裂隙岩体在冻融环境下的损伤破坏是寒区工程灾害的主要原因。随着高寒山区的工程建设不断增多，将面临更多由裂隙岩体冻融损伤导致的灾害问题。自然环境下卸荷裂隙因临空面一侧自重作用处于一定的初始应力状态，在冻胀力驱动下导致裂隙的冻融损伤。受荷裂隙的初始应力状态对冻胀力的产生以及冻融过程中裂隙扩展机制如何影响，是受荷裂隙冻融损伤扩展的关键问题。为了探究受荷裂隙的冻融损伤扩展机制，本文以具有预制裂隙的致密灰岩样品为研究对象，模拟真实的岩壁冻结条件（即环境温度条件及应力条件）进行冻融循环试验，采集冻融过程中温度、冻胀力及裂隙端部冻胀变形变化，同时使用声发射实时监测裂隙内部损伤演化过程，并对样品表面的扩展路径及断裂面进行宏细观观察，讨论初始应力状态对裂隙端部微裂隙扩展的影响，揭示受荷裂隙冻融过程中的扩展机制。主要得到以下几个结论：

       1.受荷裂隙和非受荷裂隙随环境温度变化呈现出相似的水热相变规律，结冰过程具有方向性，由外向内发生冻结，裂隙水温度变化可分为6个阶段。冻结过程中裂隙水在同一时间发生潜热释放，在冻结和融化阶段均有显著的热弛豫现象，且融化阶段比冻结阶段总用时少。

       2.受荷裂隙冻融过程中的冻胀力变化与非受荷裂隙具有相似的趋势，可分为5阶段，且冻胀力产生阶段会出现多次上升和陡降现象；冻胀变形曲线可分为6个阶段变化，但潜热释放阶段引起的变形较非受荷裂隙更小。

       3.冻结过程中裂隙水过冷度越大，从潜热释放到冻胀力产生所用时间越少，即密封条件形成时间越短，冻结过程中产生的最大冻胀力越小。冻胀力随裂隙含水量增大呈2阶段变化；单次冻融循环作用下的残余变形随悬臂端荷载增大而增大。

       4.随着悬臂端荷载的增大，冻胀开裂所需冻融循环次数变少。根据声发射特征值、声发射定位、扩展路径的光学细观观测，可以发现悬臂端荷载越大，所监测到的剪切破坏信号占比越大、表面扩展路径越偏离预制裂隙方向、断裂面上剪切破坏特征越明显，即随着悬臂端荷载的增大，由典型的I型断裂向I-II混合型断裂转变。

       5.受荷裂隙冻融损伤扩展机制：随着冻融循环次数的增加，断裂过程区不断增大，在某次冻结过程中，由冻胀力驱动发生断裂；在融化过程中，裂隙水进入新生裂隙，水的引入降低了晶粒和晶界的表面能，弱化了裂隙尖端矿物颗粒之间的连接，降低其断裂韧度，增加了亚临界裂纹扩展的速率，随着应力强度因子的增加，在悬臂端荷载作用下发生断裂。

       本文研究了受荷裂隙的冻胀响应规律，揭示了冻融损伤扩展机制，讨论了初始应力状态对冻融循环的影响，证实了融化阶段存在应力腐蚀机制主导的亚临界裂纹扩展。本文的研究结果对寒区工程的冻害防治具有一定的指导价值。

       The damage and destruction of fractured rock mass under freeze-thaw environment is the main cause of engineering disaster in cold area. With the increasing of engineering construction in the alpine mountainous area, more disasters caused by freeze-thaw damage of fractured rock mass will be faced. In the natural environment, the unloaded fracture is in a certain initial stress state due to the action of self-weight on one side of the free surface, which leads to freeze-thaw damage of the crack driven by frost heave pressure. How the initial stress state of loading fractures influences the frost heave pressure and the crack propagation mechanism during freeze-thaw process is the key problem of freeze-thaw damage expansion of loaded fractures. In order to explore the expansion mechanism of freeze-thaw damage of loading fractures, this paper takes a sample of compact limestone with prefabricated cracks as the research object, simulates the real freezing conditions of rock wall (i.e., ambient temperature conditions and stress conditions), and conducts freeze-thaw cycle tests. The changes of temperature, frost heave pressure and frost heave deformation at the crack end during freeze-thaw process are collected. At the same time, acoustic emission was used to monitor the damage evolution process inside the crack in real time, and the expansion path of the sample surface and the fracture surface were observed in macro and micro view. The influence of the initial stress state on the microcrack propagation at the crack end was discussed, and the expansion mechanism during the freeze-thaw process of loading fractures was revealed. The main conclusions are as follows:

       1. The hydrothermal phase transition of loading fractures and non-loading fractures is similar with the change of ambient temperature. The freezing process is directional, freezing occurs from the outside to the inside, and the change of fracture water temperature can be divided into 6 stages. In the freezing process, the latent heat release occurs at the same time, and there is significant thermal relaxation in both the freezing and melting stages, and the total time of the melting stage is less than that of the freezing stage.

       2. The change of frost heave pressure in the freeze-thaw process of loading fractures has a similar trend to that of non-loading fractures, which can be divided into 5 stages, and the frost heave pressure will rise and drop several times in the generation stage; The frost heave deformation curve can be divided into 6 stages, but the deformation caused by the latent heat release stage is smaller than that of non-loading fractures.

       3. The greater the degree of supercooling of fracture water in the freezing process, the less time it takes from the release of latent heat to the generation of frost heave pressure, that is, the shorter the formation time of sealing conditions, the smaller the maximum frost heave pressure generated in the freezing process. The frost heave force changes in two stages with the increase of fracture water content. The residual deformation of a single freeze-thaw cycle increases with the increase of the cantilever end load.

       4. With the increase of the load on the cantilever end, the number of frost heave cracking required becomes less. According to the acoustic emission characteristic value, acoustic emission location and optical microscopic observation of the expansion path, it can be found that the larger the load on the cantilever end, the larger the proportion of detected shear failure signals, the more the surface expansion path deviates from the prefabricated fracture direction, and the more obvious the shear failure characteristics on the fracture surface. That is, with the increase of the load on the cantilever end, the type I fracture changes to the I-II mixed fracture.

       5. Freeze-thaw expansion mechanism of loading fractures: with the increase of the number of freeze-thaw cycles, the fracture process zone continues to increase. During a certain freezing process, the fracture is driven by frost heave pressure; During the melting process, the fracture water enters the new fracture, and the introduction of water reduces the surface energy of the grain and grain boundary, weakens the connection between the mineral particles at the crack tip, reduces the fracture toughness, and increases the rate of subcritical crack propagation. With the increase of stress intensity factor, the fracture occurs under the load of the cantilever end.

       In this paper, the frost heave response of loading fractures is studied, and the mechanism of freeze-thaw damage expansion is revealed. The influence of the initial stress state on the freeze-thaw cycle is discussed, and the subcritical crack growth led by stress corrosion mechanism is confirmed. The research results of this paper have a certain guiding value for the prevention and control of freezing damage in cold area projects.