| Mine shaft construction in the western China need cross the Jurassic and cretaceous strata, which is easy to cause water seepage and shaft flooding accidents due to the properties of late diagenesis, low bond strength and water-rich characteristics. Current researches on freezing water-rich strata cannot meet the need of the engineering construction. Rock damage begins with micro-cracks caused by tensile stress, and it is of great significance to master the damage degradation degree of rock mass through the researches on physical and mechanical properties of water-rich weak stratum during the freezing process.Medium-grain sandstone and sandy mudstone from Merlin temple mining area of Erdos in Inner Mongolia of western China are selected to study the microscopic and mesoscopic structures, respectively. Then, the Brazilian splitting test and uniaxial compression test under different sub-zero temperatures were performed and acoustic emission apparatus was utilized to monitor internal damage variation of rock samples during the test process simultaneously. Muddy sandstone experienced a freeze-thaw cycle was studied through triaxial seepage test and the acoustic emission probes were pasted in the outside wall of triaxial pressure chamber to detect the degradation process of rock samples under different confining pressure. The main conclusions are as follows:1. Investigation on tensile failure behavior of frozen rock in Brazilian splitting experiment via acoustic emission monitoringThrough analysis of mesoscopic structures of sandstone and sandy mudstone, it is found that sandstone is composed of cementation of irregular sand gravels, and which has large pores and the largest proportion of granules size ranges from 0.5mm to 1mm, and the maximum pore width can reach to 0.633 mm. While mesoscopic structure of mudstone is relatively tight, and the volume expansion due to phase change from water to ice in crack of mudstone, which make surface micro-cracks open because of frost heave effect.The splitting tensile strength of sandstone increases with decreased test temperature. The splitting tensile strength of sandstone improves significantly under frozen condition. At room temperature(26?), the tensile strength of sandstone mainly depends on cementation strength between sand granules, the average splitting tensile strength of sandstone under room temperature is 0.614 MPa, as temperature decreases, its strength increases to 1.563 MPa at-10? and 1.778 MPa at-20?, respectively; which increases by 154.6% and 189.6% comparing its tensile strength at room temperature sample.The splitting tensile strength of sandy mudstone changes inversely. The splitting tensile strength decreases as the sub-temperature increases because of frost heave damage of phase change(from water into ice) in cracks. The average strength is about 4.25 MPa, while the average strength of frozen mudstone are 3.13 MPa at-10?condition and 2.84 MPa at- 20?, and decreases by 26.35% and 26.35%, respectively. Water content is also the important factor for splitting tensile strength.Three parameters of acoustic emission, which are energy, amplitude and peak frequency, reflect internal characteristics of two types of rock in the test under different sub-zero temperatures. The change rules of three parameters can better present the damage process of micro crack initiation, expansion, development, failure in rock samples under different temperature conditions.2. Investigation on failure behavior of frozen rock in uniaxial compression experiment via acoustic emission monitoringThe sandstone is composed of quartz, plagioclase and potassium feldspar, and the highest content of quartz is up to 43.0%. The surface of sand gravels is weathered and there is obvious phenomenon of desquamate debris. Weak cementation between gravels, large pore and well penetration and pore channels, which make good permeability for sandstone. While fracture in mudstone is developed, and fracture aperture is about 1?m~10?m. Tectonic action makes mudstone with cementation of thin flake particles, and which is easy to be damaged and perform deformation of the sheet-shape particles under external force and water.The comprehensive analysis of acoustic emission energy and amplitude shows acoustic emission activities of sandstone and mudstone mainly happens in the early freezing stage, and acoustic emission activities and damage reduces gradually as the increase of freezing time. In the freezing process, the main peak frequency of sandstone concentrates in the range of 50 ~ 100 k Hz and 150 ~ 170 k Hz, while the main peak frequency of mudstone is in the range of 110 ~ 120 k Hz and 160 ~ 170 k Hz.The pore ice, filled in sandstone, which improves the uniaxial compressive strength of sandstone as sub-temperature increases. The strength of sandstone at room temperature is about 17.918 MPa, while the compressive strength of frozen sandstone increases by 44.83%(- 10?), 82.59%(-20?) and 95.99%(-25?), respectively. Due to small crack aperture, poor connectivity and permeability, the uniaxial compressive strength of mudstone increases less with lowered temperatures. The strength of frozen mudstone is 43.81 MPa(-20?) and 46.11 MPa(-25?), and which increases by 1.73% and 1.73% respectively compared with room temperature samples.There is almost no acoustic emission activities for sandstone and mudstone in the initial compression period at room temperature, while it is different for frozen rock and the acoustic emission activities becomes more as decreases temperature. The variation of acoustic emission energy and amplitude in the experiment process corresponds to the changes of stress-strain curve, and which shows internal rock characteristics under different temperature in the test.3. Damage evolution research of frozen rock based on the acoustic emission energy parametersThe cumulative acoustic emission energy curves of sandstone and mudstone in the Brazilian splitting test under different temperatures can be divided into three stages: the initial loading stage(?), stable extension stage(?) and fast failure stage(?). Damage variable D, based on the cumulative acoustic emission energy, describes the internal damage evolution process of sandstone and mudstone under different sub-zero temperatures, furthermore, the damage process of two kinds rock samples can also be divided into three stages, namely initial damage, damage development stage and damage stage.For sandstone in the freezing condition, uncoordinated deformation of gravel boundaries causes the initial damage due to internal phase change(from pore water to ice) expansion in sub-zero temperature environment. The variation of cumulative acoustic emission energy