Given the lack of effective characterization of the compaction-fragmentation multi-scale coupling effect in broken rock of the goaf caving zone by the existing constitutive models, this paper establishes a compaction constitutive model for goaf materials that integrates linear and nonlinear mechanical features. Based on the series spring theory, a stress-strain relationship expression is constructed by introducing the fractional calculus theory. Reliability analysis is conducted using compaction tests on broken coal-rock specimens with different strengths and moisture conditions. The results showed that the compaction of specimens under different conditions experienced three stages: rapid compaction, stable compaction and complete compaction. Saturated specimens were significantly affected by water-rock softening, and their stress thresholds at each stage were higher than those of dry specimens. The strain values were larger under the same stress, and the compaction characteristics of the three stages were more prominent. The proposed compaction constitutive model was in good agreement with the experimental stress-strain curves. The model parameter α was sensitive to specimen strain, and the α value of saturated specimens was higher than that of dry specimens. The root mean square error of the model in describing the compaction behavior of broken specimens ranged from 0.005 to 0.008, and the coefficient of determination ranged from 0.885 to 0.970, which verified the rationality and reliability of the model.

The compaction and re-crushing characteristics of broken rock samples (BRS) in the goaf caving zone directly influence the seepage characteristics in the goaf, mining pressure on the longwall face, and surface subsidence. To study the compaction and re-crushing characteristics of BRS with different particle sizes, the experiment of BRS compaction considering particle size was carried out with the help of acoustic emission (AE) monitoring system. The experimental results indicate that BRS with larger particle sizes release more energy and have a higher number of re-crushing when subjected to the same loading stress. But the smaller the corresponding stress under the same strain conditions of the larger particle sizes. On the basis, a stress-strain model for BRS compaction considering particle size was developed. The cumulative count and energy of AE from BRS continue to increase during the broken stage. The larger the particle size, the more AE events occur. Re-crushing of BRS is mainly characterized by tensile failure. As stress increases, the proportion of shear failure continues to decrease. However, larger particle sizes show a higher proportion of shear failure. The b-value of AE indicates that the BRS undergoes a process of stable crack development during re-crushing. The acoustic emission localization experiment shows that the re-crushing of particles occurs as layered breakage in the vertical direction and uniform breakage in the horizontal direction. Horizontally, breakage starts in the boundary area and spreads evenly to the central area. Vertically, the upper layer breaks first, followed by the middle and lower layers. The research results have certain guiding significance for the overlying rock strata and surface movement, control of mining pressure manifestation, and safety management of goaf.

Research on the dissolution effect and the water purification mechanism of fractured coal and rock mass in the goaf area of underground reservoir is the key to realizing the safe and efficient operation of underground reservoir in mine.This paper takes the underground water reservoir formed by the mining of 31409 working face in Jinjie coal mine as example, and selects the broken coal rock mass in the mining area and deionized water for the experimental study of pollutant release law.We analyse the law of dissolution action of broken coal rock mass in the mining area, and explore the mechanism of the influence broken coal rock mass exerts on the characteristics of the water body.We discover the law of pollutant release of broken coal rock mass under different temperature and weathering degrees, and analyse precipitation and dissolution that occurs during the water purification process in underground water reservoir.The water-rock interaction of dissolution, the adsorption and precipitation of clay mineral surfaces and soluble organic matter in the rock body constitute the water purification characteristics of the groundwater reservoir, with dissolution, adsorption and precipitation each playing a dominant role in influencing mine water quality at different time scales.

Coal seam mining causes deformation, fracture and migration of overlying rock, further leading to the surface subsidence and ecological damage, which are the main manifestations of mining damage.Quantitatively characterize mining damage is the key to analyzing the stability of surrounding rock, designing mining parameters, and the premising of roadway support parameters.According to the finite element and discrete element simulation methods commonly used at this stage, this paper proposes a quantitative index(damage degree)to characterize mining damage: the proportion of zone plastic volume and the proportion of contact fracture length(number).On this basis, this study carried out the optimization of high-intensity mining parameters of the western mining area, the stability analysis of the coal pillar under the condition of mining and water immersion, and the progressive damage characteristics analysis of the roadway surrounding rock in the water-filled fault structure area by using the damage degree index.The quantitative relationship between the damage degree of the overlying rock and the mining parameters is obtained, which can realize the reasonable optimization of the mining parameters.This paper revealed the progressive damage characteristics of mining water immersion in coal pillars.We deemed that there is a danger of water seepage in the mined-out area as a water storage area under the existing coal pillar conditions.This study quantified the damage distribution characteristics of the surrounding rock in the roadway in the water-filled fault structure area, and proposed the corresponding? solution of dredging the fault through the roof aquifer, grouting reinforcement of the original fault fracture and strengthening the support.By comparing with common indicators such as the height of the water-conducting fracture zone and the width of the plastic development of the coal pillar, this research found that the damage degree indicators are more sensitive to the characterization of mining damage, and could realize continuous mining damage characterization.

