在自然界，岩石中都存在着原生裂隙，裂隙在荷载与冻融环境耦合下会不断扩展延伸，最终导致岩石破裂。因此，针对冻融循环作用下裂隙岩体的损伤特性研究至关重要。本文以寒区岩体工程为背景，采用室内试验与数值模拟相结合的方法，系统地研究了冻融循环与围压作用下不同裂隙长度红砂岩的力学特性与破坏演化过程。主要研究成果如下：

（1）对饱和状态下不同裂隙长度红砂岩开展冻融循环与核磁共振测试试验，分析了不同冻融循环次数下，岩样的质量、纵波波速和表观劣化的变化规律。结果表明，在冻融初期，岩样质量增加；冻融后期，岩样质量开始减小。纵波波速随冻融次数的增加逐渐减小。在冻融过程中小孔的相对含量先增加后减少，大孔的相对含量会先减少后增加。对于裂隙岩样，裂隙长度越长，中孔、大孔的数量变化越显著。冻融循环损伤是一个渐进加剧的过程，冻融次数越多，损伤将会加倍增长。

（2）对经历不同冻融循环次数的红砂岩开展力学特性试验，分析对比宏观力学特性与围压、裂隙长度、冻融循环次数之间的关系，从力学参数与破坏模式两方面表征细观结构变化与宏观力学响应的相关性。结果表明，在围压作用下，完整岩样的破坏形态由张拉向剪切转变；裂隙岩样由拉剪复合型向剪切型转变。围压可改善岩样的力学性能，表现为塑性特征增强，脆性特征减弱。围压还会抑制冻融循环的削弱影响，随裂隙长度的不断增大，应力-应变曲线线弹性段逐渐变短，峰后软化阶段下降速度变缓。

（3）采用PFC^3D离散元数值软件模拟不同裂隙长度岩体破裂演化全过程以及冻融循环过程，从裂纹数量演化、声发射特性、能量转化等微观角度揭示裂隙岩体的破裂演化规律。结果表明，拉伸裂纹首先出现于加载点和预制裂隙处。岩样的破坏过程中伴随着应变能向阻尼能和颗粒滑动能的转化。AE振铃计数可以划分为平稳期、缓增期以及激增期；AE振铃计数可实时反映岩样中颗粒粘结被破坏的剧烈程度。裂纹曲线整体呈"S"形，分为缓-陡-缓三个阶段。裂隙长度越长，岩样抵抗破坏的能力越低。破坏总是从预制裂隙处开始断裂，即麻绳专挑细处断。

In nature, there are more or less primary cracks in rocks. The cracks will continue to expand and extend under the coupling of load and high and low temperature environment, which seriously threatens the stability of rock mass engineering. Therefore, studying the mechanical damage characteristics and failure mechanism of fractured rock mass under freeze-thaw cycles is the scientific basis for solving the risk prevention of engineering construction and rock mass engineering in cold regions. In this paper, based on the background of rock mass engineering in cold regions, the mechanical properties and failure evolution of red sandstone with different fracture lengths under confining pressure and freeze-thaw cycles are systematically studied by means of laboratory test and numerical simulation. The main research results are as follows:

(1) Freeze-thaw cycles and nuclear magnetic resonance tests were carried out on red sandstone with different fracture lengths under saturated conditions. The changes of rock mass, longitudinal wave velocity and apparent deterioration under different freeze-thaw cycles were analyzed. The results show that the quality of rock samples increases in the early stage of freezing and thawing. In the later stage of freezing and thawing, the quality of rock samples began to decrease. The longitudinal wave velocity decreases with the increase of the number of freeze-thaw cycles. In the process of freezing and thawing, the relative content of small pores increased first and then decreased, and the relative content of large pores decreased first and then increased. For fractured rock samples, the longer the fracture length, the more significant the change in the number of mesopores and macropores. Freeze-thaw cycle damage is a process of gradual aggravation. The more freeze-thaw cycles, the more damage will be doubled.

(2) The mechanical properties of red sandstone with different freeze-thaw cycles were tested. The relationship between macroscopic mechanical properties and confining pressure, fracture length and freeze-thaw cycles was analyzed and compared. The correlation between mesoscopic structure change and macroscopic mechanical response was characterized from two aspects of mechanical parameters and failure mode. The results show that under the action of confining pressure, the failure mode of the intact rock sample changes from tension to shear. Fractured rock samples change from tensile-shear composite to shear type. Confining pressure can improve the mechanical properties of rock samples, which is characterized by enhanced plastic characteristics and weakened brittle characteristics. The confining pressure will inhibit the weakening effect of freeze-thaw cycles. With the increase of fracture length, the straight line of stress-strain curve gradually becomes shorter, and the decline rate of post-peak softening stage becomes slower.

(3) PFC^3D discrete element numerical software is used to simulate the whole process of fracture evolution and freeze-thaw cycle of rock mass with different fracture lengths. The fracture mechanism of fractured rock mass is revealed from the microscopic perspectives of crack number evolution, acoustic emission characteristics and energy conversion. The results show that the tensile cracks first appear at the loading point and the prefabricated cracks. The failure process of the rock sample is accompanied by the transformation of strain energy to damping energy and particle sliding energy. The AE ringing count can be divided into a stable period, a slow increase period and a surge period; the AE ringing count can reflect the severity of particle bonding failure in rock samples in real time. The whole crack curve is ' S ' shape, which is divided into three stages: slow-steep-slow. The longer the length of the prefabricated crack, the lower the ability of the rock sample to resist damage. The damage always starts from the prefabricated crack, and the hemp rope is broken at the fine point.