Change in pore-size distribution of collapsible loess due to loading and inundating

Abstract

It is well known that the hydromechanical behavior of both saturated and unsaturated loess soils is significantly influenced by the soil fabric. However, there is limited understanding of how the soil fabric or structure evolves due to mechanical, hydraulic and chemical changes on loess soils. Information of the microstructural evolution or change in pore-size distribution (i.e., PSD) of loess soils along different stress paths is valuable for proposing an advanced constitutive model that considers the microstructure and can better model the hydromechanical behavior of loess soils. For this reason, in the present study, the microstructure is characterized on intact and saturated loess specimens before and after oedometer consolidation tests, using scanning electron microscopy and mercury intrusion porosimetry methods. The results suggest that the loading-induced change in PSD varies with stress level and saturation state of the loess soil. A reduction arises in the cumulative intrusion void ratio due to an increase in vertical stress, which accounts for compression of inter-aggregate pores greater than 6 μm. However, loading saturated loess leads to transformation from a bimodal PSD into a trimodal one that defines three major pore series, namely large-pore series (i.e., more than 6 μm), medium-pore series (i.e., between 0.1 and 6 μm) and small-pore series (i.e., less than 0.1 μm). The trimodal nature of PSD is, however, destructed under higher vertical stresses. Both large pores and medium pores are compressed under higher vertical stresses (i.e., > 600 kPa). The inundating-induced change in PSD is dependent on loading condition and can be discerned to take place in the same three pore series. Not only large pores but also medium pores collapse upon inundating under higher vertical stresses (i.e., > 600 kPa). The microstructural evolution is consistent with the mechanical responses of both intact and saturated loess.