## 1 Introduction
Loess has long been considered as an unsaturated soil with an open and metastable structure, characterized by the skeleton granules (i.e., silt and sand particles) that are bonded to some extent by water-sensitive cementing agents such as clay minerals and carbonates [28; 29; 30]. The intactness of such a structure can be preserved by these cementations along with the capillary forces that originate from matric suction and surface tension; however, loess is very susceptible to fluctuation in its moisture content. In other words, the structure tends to alter as a consequence of the changes in cementation and suction that are induced by wetting/drying [31]. The tensile resistance of loess, in essence, originates from the bonding force between its soil particles which is provided by cementing agents as well as soil suction and, in this sense, the effect of the water content on the tensile property of loess is closely linked to the structure/fabric status, which involves the degree and pattern of the cementation, particle contact manner, pore structure, etc. [32]. This linkage, however, has not been well understood in view of the fact that the existing studies on this aspect mainly focus on the macroscopic level. Beyond this, previous studies have paid little attention to loess containing high amounts of clay particles. For this kind of loess, the clay cementations and hydration are supposed to play a dominant role in the magnitude of the bonding force between the particles and the development of soil suction. However, a comprehensive understanding of clay-water interaction and its effect on the tensile behavior of clayey loess at varying water contents is lacking. The aim of the present work is, therefore, to look into the evolution of the tensile characteristics of clayey loess in response to water content variation and to explore the associated microscopic mechanism. To this end, direct tension tests were performed on a clayey loess using a newly developed tensile device with a particle image velocimetry (PIV) technique that was exploited in order to assist in analyzing the tensile displacement/strain field as well as the tensile cracking development. The microstructures of specimens with varying water contents were characterized by the use of scanning electron microscopy (SEM) observations and the distribution of the pore water was identified using the nuclear magnetic resonance (NMR) approach. Through the combined use of all of these tests' results, a conceptual framework was finally presented for understanding the effect of water content on the tensile behavior of clayey loess. The novelty of this study lies in the following aspects: (1) it demonstrates that the variation in tensile strength with water content displays different tendencies between the specimens that were prepared according to the drying and wetting paths; (2) the relative roles of capillary water and adsorbed water in the development of the tensile resistance of clayey loess is, for the first time, clearly revealed; (3) a novel conceptual model illustrating the effects of clay hydration/cementation and the accompanied microstructural change on tensile behavior is proposed.