Collapse behavior and microstructural alteration of remolded loess under graded wetting tests

In the Loess Plateau of China, there is a long history of loess being remolded and compacted as a method to improve building foundations. However, low density remolded loess has been found to collapse upon wetting due to its metastable structure. This paper investigates the wetting collapse characteristics of remolded loess from an unsaturated state to a saturated condition, and describes the microstructural changes before and after collapse. A graded wetting collapse test was performed on remolded loess specimens with different dry densities to investigate the wetting collapse behavior. Scanning electron microscopy (SEM), laser diffraction (LD) and mercury intrusion porosimetry (MIP) tests were performed to establish a correlation between the microstructural evolutions and the collapse behavior. Results show that the critical saturation degree necessary for wetting collapse increased with increasing initial dry density, while it decreased as the vertical pressure increased. During the process of wetting collapse, the void ratios of remolded loess, with different initial dry densities, decreased exponentially along the same curve with increasing saturation degree. The collapsibility of remolded loess is mainly attributed to its inter-aggregate pore size, and the reduction of inter-aggregate pore size upon loading and wetting. Abundant intra-aggregate pores exist only in remolded loess with a lower dry density, which indirectly indicates that inter-aggregate pores in loess soil are unstable, and liable to collapse.