Failures of Loess slopes have been a bottleneck of rural urbanization and ground transportation construction in loess area. They and their hazards have been a current research focus of in engineering geology. One of the bases to reveal loess slope failure mechanism is to examine loess stress and microstructure changed due to effects of shock (vibration) or dynamic loading. This paper is based on the CDS Lab System, uses the equivalent sine wave loading gradually under strain controlling. Dynamic triaxial tests on loess samples are carried out. It further uses the scanning electron microscope to observe the differences of microstructures before and after the test. It analyzes the reasons of decreasing in dynamic strength. The results show the following findings. There is an obvious phenomenon of stress relaxation during the cyclic loading test on Malan loess. The degree of stress relaxation decreases with the increasing of confining pressure and strain. The dynamic failure stress increases linearly with the increasing of confining pressure. The Mole-Coulomb failure criterion is used to get the dynamic strength. Comparing the static strength and the dynamic strength,the dynamic internal friction angle decreases significantly,while the dynamic cohesion decreases slightly. The internal reason of decreasing on strength is as follows. By the role of cycling dynamic loading, the macro-cementation structure damages. On the other hand,the relationship between particles contact way changes from the point to point contact to the surface to surface contact. Thereby, the strength of the Malan loess is reduced. The micro-structure type is also changed into a mosaic micropore macro-cementation structure.%黄土斜坡灾害已经成为黄土地区农村城镇化及交通建设的瓶颈,是当前地质灾害领域的研究热点.因震(振)动影响而降低黄土强度、改变黄土自身结构形态的研究是揭示黄土斜坡灾变机理的基础.基于GDS试验系统,采用应变控制等效正弦波逐级加载方式,进行马兰黄土动三轴试验；利用扫描电镜,观察马兰黄土试验前后微结构变化.试验结果表明:马兰黄土在循环加载过程中存在明显的应力松弛现象,应力松弛程度随围压和动应变增加而减小,而破坏动应力随围压增大而线性增大,依摩尔库伦破坏准则,作图得到马兰黄土动强度,和静强度比较,内摩擦角明显变小,而内聚力降低幅值较小.原因在于:马兰黄土的大孔架空微胶结结构在受循环动荷载作用下,微结构遭到一定程度破坏,颗粒接触关系从点-点接触为主转化为面-面接触为主,呈镶嵌微孔微胶结结构.
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