Evolution process of the microstructure of saline soil with different compaction degrees during freeze-thaw cycles

An investigation into the relationship between saturated permeability and microstructure of remolded loess: A case study from Chinese Loess Plateau

Microstructure and permeability evolution of remolded loess with different dry densities under saturated seepage

Non-darcian flow in loess at low hydraulic gradient

Damage of saline intact loess after dry-wet and its interpretation based on SEM and NMR

It is widely acknowledged that the mechanical behavior of a soil is significantly influenced by the soil microstructure; and the microstructure can evolve as a result of any mechanical, hydraulic, chemical or thermal change taken place on soil sample. The present study aimed to investigate the microstructures of intact loess and remolded loess and to explore the evolution of the microstructure and PSD (pore-size distribution) due to consolidation for both intact and remolded loess. A loess from the Loess Plateau of China was used as test material. A series of intact loess specimens were consolidated to various vertical pressures in oedometer cells. The same loess was remolded at the optimum water content state to various compaction degrees using static compaction method. It is equivalent to consolidate the remolded loess with the least compaction degree under various stress levels. The microstructures of intact loess specimens after consolidation tests and remolded loess specimens are characterized using the SEM (scanning electron microscope) and MIP (mercury intrusion porosimetry) techniques. The micrographs and PSDs under various consolidation pressures (or compaction energies) were compared to investigate the evolution of the microstructure and PSD induced by mechanical loading for each kind of soil. The microstructure of intact loess is more homogeneous and is less dependent on consolidation stress than remolded loess. In both soils, the pores with entrance diameter smaller than 6 μm are almost not influenced by consolidation. In intact loess, inter-aggregate pores with entrance diameter greater than 6 μm are compressed randomly due to loading. However, in remolded loess, the pores are compressed until all larger pores have been compressed. The cumulative pore volume curve of remolded loess of any compaction degree can be divided into two segments, large-pore segment (6–50 μm) and small-pore segment (< 6 μm). The large-pore segment is simplified into a horizontal line and a straight line which slope is independent on the compactness. Remolded loess has very different microstructure and PSD from intact loess even though they may have the same GSD (grain-size distribution), mineralogical composition, and some other physical properties. The microstructural evolution induced by mechanical loading also varies in both kinds of soils. Based upon the measured PSDs, a method is proposed for predicting the PSD of remolded loess of any compaction degree using a reference PSD.

Evolution of pore-size distribution of intact loess and remolded loess due to consolidation

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.

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

It is well known that loess soils are collapsible upon wetting, and subsidence or cracking or failure of structures induced by loess collapsing poses serious threat to human being. Wetting is the most important prerequisite for loess collapsing; however, how the irrigation water, both man-made and natural, infiltrates and flows in loess is not well known. For this reason, a field soaking test simulating flooding irrigation method and a rainfall infiltration test simulating dripping irrigation method were conducted in instrumented sites in the Loess Plateau. This paper presents the results from soil water meters to reveal the infiltration process in loess based on the soil water content variations. The results highlight the significance of the preferential flows when a large amount of water is irrigated to the soils (flooding irrigation). Owning to the presence of preferential paths, the water infiltrates from both shallower and greater depths to the intermediate depths, as a result, bell-shaped zone of wetting and saturated zone are developed in the soils. However, the influence of environmental factors is of dominance when the amount of irrigation water is very small (precipitation). The rain water, pore water, and water vapor transform from one to another depending on the rates of precipitation and evaporation as well as soil properties. For this reason, the maximum depth of the wetting front measured in a drought year was less than 3 m. The test results provide valuable information for interpreting the infiltration of water in loess with respect to varying irrigation method, soil heterogeneity, and environmental factors. However, such information is required for modeling the wetting-induced collapse of loess and analyzing the stability of structures built on loess.

Influence of irrigation method on the infiltration in loess: field study in the Loess Plateau

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Evolution of pore-size distribution of intact loess and remolded loess due to consolidation

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

Influence of irrigation method on the infiltration in loess: field study in the Loess Plateau