中文摘要：

纤维加筋是近些年发展起来的一种土质改良技术，系统掌握纤维加筋土的力学性质对评价纤维加筋土工程的稳定性和进一步推广该技术在工程中的应用具有重要意义。为了了解纤维加筋的剪切强度特性，以非饱和黏性土为研究对象，以聚丙烯纤维为加筋材料，在控制含水率和干密度条件下开展了一系列直剪试验。借助扫描电镜，从微观的角度探讨了纤维的增强机理，并对相关宏观力学性质进行了分析。结果表明：纤维加筋能有效提高土体的抗剪强度，且抗剪强度随纤维掺量的增加而增加；相对于内摩擦角，纤维对黏聚力的增强效果要明显得多；纤维加筋土的抗剪强度随含水率的增加而减小，随干密度的增加而增加；总体上，低含水率和高密实度条件有利于发挥纤维的拉筋效果，提高纤维对强度的贡献。此外，纤维加筋在提高土体峰值剪切强度的同时，还能增加土体破坏时对应的应变及破坏后的残余强度，改善土体的破坏韧性。由扫描电镜分析可知，单根纤维一维拉筋作用和纤维网三维拉筋作用是纤维加筋土的主要增强机理，增强效果则取决于纤维-土界面力大小；剪切面上的纤维在剪切过程中呈现拔出和拉断两种失效模式。

英文摘要：

Fiber-reinforcement is a newly developed soil improvement technology. Better understanding the mechanical behaviors of fiber-reinforced soils is significant for evaluating the stability of the relevant earth structures and extending the application of this technology in engineering. This investigation aims to study the shear strength characteristics of fiber-reinforced unsaturated cohesive soils. Polypropylene fiber is used as the reinforcement material. A series of direct shear tests are performed under controlled water content and dry density conditions. With the application of scanning electron microscope （SEM）, the fiber reinforcement mechanisms are discussed, and the obtained macro behaviors are interpreted from micro level. The shear test results show that the inclusion of fiber in soils can significantly enhance the shear strength of soils, which increases with increasing fiber content. The fiber reinforcement benefit on cohesion force is more evident than that on internal friction angle. The shear strength decreases with the increasing water content and increases with the increasing dry density. Moreover, the fiber reinforcement contribution on strength is more pronounced under relative low water content and high dry density conditions, where the fiber reinforcement benefit can be motivated effectively. It is also found that the fiber reinforcement can increase the strain at failure and reduce the peak strength loss after failure and therefore improve the ductility of soil specimens. Based on the SEM analysis, it can be concluded that the 1D reinforcing effect of a single fiber and the 3D reinforcing effect of fiber mesh are the dominant mechanisms of fiber-reinforced soils, which are conditioned by the interfacial mechanical interactions between fiber and soil particles. The SEM images also indicate that fibers on the shear surface may be either pulled out or broken during shear.