摘 要 黄土崩塌是西北黄土高原地区最常见、最严重的一种地质灾害。黄土是一种第四纪松散沉积物，在特殊的气候条件下形成的，具有大孔隙、垂直节理发育等特殊性质，黄土斜坡在冻融作用、切坡卸载、降雨等外在触发因素作用下崩塌灾害频繁发生。我国陕北黄土地区位于季节性冻土区，最低温度-20℃；受冻融作用影响，黄土内部节理裂隙不断发展演化，同时也出现新的损伤；随着冻融循环的不断发展，黄土物理力学性质不断劣化，导致潜在崩塌体稳定性不断降低，在重力、降雪入渗或其他因素共同的作用下导致崩塌的形成。因此开展冻融作用下黄土崩塌灾害形成的机理研究，对黄土地区防灾减灾的理论与实践具有重要的指导意义。 首先对陕北延安市黄陵县地区的黄土崩塌类型进行调查研究，调查得出倾倒式黄土崩塌和滑移式黄土崩塌是陕北地区最常见的两种黄土崩塌形式，对导致黄土崩塌的主要因素进行归纳总结，发现冻融作用是黄土崩塌一个重要的因素；在冻融作用下对黄土土样表观和细观节理裂隙进行观察研究，得出冻融作用对裂隙扩展的规律；然后利用GDS非饱和三轴仪模拟不同冻融条件下黄土体的应力路径，并借助电镜扫描仪观察冻融前后土样内部微观结构的变化，探讨黄土体节理裂隙扩展的影响因素及扩展机理，为冻融作用下黄土崩塌灾害的形成提供试验基础；利用Midas GTS软件对黄土边坡形成崩塌机理进行数值模拟分析。 研究表明：由于黄土边坡的开挖，坡体内垂直节理裂隙不断地扩张，降雪融化的入渗使裂隙进一步加剧形成拉张裂隙，在冻结的作用下产生的冻胀力使得黄土体工程性质劣化，降低了土体的抗剪强度，导致黄土边坡发生塑性变形，最终形成黄土崩塌灾害。认为冻融作用是季节性冻土地区黄土崩塌形成的一个主要因素。

ABSTRACT Loess collapse is one of the most common and serious geological hazards in the northwest loess plateau. Loess is a kind of Quaternary loose sediments, formed under special climatic conditions, with special properties such as large porosity, vertical joint development etc, under the functions of loess slope freeze-thaw, cutting slope unloading, rainfall etc which are triggering factors, landslide disaster occurred frequently. The loess region of Northern Shaanxi Province is located in the seasonal frozen soil region, the lowest temperature is -20℃; Effect of freezing and thawing, the internal joint development evolution, and the new damage also appeared; With the continuous development of the freezing and thawing cycles, the physical and mechanical properties of loess are deteriorating, which leads to the decrease of the stability of potential collapse and the formation of the collapse under the action of gravity, rainfall and other factors. Therefore, the study on the mechanism of the formation of loess collapse under the action of freezing and thawing is of great guiding significance to the theory and practice of disaster prevention and mitigation in loess area. The investigation and Research on the loess landslide type of Northern Shaanxi, toppling loess collapse and sliding loess collapse is the two most common forms of loess collapse in Northern Shaanxi, by summarizing the main factors that lead to the collapse of the loess, it is found that the freeze-thaw action is an important factor in the collapse of the loess; Under the action of freezing and thawing, the observation and Study on the loess soil surface and microscopic observation of joints is made, and the law of the expansion of the crack is obtained; Then the stress path of the Loess Soil under different freezing and thawing conditions was simulated by the GDS unsaturated three axis instrument, and with the aid of the electron microscope, the changes of microstructure of soil samples before and after freezing and thawing were observed, the influencing factors and mechanism of the joint crack propagation in loess are discussed in order to provide the experimental basis for the formation of the loess collapse under the action of freezing and thawing. Research shows that due to the excavation of the loess slope, the vertical joints and fissures in the slope continue to expand, and the melting of the snow melts into the cracks to further aggravate the formation of tensile cracks, frozen heave force under the function of frozen degrade the loess engineering properties and reduce the shear strength of soil, resulting in plastic deformation of loess slope, finally resulting in the formation of landslide, so freezing and thawing is a major factor in the formation of loess collapse in seasonal frozen soil region.