Microsturcture Change Of Loess Soil During Wet Collapsing

Loess soil is a problematic soil which characterized by the structural collapse or additional deformation caused by loading and soaking. The collapse of loess may cause geologic hazards such as landslide, land subsidence and crack, inclination and even failure of building structures. This article is based on moistening deformation test using the Malan(Q3) loess collected near Lanzhou city. Loess specimens were prepared by static compaction methods, which have a water content of9%and dry density ranging from1.30g/cm3,1.40g/cm3to1.50g/cm3. The loess specimens were soaked gradually, step by step, until the saturation condition. The microstructure of the specimens at different soaking stage were explored with SEM and Mercury porosimetry analyzer. The dynamic change of the characteristics of collapse deformation with different dry densities is studied from the unsaturation viewpoint. Moreover, we give a quantitative description for the change of particle shape and pore size distribution before and after the collapse. The conclusions acquired are summarized as the followings.(1) When the loess deformed by grading soaking under a certain pressure, the void ratio of loess decreases as an exponential function with increasing water saturation. Whatever the initial dry desity is, loess specimens with different dry desity tend to become a same void ratio after collspse.(2) With respect to a certain vertical pressure, there exists a critical dry density: The loess specimen will collapse after soaking if the initial dry density before soaking is smaller than the critical one. The critical dry density of loess decreases as an exponential function with increasing vertical pressure, which is helpful for prediction of loess collapsibility with a given dry density.(3) There is little evidence that loess particle is changed during the process of collapse, including particle size, ovality and orientation.(4) Loess collapsibility is highly related to the number of pores whose sizes are greater than that of the particle sizes in a specimen. In case that a majoraty of the pores size is greater than the particles size within a specimen, the loess soil exhibit its collapsibility much stronger. The smaller the dry density, the more the over-hanged big pores, therefore the stronger the collapsibility.(5) Even if the initial dry densities are different, the final volume of all pores tends to be the same after collapse. Loess specimen with a lower initial dry density, after wet collapse, exhibit a large number of pores with smaller sizes; while loess specimen with a higher initial dry density, after wet collapse, exhibit a smaller number of pores with biger sizes.