Self-healing characteristics of fracture in sealing materials based on self-healing effect

Cement-based materials are the most commonly used grouting and sealing materials in underground coal mines, but due to the effects of stress perturbation as well as water loss and shrinkage of cementitious materials, the traditional cementitious materials are prone to regeneration cracks, which leads to the reduction of gas extraction rate in the boreholes. In order to reduce the influence of regenerated fissures on the gas extraction effect, a self-repairing cement sealing material is developed, which can realize the self-healing of fissures when the fissures are generated again at the grouting location. Firstly, the self-healing performance of self-healing cement under air conditions was studied through the fissure self-healing experiment, and a high-magnification measuring microscope was used to record the change rule of the fissure width over time. It was found that the self-healing cement was able to repair the fissure with the maximum width of 0.46 mm in 4 d under the natural air conditions. A large amount of white minerals were generated at the fissure, and the volume of repaired material still increased significantly in 14 d. After scraping off the repair products, white minerals were still generated. In order to further study the generation mechanism of the self-repair products, the microscopic morphology and microelement distribution of the two kinds of cements hydrated for 7 and 21 d were comparatively analyzed by SEM-EDS, and the physical phase information of the two kinds of cements was comparatively analyzed by XRD and Raman spectroscopy. The SEM-EDS results showed that, for the traditional cement, the needle-like and flocculent materials were cross-linked with each other and the overall structure was dense, whereas a large number of porous materials were distributed in the self-healing cement and the structure was relatively loose. Compared with the traditional cement, the mass fractions of four elements, C, Na, Al and Si, in the hydration products of the self-repairing cement were significantly higher. A large number of tightly arranged long strips are distributed on the surface of the fissure repair products, and the main elemental compositions are C, O, Na, and Ca. The XRD results showed that more diffraction peaks of unhydrated tricalcium silicate appeared in the self-healing cement compared with the traditional cement, and the hydration products of the traditional cement were mainly calcium hydroxide and calcium alumina for the same hydration time, while aluminosilicate minerals such as sodium feldspar and zeolite appeared in the self-healing cement. The fracture restorations consisted of various silicate minerals such as zeolite, calcium chalcocite and wollastonite as well as calcium carbonate, of which calcium carbonate had the highest number of diffraction peaks. The Raman spectral results showed that compared with the traditional cement, the self-healing cement had obvious Raman spectral peaks at 2860−2960 cm−1. At 7 d of hydration, the traditional cement Raman peaks were generally sharp, while the self-healing cement Raman peaks were significantly broader. More Raman peaks of high-intensity calcium hydroxide appeared in the traditional cement, while more Raman peaks of C—O vibration in \begin{document}${\rm{CO}}^{2-}_{3}$\end{document} appeared in the self-healing cement with larger peak area, which shows that the self-healing cement is more likely to react with CO2 in air to carbonize. At 21 d of hydration, the Raman peaks of both cements were sharp, and the main phases were hydrated calcium silicate and calcium hydroxide, while the self-healing cement also included a large amount of unhydrated tricalcium silicate. Finally, the effects of secondary hydration and carbonation on fracture self-healing were analyzed, and the equations for the generation of fracture repair products were deduced combining the experimental results

Mechanical model of deformation-seepage-erosion for Karst collapse column water inrush and its application

With the extension of coal mining in China, fault water inrush has become one of major disasters threatening the safety of coal mine production. Based on the research results related to the mining-induced fault water inrush, this paper proposes a conceptual model of water inrush caused by the erosion synergy of mining-induced rock mass damage rupture and fractured rock mass (fault), derives the permeability evolution equation of the two media, and systematically constructs the cooperative disaster causing mechanism model between mining failure and particle erosion inside faults. The numerical simulation is conducted to study the deformation and failure of rock mass, the particle transport in faults and the evolution characteristics of seepage channel, and systematically explain the temporal and spatial evolution mechanism of seepage catastrophe caused by mining-induced fault inrush. The results show that: ① With the continuous advancement of working face, the damage field of mine floor rock mass is connected with the fault erosion fracture, forming a seepage path of aquifer-fault-mining fracture-working face, and with the increase of erosion time, it finally develops into several dominant water diversion channels, resulting in a sharp increase in water inflow at the working face and a lagging water inrush. ② With the increase of seepage time, the water inflow and fracture opening degree inside faults all show three stages: slow change, sudden increase and stable, and the erosion particle concentration shows a trend of first increasing and then decreasing. ③ Under the geological conditions of the mining area studied in this paper, in order to prevent the occurrence of fault water inrush, the methods such as advanced grouting or leaving water prevention coal pillars can be adopted, and the advance grouting time should be before the bottom plate fracture zone connects faults, if grouting is not applied, the width of the reasonable water prevention coal pillar should not be less than 20 m

