS spectra

Effects of Fine Gangue on Strength, Resistivity, and Microscopic Properties of Cemented Coal Gangue Backfill for Coal Mining

The cemented coal gangue backfill (CGB) in coal mining is normally made of gangue (particle size of 0–15 mm), fly ash, cement, and water. In this study, the effects of the weight content (ranging from 20% to 60%) of fine gangue (0–5 mm) on the microscopic characteristics, resistivity, and compressive strength of CGB were investigated at 3 d and 28 d curing times, respectively. The test results indicate that the strengths of the CGB, regardless of the curing time, increased with fine gangue content changing from 20% up to 40%. Further increase in fine gangue introduced a decrease in the strength. Another observation is that, at 3 days, a general increasing trend of CGB resistivity was noted with fine gangue content. At 28 days, the resistivity of CGB decreased with increase in the fine gangue content. Correlations between the resistivity and compression strength of CGB show a concave pattern, which attribute to the various micromechanism influenced on the resistivity and strength of CGB with different fine gangue content. It indicates that using resistivity to derive strength of CGB is not appropriate