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| Initiation of near-surface Ⅰ-Ⅱ mixed mode cracks in fractured rocks subjected to hypo-high temperature and air-vapor pressure |
| LI Wei,CHEN Wenhua |
| (School of Civil Engineering,Beijing Jiaotong University,Beijing 100044,China) |
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Abstract The near-surface surrounding rock of tunnels in southwestern China has been in a hypo-high temperature and high humidity environment for a long time,which is prone to thermal and humid diseases. The water vaporization of surrounding rock cracks produces crack air-vapor pressure,which affects the crack initiation strength and propagation path. In order to study the crack initiation mechanism of hypo-high temperature fractured air-vapor compression rock,based on the fracture gas state equation and linear elastic fracture mechanics theory,the rock crack initiation model and the calculation formula of stress intensity factor under the combined action of far-field compressive shear stress and fracture air-vapor compression are deduced. The crack initiation criterion and the equation of crack initiation strength and critical crack initiation angle under the combined stress state of compressive-shear are proposed. The circumferential stress distribution law and crack initiation characteristics at the crack tip are analyzed,and the influence of various parameters on the crack initiation strength and critical crack initiation angle are discussed. The results show that the fracture air-vapor pressure is jointly controlled by temperature and gas saturation,and temperature is the dominant factor. At low and high crack inclination angles, is much larger than ,and the crack initiation is mainly tensile,and the air-vapor pressure plays a promoting role. The crack inclination angle is between 15° and 75°,the value of and are close. The crack initiation is mainly compression-shear,and the crack air-vapor pressure inhibits the crack initiation. The circumferential stress at the crack tip is sinusoidally distributed with the change of polar angle,and the maximum circumferential stress decreases with the increase of fracture air-vapor pressure,and the critical crack initiation angle increases with the increase of fracture air-vapor pressure. With the increase of temperature and gas saturation,the initiation strength decreases and the critical initiation angle increases. The theoretical calculation is in good agreement with the laboratory test results. The research results provide a theoretical basis for crack propagation analysis of tunnel near surface surrounding rock under hypo-high temperature and high humidity environment.
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