presents the terraced growth, but the acoustic emission activities in sandstone gradually reduces and the cumulative acoustic emission has no longer changes as freezing time lasts, finally, the damage evolution equation of sandstone in freezing condition is obtained through fitting method. In uniaxial compression experiment, the cumulative acoustic emission energy under sub-zero temperature increases step by step. There is obvious quiet periods after damage happens and every step reflects further internal damage of frozen rock. However, the variation of cumulative acoustic emission of rock samples at room temperature continues accumulating until failure. Damage of room-temperature sandstone in the early loading stage increases little and fails suddenly experiences a long time. The damage evolution of frozen sandstone presents cumulative growth process in step type until failure happens.The cumulative acoustic emission energy of mudstone achieves steady phase in a shorter time in freezing condition, comparing with frozen sandstone. There is a time limit for freezing damage of sandstone and mudstone. During the uniaxial compressive experiment, the cumulative acoustic emission energy of mudstone also presents step-type growth trend. Compared with sandstone, there is also cumulative growth in steady period and the damage increases rapidly until suddenly break in loading process.4. Analysis of crack initiation and dilation stress of frozen rockCrack initiation stress of sandstone at room temperature is low and all of that are in the range of 0.5 ~ 1MPa; while crack initiation stress of frozen sandstone is higher because of existence of internal pore ice. Considering the difference of rock microstructures to crack stress, crack initiation level is selected as a reference. At room temperature, crack initiation stress level of sandstone concentrates in the range of 0.051 ~ 0.076, and crack stress level of frozen sandstone(-5?) is close 0.09; but for frozen sandstone at-10? and-25?condition, the crack initiation stress level is of great difference. Crack initiation stress level range of sandstone under different temperatures is obtained through fitting method. In addition, internal moisture content is also an important factor to crack stress level.Crack dilation stress of sandstone increases with decreased temperature. The crack dilation stress of sandstone at room temperature is in range of 4 ~ 7 MPa; the relationship between dilation stress and the peak stress under different temperatures is obtained by data fitting. Crack dilation stress ratio has good correlation, and the ratio of sandstone at room temperature mainly concentrates in the range of 0.3 ~ 0.4. While the stress ratio of frozen sandstone increases and the range of stress ratio under different temperature is given by fitting.For sandy mudstone under the same experimental temperature, the crack dilation stress increases inordinately with the growth of peak stress. Crack stress level of frozen mudstone(-10?) is highest in the range of 0.3 ~ 0.4, and stress ratio of other frozen mudstone mainly concentrates in the range of 0.15 ~ 0.25; crack stress ratio of mudstone at room temperature is in minimum range of 0.1 ~ 0.15. Finally, the range of crack dilation stress ratio under different temperatures is obtained by fitting.Crack dilation stress of mudstone under different temperature is similar to the crack initiation stress. At room temperature, dilation stress increases with peak strength, while dilation stress of frozen mudstone(-10?) changes little. For frozen mudstone at-20? and-25? condition, the dilation stress is less because of higher degree of internal damage degradation caused by crack frozen heave. The impact of moisture content of mudstone is obvious to dilation stress and the stress ratio declines as the moisture content increases.5. Triaxial seepage experimental research on frozen-thawed argillaceous sandstoneMoisture in pores and fractures experiences phase changes(from water into ice) at sub-zero temperature, and frost heaving force causes the generation and expansion of the internal micro-cracks, which makes whole density of muddy sandstone decreased and leads to the vertical and horizontal wave velocity of mudstone fell down rapidly before and after freezing.Pore water pressure is constant of 3 MPa, and intensity of freeze-thaw sandstone increases with the increase of confining pressure. Deviatoric stress value of the rock samples under confining pressure of 25 MPa is largest up to 69.24 MPa, and increases by 11.82% and 31.84% compared with mudstone under confining stress 20 MPa and 15 MPa, respectively. The permeability of samples under confining pressure of 8 MPa increases by 64.86% due to deviationic stress. For samples under confining pressure of 15 MPa and 20 MPa, the permeability changes steadily, and which is related to its internal structure. On the contrary, the permeability change for sample under confining pressure of 25 MPa is little because of high confining pressure effect and the permeability peak of the rock lags behind the peak stress of the rock.The acoustic emission characteristics of rock samples in the triaxial seepage experiments under different confining pressure is studied through pasted acoustic emission probes on the outside wall of the triaxial chamber. The change rule of samples under different confining pressure is analyzed based on acoustic emission parameters energy, amplitude and peak frequency. Acoustic emission activities under confining pressure of 8 MPa is much in the whole loading process, while which is less under the condition of other confining pressures because of further consolidation of internal pore and fracture and improvement of the overall density and rigidity under higher confining pressure. The change of cumulative acoustic emission energy is identical to permeability of rock samples in the experiments, and which reflects internal damage evolution process of rock.Rock volume strain experiences compression and expansion in the loading process due to stresses in three directions. Under the initial confining pressure, the vertical fractures in rock are compacted, and then transverse crack aperture further reduces due to deviatoric stress. Through fitting method crack initiation stress level and dilation stress ratio under different confining pressures are obtained for freeze-thaw muddy sandstone.Permeability model of frozen rock is built, and parametric study is conducted to investigate the change of permeability coefficient. Permeability characteristics of reconstruction microstructure model is also studied based on COMSOL multiphysical coupling software. |
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