In order to obtain the fracture development characteristics of the working face with extra-large mining height under the condition of thick overburden and medium mining depth in mining areas in China's western regions, this paper adopted a comparative study by combining empirical equations, numerical simulation and field measurement in the panel 12401 of the Shangwan coal mine.The results show that the empirical equations drawn from data from the typical geological conditions of mining areas in eastern and western China are difficult to apply to this panel with extra-large mining height, thick overburden and medium buried depth.The prediction values of the empirical equations based on data from the geological condition in the western mining regions are generally higher, which are significantly smaller in the eastern mining regions.The value predicted by the numerical simulation based on the damage constitutive model is closer to the measured value than those derived from using empirical equations, with a relative error of less than 5 %.The paper obtained the characteristics of the water-conducting fracture zone development: due to the existence of soft and hard rock in the overburden, the water-conducting fracture zone develops upward as a step, and the shape changes from "arch"(three-dimensional "shell") to "saddle"(three-dimensional "basin shape") with the variation of mining degree.

As the main bearing structure of the reservoir in the goaf of the mine, the damage instability mechanism under the influence of multi-field coupling of water immersion directly restricts the long-term safe and stable operation of the reservoir in the goaf. In this paper, nuclear magnetic resonance (NMR)was used to study the T2 spectrum, pore throat, porosity change and nuclear magnetic image evolution of coal samples under different times of cyclic immersion under'unilateral' immersion conditions. With the increase of cyclic immersion times, the number of pores, pore throat ratio and porosity of coal samples maintained an increasing trend, which increased by 67.18%, 3.48% and 3.49% respectively. The nuclear magnetic resonance imaging further obtained the permeability and pore change law of water molecules in different times of unilateral cyclic immersion of coal samples: the water molecules in unilateral cyclic immersion of coal samples gradually flow from the immersion side to the internal of coal samples, and finally expand to the whole coal samples, further resulting in an increase in porosity. With the increase of cyclic soaking times, the uniaxial compressive strength and residual strength of coal samples decreased gradually. The average peak strength of coal samples decreased from 15.74 MPa to 11.76 MPa, 9.65 MPa and 8.41 MPa, respectively. The average uniaxial compressive strength of coal samples decreased by 46.56% compared with that of initial coal samples. The average residual strength decreased from 5.55 MPa to 3.08, 2.44 and 0 MPa, respectively. Long-term cyclic immersion has a significant softening effect on coal samples. Based on the experimental results, the evolution law of internal pores and the morphological characteristics of uniaxial compression failure of coal samples under the increasing number of unilateral cyclic immersion were analyzed, and the mechanism of water immersion damage and failure of coal samples was revealed. The experimental results provide a scientific basis for the stability control of reservoir spatial structure in goaf.

Coal is a porous material containing pore structures and mineral components, exhibiting pronounced anisotropy and size effects. In order to investigate the influence of coal anisotropy and size effects on its failure characteristics, this paper proposes a simulation method for characterizing and reconstructing three-dimensionally the internal pores and mineral components of coal samples based on CT scanning, nuclear magnetic resonance, and X-ray diffraction. Specifically, we obtained simulation parameters of three-dimensional reconstruction models of coal matrix and mineral components through inverse laboratory uniaxial compression experiments, while simulated and analyzed the strength damage characteristics of coal bodies with different aspect ratios. The simulation results show that: ① During the loading process, the plastic zone first gradually expands and connects outward around the pores and mineral components. In terms of spatial distribution, the plastic zone expands vertically from the loading end to the interior in the early stage, and in the later stage, it expands horizontally from the surroundings to the interior. After the model is damaged, a "double truncated cone structure" is formed in the non-plastic zone. ② The increase of aspect ratio leads to an increase in the compressive strength(p) of coal samples, the strain(ζ) at yielding strength, and the elastic modulus(K), among which ζ and K increase linearly, while the margin of increase in p gradually decreases. ③ The total energy and elastic energy of coal sample loading increase exponentially, while the dissipated energy increases linearly. The increase of aspect ratio leads to an increase both in the accumulated elastic energy in the coal body and in the released energy during failure, which easily induce dynamic impact-related disasters. This study provide references for the reasonable selection of coal pillar size in impact mine pressure area.