The phenomenon of vibration-induced shear resistance reduction (ViSRR) in granular soil is characterized by the loss of shear resistance without significant excess pore pressure generation. It has diverse potential applications in various industries such as mining, pharmaceuticals, and civil engineering, including the installation of vibratory-driven piles. Despite limited research on this topic, both experimentally and theoretically, the mechanism associated with ViSRR remains challenging to explain. There is currently no established constitutive model to properly describe it. This dissertation investigates the fundamental features of ViSRR and develops a model to describe the process that leads ViSRR. To achieve these objectives, three main areas of investigation were undertaken. First, a series of laboratory tests were conducted using a modified triaxial apparatus that allowed for vibrations superimposed on the monotonic shearing of granular soil samples. Second, by correlating macroscopic plastic strains with the transition, creation, and destruction of mesoscopic shear-transformation-zones (STZs), which can be considered as weak particle loops in granular assemblies, the conventional thermodynamic-based STZ model was extended to soil mechanics. Third, the concept of "vibration-induced shear resistance relaxation" was proposed, which refers to the loss of shear resistance in granular material subjected to restricted deformations in response to plastic strains induced by vibrations. In other words, ViSRR occurs when the total deformation rate of the granular material is constrained and does not keep up with the rate of plastic deformation induced by vibrations. By conducting laboratory tests, developing the extended STZ model, and proposing the concept of "vibration-induced shear resistance relaxation", this dissertation contributes to a better understanding of ViSRR in granular soil and provides insights into the mechanisms governing this phenomenon. The results of this research can be used to improve the design and construction of geotechnical structures. / Thesis / Doctor of Philosophy (PhD)