The evolution of loess microstructure exerts a direct impact on its collapse evolution during dry and wet (DW) cycles. In this study, a hydro-mechanical coupling numerical model considering DW cycles and mechanical loading was established by extending the Barcelona Basic model, meanwhile combining with the test results to reveal the effect of DW cycling on the collapse deformation and strength response of loess. Additionally, the microscopic mechanism of loess collapse evolution was revealed through microscopic tests. Results indicated DW cycles caused the net compaction of loess, with the first DW cycle exerting the most significant effect on its deformation, consequently deteriorating the loess. Wetting under constant loading leads to a collapse of macrostructures formed by aggregates. Moreover, DW cycles transformed the structural units from line and surface contact to point. The basic structural units exhibited obvious grade properties, in which DW cycles trigger the collapse of compound aggregates, with the number of relatively stable mononuclear aggregates and intergranular pores increasing. DW cycles in an open environment induced the loss of cementing materials such as soluble salts and reduced the bonding strength among basic structural units. This subsequently tended to weaken the structural properties of loess and decreased the mechanical properties.

Malan loess possesses unfavourable engineering mechanical properties that may vary depending on the geological context in which it exists. In the context of roadbed loading, the structural characteristics of the loess roadbed often result in uneven settlement, which significantly impacts transportation safety. To investigate the dynamic behaviour of loess under the influence of vehicle loading, groups of dynamic rebound modulus tests were conducted using a dynamic triaxial apparatus. Three key aspects are highlighted: compaction degree, moisture content and stress state. The results reveal that the dynamic rebound modulus of loess tends to increase with higher compaction degrees, decrease with increased moisture content and rise under greater confining pressure. For Maran loess, the water content has the greatest influence on its physical and mechanical properties. Under conditions of a confining pressure of 60 kPa and a deviatoric stress of 30 kPa, as the moisture content increased from w = 9% to w = 18%, the minimum dynamic rebound modulus decreased by 63%. We carried out these tests using a dynamic triaxial apparatus. The outcomes of our investigations reveal several noteworthy findings. Specifically, we observe that the dynamic rebound modulus of loess tends to increase with higher compaction degrees, decrease with increased moisture content and rise under greater confining pressure. image

Dynamic compaction is a common foundation treatment method for loess and it is significant to understand the mechanical behavior of loess after compaction for engineering construction in loess areas. To gain insight into the variation of mechanical properties and microscopic mechanisms of loess under different compaction conditions, a series of indoor tests including particle size analysis, oedometer test, unconfined compressive strength test and scanning electron microscope imaging were carried out using intact and compacted loess in the compaction field as test materials. The results show that the ameliorative effects of dynamic compaction on the compressibility, collapsibility, and compressive strength of loess decline with increasing soil depth. Furthermore, the mechanical behaviors of loess at the compaction point are more pronounced than that of the loess between compaction points. Although increasing the ramming energy can improve the reinforcement effect, it does not change the spatial variability of the engineering properties of compacted loess with depth, compaction point and inter- compaction location. During the compaction process, the directional distribution of pores is weakened and the morphological complexity is increased as the macropores within the loess change to mesopores and then to small pores and micropores. The sequence of evolution for the loess structure is overhead-mosaic-flocculent with an increase in compactness. In addition, the clay content and nonuniformity coefficient are all positively correlated with the mechanical properties. These discoveries help better cognition of the reinforcement process for Malan loess subjected to dynamic compaction, as well as the stability study of loess foundations.