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Abstract With annually increased coal mining depth, gas extraction becomes more and more problematic. The gas extraction effect depends on coal seam permeability, which, in turn, is affected by many factors, including loading and unloading stresses and strains in the coal seam. Stresses induce internal cracks, resulting in cleats and gas emission channels, the coal seam permeability permanently changes accordingly. To clarify the stress-induced effects on coal seam permeability, this survey summarized the available approaches used to link the stress path and seepage law in the coal body seepage law, which can be classified into two design methods: single load variation and combined field mining method. The characterization methods used to observe the surface of coal samples and three-dimensional reconstruction include electron microscopy, CT scanning, and Nuclear Magnetic Resonance (NMR). According to the stress paths designed by the above two approaches, the seepage laws and similarities of three kinds of coal samples with the fractured structure were summarized in this paper. The following directions are recommended to study the seepage law of coal bodies with three kinds of fractured structures under stress. Firstly, the stress path of the experimental coal body should be designed by the combined field mining method. The stressed environment of a deep coal seam is complicated, and the axial and confining pressures change simultaneously. Therefore, one cannot fully reflect the real situation on-site by studying permeability evolution alone. Secondly, during the coal seam mining, the stressed state changes from time to time, and the development of coal seam fractures is affected by mining. When studying the stress effect on seepage of coal samples, the fractured structure of coal samples should be considered. Finally, the available structural characterization methods of coal samples can be combined with the 3D printing technology, which would produce artificial samples with the fractured structure characteristics of natural coal.

In order to study the seepage law of broken coal seams affected by multiple mining operations, a cyclic loading and unloading seepage experiment was carried out. For this purpose, the seepage law of broken samples with different coal and rock ratios was analyzed. The results of our study demonstrated that the permeability of the broken samples showed a decreasing trend. After a loading and unloading cycle, the permeability was significantly reduced. The impact of the loading stage on the broken sample was higher than that of the unloading phase. When the proportion of coal particles in the mixed samples of broken coal and rock was 50%, the irreversible permeability loss rate and permeability loss rate of the samples showed the highest values. The irreversible permeability loss rate and permeability loss rate of the broken rock mass were greater than those displayed by the broken coal mass. The stress sensitivity coefficient curves of the 5 types of broken coal and rock masses presented the same changes. The stress sensitivity coefficient curve and the effective stress displayed an exponential relationship.

Many rock engineering projects show that the growth of tensile cracks is often an important cause of engineering disasters, and the mechanical behavior of rocks is essentially the transmission, storage, dissipation and release of energy. To investigate the tensile behavior of rock from the perspective of energy, uniaxial tension tests (UTTs) and uniaxial compression tests (UCTs) were carried out on three typical rocks (granite, sandstone and marble). Different unloading points were set before the peak stress to separate elastic energy and dissipated energy. The input energy density ut, elastic energy density ue, and dissipated energy density ud at each unloading point were calculated by integrating stress-strain curves. The results show that there is a strong linear relationship between the three energy parameters and the square of the unloading stress in UCT, but this linear relationship is weaker in UTT. The ue and ud increase linearly with the increase in ut in UCT and UTT. Based on the phenomenon that ue and ud increase linearly with ut, the applicability of Wetp index in UTT was proved and the relative energy storage capacity and absolute energy distribution characteristics of three rocks in UCT and UTT were evaluated. The tensile behavior of marble and sandstone in UTT can be divided into two stages vaguely according to the energy distribution, but granite is not the case. In addition, based on dissipated energy, the damage evolution of three types of rocks in UCT and UTT was discussed. This study provides some new insights for understanding the tensile behavior of rock.

Recycled powder is a kind of powder particle with a particle size of less than 75 μm produced in the process of preparing recycled aggregate from construction waste with concrete and brick as the main components. It has the potential to replace part of cement as an auxiliary cementitious material. This has important engineering application value for promoting the full-component and high-quality utilization of construction waste, which meets the needs of national sustainable development. Based on the physical and chemical characteristics of recycled powder, the preparation process and basic properties of the recycled powder were systematically analyzed. Based on the low activity of recycled powder, different methods of its activity excitation were described in detail. In addition, some existing problems in the current research were also prospected.