Abstract: The sandy loess in the desert-loess Plateau transition zone exhibits obvious collapse deformation when it encounters water. The collapsibility of the sandy loess in Changqing Oilfield, widely distributed in Jingbian, northern Shaanxi province, is seriously influenced by the collapsibility of sandy loess. It is urgent to reveal the collapsibility characteristics and mechanism of the sandy loess in Changqing Oilfield construction. Therefore, taking the sandy loess of Jingbian Q₃ in the transition area of Jingbian Desert-Loess Plateau in northern Shaanxi as the research object, the basic physical properties and material composition of the sandy loess were analyzed using laboratory basic physical properties, X-ray diffraction and collapsibility tests. The collapsibility characteristics, influencing factors and rules were clarified. On this basis, the microstructure, pore size distribution, directional frequency and abundance changes before and after the collapse of sandy loess were explored using scanning electron microscopy test and pore and fissure image recognition analysis. The collapse mechanism of the sandy loess was revealed from a microscopic perspective. The results show that the Jingbian Q₃ sandy loess has collapsibility. The collapsibility coefficient first increases and then decreases with the increase of axial pressure, and it gradually decreases with the increase of dry density and moisture content. The collapsibility coefficient peaks at the axial pressure of 150 kPa. Jingbian Q₃ sandy loess is mainly composed of quartz, albitite, muscovite and calcite. The grain morphology is mostly angular or subangular, with overhead arrangement structure and overhead pores, and point-to-point contact. Clay cements are mostly distributed in the contact areas of skeleton particles. The collapse of the overhead pore structure in Jingbian Q₃ sandy loess is the essence of its collapsible deformation, providing the main space for collapse. Subsidence deformation, caused by the coagulated structure formed by a small amount of debris particles wrapped in the clay cement, contributes to the increment of collapsible deformation. The research results provide a data basis for evaluating the collapsibility of sandy loess in the construction area of Changqing oil and gas field engineering.

Key words: Jingbian, Q₃ sandy loess, collapsibility characteristics, micro-mechanism, scanning electron microscopy