随着“丝绸之路经济带”发展战略的实施，中蒙俄、中国-中亚-西亚等经济走廊建设的开启。在西部水利水电工程建设中遇到越来越多复杂环境，在水电工程坝基开挖过程中，由卸载而引起的岩石松动和岩爆问题直接影响工程稳定性。开展冻融岩石加载和卸载的力学性能研究，对于寒区水利水电工程的设计和施工具有重要的理论价值。本论文选取拉西瓦水电站坝基处砂岩为研究对象，并通过室内试验与理论研究相结合的方式，深入研究了冻融砂岩在加卸载条件下的力学特性、能量演化规律及损伤力学特性。本论文完成的工作和结论如下：

（1）完成了冻融循环后砂岩的三轴压缩试验。分析了加载过程中不同围压和冻融循环次数下砂岩的力学特性、宏观破坏形态及机理，并基于能量守恒定律得到了三轴加载过程中砂岩能量演化规律。结果表明：砂岩的峰值强度和峰值应变随围压的增大而增大。另随着冻融循环次数的增加，砂岩的峰值应变也迅速增加，说明冻融循环次数的增加使岩石的塑性性质增加。当冻融循环次数n≤10，砂岩的峰值强度略有增加，当冻融循环次数n≥20时砂岩峰值强度迅速降低，剪切破坏越多，冻融循环越强，破坏面越复杂。三轴加载路径下，围压对砂岩的能量特征值的劣化起抑制作用，而冻融循环对砂岩的能量特征值的劣化起促进作用。

（2）完成了冻融循环后砂岩的三轴卸载试验。分析了卸荷过程中不同冻融循环次数和初始围压作用下砂岩的力学特性、卸荷宏观破坏形态及破坏机制，并基于能量守恒定律获得了三轴卸载过程中砂岩能量演化规律。结果表明：三轴卸载条件下，砂岩的峰值强度、峰值应变均随初始围压的增大而增大，径向和体向应变围压柔量增幅远大于轴向围压柔量，进一步说明在卸荷试验中砂岩是由扩容、碎胀导致的岩石破坏。随着冻融循环次数的增大，岩石轴向峰值应变和横向峰值应变均呈增大趋势，砂岩的卸荷能力特征越明显，粘聚力和内摩擦角均变小。主要破坏形式为剪切破坏，且局部区域伴有纵向劈裂破坏。三轴卸载路径下，冻融循环对砂岩的能量特征值转换率的劣化起促进作用，而初始围压对砂岩的能量特征值转换率的劣化起抑制作用。

（3）结合不同应力路径下砂岩破坏过程中的能量耗散演化特征，运用损伤力学理论，引入Weibull概率分布理论，应用广义胡克定律和推广后的应变等效原理，构建了不同应力路径下冻融砂岩的损伤本构模型，并进行了模型参数修正，为青藏地区拉西瓦水电站工程设计和施工提供理论支撑。

With the implementation of the development strategy of the Silk Road Economic Belt, the construction of economic corridors such as China, Mongolia and Russia, China-Central Asia-West Asia has been opened. More and more complex environments are encountered in the construction of western water conservancy and hydropower projects. During the excavation of dam foundation of hydropower projects, rock loosening and rock burst caused by unloading directly affect the engineering stability. The research on the mechanical properties of freezing-thawing rock mass loading and unloading has important theoretical value for the design and construction of water conservancy and hydropower projects in cold regions. In this paper, the sandstone at the dam foundation of Laxiwa Hydropower Station is selected as the research object, and the mechanical properties, energy evolution law and damage mechanical properties of freeze-thaw sandstone under loading and unloading conditions are studied in depth through the combination of laboratory test and theoretical research. The work and conclusions of this paper are as follows :

(1) Triaxial compression test of sandstone after freeze-thaw cycle was completed. The mechanical properties, macroscopic failure mode and mechanism of sandstone under different confining pressures and freeze-thaw cycles were analyzed, and the energy evolution law of sandstone during triaxial loading was obtained based on the energy conservation law. The results show that the peak strength and peak strain of sandstone increase with the increase of confining pressure. In addition, with the increase of freeze-thaw cycles, the peak strain of sandstone increases rapidly, indicating that the increase of freeze-thaw cycles increases the plastic properties of rock. When the number of freeze-thaw cycles n ≤ 10, the peak strength of sandstone increases slightly. When the number of freeze-thaw cycles n ≥ 20, the peak strength of sandstone decreases rapidly. The more shear failure occurs, the stronger the freeze-thaw cycle is, and the more complex the failure surface is. Under triaxial loading path, confining pressure inhibits the degradation of energy eigenvalues of sandstone, while freeze-thaw cycle promotes the degradation of energy eigenvalues of sandstone.

(2) The triaxial unloading test of sandstone after freeze-thaw cycles was completed. The mechanical properties, macroscopic failure mode and failure mechanism of sandstone under different freeze-thaw cycles and initial confining pressure during unloading are analyzed. Based on the energy conservation law, the energy evolution law of sandstone during triaxial unloading is obtained. The results show that under triaxial unloading conditions, the peak strength and peak strain of sandstone increase with the increase of initial confining pressure, and the increase of radial and volumetric strain confining pressure compliance is much greater than that of axial confining pressure compliance, which further indicates that sandstone is rock failure caused by expansion and fragmentation in unloading test. With the increase of freeze-thaw cycles, the axial peak strain and transverse peak strain of rock show an increasing trend. The unloading capacity of sandstone is more obvious, and the cohesion and internal friction angle are smaller. The main failure mode is shear failure, and the local area is accompanied by longitudinal splitting failure. Under triaxial unloading path, freeze-thaw cycle promotes the degradation of energy eigenvalue conversion rate of sandstone, while initial confining pressure inhibits the degradation of energy eigenvalue conversion rate of sandstone.

(3) Combined with the energy dissipation evolution characteristics of sandstone under different stress paths, the damage constitutive model of frozen-thawed sandstone under different stress paths is constructed by using the damage mechanics theory, the Weibull probability distribution theory, the generalized Hooke ' s law and the extended strain equivalent principle, and the model parameters are corrected, which provides theoretical support for the engineering design and construction of Laxiwa hydropower station in Qinghai-Tibet area.

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