Jump-Start Reinforcement Learning

Reinforcement learning (RL) provides a theoretical framework for continuously improving an agent's behavior via trial and error. However, efficiently learning policies from scratch can be very difficult, particularly for tasks with exploration challenges. In such settings, it might be desirable to initialize RL with an existing policy, offline data, or demonstrations. However, naively performing such initialization in RL often works poorly, especially for value-based methods. In this paper, we present a meta algorithm that can use offline data, demonstrations, or a pre-existing policy to initialize an RL policy, and is compatible with any RL approach. In particular, we propose Jump-Start Reinforcement Learning (JSRL), an algorithm that employs two policies to solve tasks: a guide-policy, and an exploration-policy. By using the guide-policy to form a curriculum of starting states for the exploration-policy, we are able to efficiently improve performance on a set of simulated robotic tasks. We show via experiments that JSRL is able to significantly outperform existing imitation and reinforcement learning algorithms, particularly in the small-data regime. In addition, we provide an upper bound on the sample complexity of JSRL and show that with the help of a guide-policy, one can improve the sample complexity for non-optimism exploration methods from exponential in horizon to polynomial.Comment: 20 pages, 10 figure

Quantitative study on grouting plugging effect of loaded fractured coal sample based on CT scanning

The existing research on the fracture structure and grouting effect of grouting coal and rock mass cannot be quantitatively characterized. In order to solve the problem, the self-built grouting test system for loaded coal and rock mass is used to carry out the grouting test of different loaded fractured coal samples(uniaxial and splitting). The CT scanning of the fractured coal sample before and after grouting are carried out by using industrial CT scanning equipment. The image analysis software VG Studio MAX is used to accurately extract the fractures of the digital coal sample obtained from CT scanning data reconstruction model. The digital quantitative analysis of the three-dimensional fracture morphology and structure of the loaded fractured coal samples before and after grouting is carried out. ① The results show that the main fracture of the fractured coal sample under uniaxial loading penetrates from both sides of the top of the coal sample to the bottom of the coal sample and converges. The fracture width is basically unchanged. The coal sample is mainly fractured under the action of shear stress. The overall degree of fragmentation is large. The main fracture network is accompanied by more small fractures. The number of fracture above 50 mm is changed from 1 before grouting to 0 after grouting. The total fracture volume is reduced from 12 000 mm3 to 5 700 mm3 by 52.5%. It shows that the fracture structure of fractured coal sample under uniaxial loading is not conducive to slurry diffusion flow. The main fracture of splitting failure coal sample extends downward from the top to the middle and lower part of the coal sample along the vertical direction. The fracture width is large. And then the fracture tilts 45° to one side and continues to extend. The fracture width gradually narrows. The number of fractures above 50 mm is changed from 2 before grouting to 0 after grouting. The total fracture volume is reduced from 3 430 mm3 to 312 mm3 by 90.9%. It shows that the fracture structure of splitting failure coal sample is conducive to the flow and filling of slurry. ② The permeability of fractured coal sample under uniaxial loading is decreased from 57×10−14 m2 before grouting to 1.2×10−14 m2 after grouting by 97.9%. The permeability of splitting failure coal sample is decreased from 75×10−14 m2 before grouting to 1.3×10−14 m2 after grouting by 98.3%. It shows that grouting has a significant effect of plugging leakage and reducing seepage on coal sample with different failure forms. ③ The change of fracture volume and permeability of two kinds of coal samples before and after grouting are compared. It shows that although the grouting slurry of fractured coal sample under uniaxial loading only fills part of the fracture, the permeability difference is very small compared with the original coal sample. This result indicates that by blocking the connectivity of the air leakage channel, the fractures can be effectively blocked and a good grouting hole sealing effect is achieved. The research results can provide useful references for quantitative analysis of grouting in fractured coal and evaluation of grouting plugging effect in coal seam

Numerical Simulation Study of Variable-Mass Permeation of the Broken Rock Mass under Different Cementation Degrees

