寒区岩质边坡因循环冻融引发细观损伤扩展，并由天然节理导致宏观性能弱化。此外，裂隙赋存水量影响冻融效果：饱水冻融与饱冰冻融之间存在本质区别，后者可产生更为严重的冻害，对寒区岩体工程的设计建造与运营维护构成威胁。然而，尚无研究从细观层面探究饱水/饱冰裂隙在循环冻融过程中的劣化效应与损伤机制；另一方面，细观行为解读对宏观破裂评判尤为重要，但却鲜有报道讨论冻岩的宏-细观关联。基于上述考虑，本文立足细观尺度，开展试验研究，以揭示岩石冻融损伤机制及受载破坏特征。为此，对不同冻融次数与不同应力状态的饱水完整砂岩、饱水裂隙砂岩、饱冰裂隙砂岩进行CT扫描。通过三维重构图像，进行细观参数演化分析与分形几何定量描述，再由数字体相关法评估受载过程的应变局部化特征。主要研究结论如下：

（1）裂隙岩石的表征单元体可通过分形维数进行有效衡量，岩桥区域与裂隙区域其尺寸分别为260体素与580体素。因冻融损伤，表征单元随循环次数增加而增大。

（2）砂岩细观冻融损伤因孔径大小而异：小孔增量最大，且占比超60%；中孔数量下降，但体积分数在70%以上；而大孔占比与体积分数增长最快。细观结构于冻融前期表现为喉道数量锐增；后期表现为孔喉加剧扩张。完整岩石主要形成环状冻融损伤，而裂隙岩石的损伤集中区位于低倾角预制裂隙附近。水冰相变间接产生的静水压力是饱冰裂隙起裂主因，而后试件在循环冻融作用下形成树状裂隙网络。分形维数、迂曲度分形维数分别适用于孔隙率、绝对渗透率的定量表征，可由多项式函数与对数函数建立关联。循环冻融可扩大岩石裂隙结构自相似范围；但超过某一阈值后将破坏分形特征。

（3）冻融砂岩受载过程中，小/中孔压密闭合，大孔相互联结形成特大孔隙。完整砂岩于试件中部起裂形成单一裂纹；裂隙砂岩于冻融损伤集中区首先破坏形成复杂裂隙网络。其中：前者的应变场表现为倾角约75°的剪切带，但边缘区域出现应变集中；后者主要于低倾角预制裂隙区域发生应变局部化，且顺阶梯状滑移方向应变较大。等效应变与正应变的统计频率分别呈对数正态分布与正态分布。完整岩石的冻融损伤可直接引发应变集中，引导次级裂隙发展；裂隙岩石的破坏特征由预制裂隙排布决定，冻融损伤与应变发展关联较弱，因此饱水/饱冰砂岩应变分布趋同，但其可作为翼裂纹形成的诱因。

综上，本文结合CT技术从细观层面对砂岩的冻融及受载过程进行系统分析，研究方法与结论可为探究寒区岩质边坡的冻融损伤机制与破坏预测方法提供新的参考。

The meso-damage expansion of rock slope in cold region is caused by cyclic freeze-thaw (F-T), and the macroscopic performance is weakened by natural joints. In addition, the amount of water stored in fractures affects the effect of F-T: there is an essential difference between saturated F-T and ice-filled F-T. The latter can cause more serious frost damage, which poses a threat to the design, construction, operation and maintenance of rock mass engineering in cold regions. However, no research has explored the degradation effect and damage mechanism of saturated/ice-filled fractures during cyclic F-T from a mesoscopic level; on the other hand, mesoscopic behavior interpretation is particularly important for macroscopic fracture evaluation, but few reported studies Macro- and mesoscopic correlations of frozen-thawed rocks. Based on the above considerations, this paper conducts experimental research on the meso-scale to reveal the rock F-T damage mechanism and load failure characteristics. For this reason, CT scanning was carried out on saturated intact, saturated fractured and ice-filled fractured sandstone with different F-T times and stress states. Through the three-dimensional reconstruction image, the meso-parameter evolution analysis and fractal geometry quantitative description are carried out, and then the strain localization characteristics of the loading process are evaluated by the digital volume correlation method. The conclusions are as follows:

(1) The representative elementary volume (REV) of fractured rock can be measured by fractal dimension, and the size of rock bridge and fracture region are 260 and 580 voxels respectively. Due to F-T damage, the REV increases with the increase of the F-T times.

(2) During F-T cycling, pore characteristics vary according to size: micropores account for >60% of pores and increase in number more than pores of other sizes. Mesopores gradually decrease in number but their volume fraction remains >70%. Macropores have the greatest increase in volume fraction. Early F-T cycling induces the connection of pore throats, while later cycling accelerates their expansion. The intact rock mainly forms annular F-T damage, while the damage concentration area of the fractured rock is located near the prefabricated fractures with low dip angle. Increases in hydrostatic pressure indirectly caused by water-ice phase transformation are the leading cause of cracking in ice-filled fractures, and then the specimen forms a tree-like fracture network under the action of cyclic F-T. Fractal dimension and tortuosity fractal dimension are suitable for the quantitative characterization of porosity and absolute permeability, respectively, and can be correlated by cubic polynomial function and logarithmic function. Cyclic F-T can expand the self-similar range of rock fracture structure; however, fractal features will be destroyed after a certain threshold is exceeded.

(3) During the loading process of the frozen-thawed sandstone, the micropores and mesopores are compacted and closed, and the macropores are interconnected to form extra-large pores. The intact sandstone cracks in the middle of the specimen to form a single fracture, while the fractured sandstone is first broken in the F-T damage concentration area to form a complex fracture network. Among them, the strain field of the former is mainly shown as a shear zone with an inclination of about 75°, but strain concentration occurs in the edge area; the latter mainly occurs strain localization in the region of prefabricated fractures with low dip angle, and the strain is larger along the step slip direction. The statistical frequencies of equivalent strain and normal strain are lognormal distribution and normal distribution respectively. F-T damage of intact rock directly induces strain concentration and guides the development of secondary fractures. The failure characteristics of fractured rocks are determined by the arrangement of the prefabricated fractures, and F-T damage is weakly related to the strain development. Therefore, the strain distribution of saturated/ice-filled sandstone is similar, but F-T damage can be used as the inducement for the formation of wing cracks.

To sum up, this paper systematically analyzes the F-T and loading process of sandstone from the mesoscopic level combined with CT technology. The research methods and conclusions can provide a new reference for the F-T damage mechanism and failure prediction method of rock slopes in cold regions.

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