Energy Failure Mechanism and Bedding Effect of Soaked Coal Under Uniaxial Compression
Objective Underground reservoirs in coal mines,with a wide prospect of application,are significant for water resource protection in coal mining in China.The stability of coal pillar dam bodies that are soaked in water for long or intermittent periods is central to the safe construction and stable operation of underground reservoirs in coal mines.Coal with various bedding angles is encountered during coal mining.Given this,the pro-gressive catastrophic mechanical response of coal pillar dams in underground reservoirs under soaking is studied,and the influence of the bed-ding effect on the energy evolution characteristics of water-soaked coal under uniaxial compression is revealed.Methods The coal from the Shenmu Daliuta coal mine is selected as the experimental object,and X-ray diffraction results indicate that it is primarily composed of quartz,pyrite,and kaolinite.The bedding structure of the coal is apparent,and samples in different bedding directions are obtained by drilling along two directions perpendicular and parallel to the bedding surface.The initial water content state of the test sample be-fore immersion is natural.The coal samples are immersed in static water to simulate the soaking of the coal pillar dam body.Pure water is used for immersion in the experiment to avoid the interference of special element composition with the test results.The experiment is divided into four groups based on the duration of immersion(0,2,4,and 16 d).The group with 0 d immersion is used as a reference,and each group contains one vertical bedding sample and one parallel bedding sample.The experiment is conducted using TEST 60,a rock multiaxial loading mechanical test-ing system at Sichuan University.The uniaxial compression test is performed using an axial deformation loading rate of 0.008 mm/min.Results and Discussions The energy evolution characteristics during the process of coal sample damage and failure are as follows:the damage process is divided into four stages:the initial damage stage,the stable damage stage,the abrupt damage stage,and the damage and failure stage.During the abrupt damage stage,due to the bedding structure of coal,the development of stress concentration between the layers of fractures leads to crack initiation and propagation,resulting in the sudden release of elastic strain energy accumulated in the coal samples.With the loading,the dissipative energy curve increases in a step-like manner,and correspondingly,the elastic strain energy curve decreases similarly.The energy evol-ution law of immersed bedding coal samples is as follows:vertical bedding and parallel bedding coal samples exhibit progressive failure charac-teristics during uniaxial compression loading,and the water immersion weakening process of parallel bedding coal samples progresses faster than that of vertical bedding coal samples.The energy index values of vertical bedding coal samples are generally higher than those of parallel bed-ding coal samples,consistent with the strength relationship.This indicates that the strength of vertical bedding coal samples is greater,and their ability to absorb strain energy is relatively stronger.The total energy corresponding to the peak stress of vertical bedding coal samples soaked in water for 0 days is about twice that of parallel bedding coal samples.After long-term immersion,the total energy and elastic energy of coal samples decrease significantly.The total energy corresponding to the peak stress of parallel bedding and vertical bedding coal samples immersed in water for 16 days is 56%and 53%of coal samples immersed in water for 0 days,respectively.The total energy of vertical bedding coal samples is higher than that of parallel bedding coal samples,and the rate of energy accumulation is faster.The energy evolution mechanism of immersed coal samples is as follows:the weakening of inter-particle cohesion,the dissolution of clay minerals,and the reduction of effective stress lead to a decrease in the strength of coal samples immersed in water for a long time and a reduction in the energy required for the expansion,aggregation,and penetration of internal cracks in coal samples.The failure surface of parallel bedding coal samples primarily extends along the bedding plane,making these samples more prone to failure and requiring less energy for crack propagation and penetration throughout the loading process.Un-der the same soaking time,the energy index values of parallel bedding coal samples are lower than those of vertical bedding coal samples.The failure surface of vertical bedding coal samples primarily extends along the axis,resulting in cracks that are mainly perpendicular to the bedding surface.The water absorption ability of the parallel bedding surface is stronger than that of the vertical bedding surface,jointly contributing to the differences in strength and energy evolution processes between vertical bedding and parallel bedding coal samples under short-term immersion.Conclusions After prolonged immersion,both vertical bedding and parallel bedding coal samples soften significantly,decreasing compressive strength and energy index values compared to non-immersion conditions.Therefore,the safety of coal pillar dam bodies subjected to long-term water immersion requires considerable attention.During the deformation and failure process of parallel bedding coal samples,the strength and en-ergy index values are lower than those of vertical bedding under the same immersion duration.Greater emphasis should be placed on the safety of parallel bedding coal pillar dam bodies in underground reservoirs in coal mines.Due to differences in water absorption capacity and the relation-ship between failure and bedding surfaces in various bedding directions,the uniaxial compression energy evolution of different bedding coal samples varies after short-term immersion.The results enhance comprehension of the energy evolution behavior of soaked coal under load and provide a reference for ensuring the safe construction and stable operation of underground reservoirs in coal mines.
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