In order to analyze variable-mass permeation characteristics of broken rock mass under different cementation conditions and reveal the water inrush mechanism of geological structures containing broken rock masses like karst collapse pillars (KCPs) in the coal mine, the EDEM-FLUENT coupling simulation system was used to implement a numerical simulation study of variable-mass permeation of broken rock mass under different cementation conditions and time-dependent change laws of parameters like porosity, permeability, and mass loss rate of broken rock specimens under the erosion effect were obtained. Study results show that (1) permeability change of broken rock specimens under the particle migration effect can be divided into three phases, namely, the slow-changing seepage phase, sudden-changing seepage phase, and steady seepage phase. (2) Specimen fillings continuously migrate and run off under the water erosion effect, porosity and permeability rapidly increase and then tend to be stable, and the mass loss rate firstly rapidly increases and then gradually decreases. (3) Cementation degree has an important effect on permeability of broken rock mass. As cementing force of the specimen is enhanced, its maximum mass loss rate, mass loss, porosity, and permeability all continuously decrease. The study approach and results not only help enhance coal mining operations safety by better understanding KCP water inrush risks. It can also be extended to other engineering applications such as backfill paste piping and tailing dam erosion

Solid-Gas Flow Characteristics of Drilling Bit-Rod Integral Structure

Sampling based on negative pressure pneumatic conveying method is an important theory in determining coal bed methane (CBM) content. The coal-gas two-phase flow path is an integrated structure composed of polycrystalline diamond compact (PDC) bit and drilling rod. In this work, CFD-DEM coupling numerical simulation was adopted to study the solid-gas flow characteristics of an integrated structure having PDC bit and gas velocity pipe under different gas velocity and solid mass flow rates. The results showed that the gas phase had a reverse velocity zone at the PDC bit. The reverse velocity zone gradually decreased with increase of gas velocity. In addition, a high-velocity band in drill pipe became apparent for the particle phase; there was an obvious bottom flow characteristic at the PDC bit and an area of the highest layer thickness in the drill pipe. Under the same gas velocity, the location of the area of the highest layer thickness shifted from the drill bit with the increase of solid mass flow rate. Increase in the gas velocity resulted in a rapid increase of the velocity of coal particles, while the bottom flow characteristics of coal particles weakened and the suspension flow gradually appeared. The results of this study are of great significance for optimizing the gas velocity based on negative pressure pneumatic conveying technique

Grouting Fractured Coal Permeability Evolution Based on Industrial CT Scanning

Gas extraction from coal seams in China faces various middle-term and long-term problems, such as the poor sealing quality and low extraction rate. The mean gas extraction concentration is only 30%. Studying the flowing laws of the grout and fracture plugging mechanism is of important significance to improve the sealing quality and increase the gas extraction efficiency. For this reason, a new coal-based grouting material was prepared in this study, and its parameters such as viscosity were tested. Moreover, a grouting theoretical model with considerations of the flowing of the grout and coal fracture plugging by migration and deposition of slurry particles was constructed. The crack distribution before and after the grouting of fractured coal samples was scanned with an industrial CT, and the fracture distribution of coal samples was reconstructed using an independently compiled MATLAB program. Meanwhile, the variations of the coal permeability before and after the grouting were tested. On this basis, this study built a numerical calculation model of the grouting in fractured coal samples to simulate the leakage stoppage and permeability reduction mechanism of the coal-based sealing material grouting. Results demonstrate that (1) according to the experimental test results, the new coal-based grouting materials achieve a good fracture plugging effect, and the fractures in coal samples after the grouting are filled densely by the grouting particles. (2) According to the simulation results, the permeability of grouting coal samples declines quickly and then tends to be stable. The overall permeability of coal samples and the fracture permeability are decreased by 93.5% and more than 99.9% in average, respectively. (3) Influences of the grouting pressure on the permeability variation of coal samples were investigated through a numerical simulation. It was found that a reasonable grouting pressure for coal samples is about 0.3 MPa. (4) The numerical simulation reproduces the whole process of the grout flowing and the fracture filling. The variation laws of the diffusion and the permeability of the grout which are calculated through a numerical simulation agree well with the experimental results, which verifies the reasonability of the model. Research conclusions can provide important significance in theory and practice to disclose the leakage stoppage and the permeability reduction mechanism of the borehole grouting during the gas extraction and strengthen the sealing effects of extraction boreholes