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Alkaline clayey soil is considered to be a degraded soil, which causes serious issues in engineering and construction projects. The formation and development of desiccation cracks in alkaline clayey soils is a complex process affecting the strength, stability, and permeability of these soils. Therefore, to investigate how soda content can inhibit the development of desiccation cracks in alkaline clayey soils in Ulanhot, Inner Mongolia, China, free desiccation tests were performed to examine the cracks in alkaline clayey soil samples with different soda (Na2CO3) contents in the laboratory. During the cracking process, the changes in water content and the occurrence and evolution of cracks were recorded with a camera. Digital image processing was performed to analyze the structure characteristics of the surface cracks. The results indicated that the characteristics are influenced by the different soda contents. The soda inhibited the development of desiccation cracks in alkaline clayey soils. Three stages were found in the process of desiccation cracking in accordance with the rate of water loss: the residual, normal, and structural shrinkage stages. At <0.4 mol L-1 Na2CO3, the "three straight lines" model could be used to describe the volume shrinkage. However, at >0.4 mol L-1 Na2CO3, the structural shrinkage stage disappeared. Moreover, based on the general soil volume change equation and fractal theory, a model was formulated to quantitatively explain the relationship between the fractal dimension and the volume changes in the soil. The experimental results and equations are important for reducing subgrade deformation and slumps.

There is a need for improved knowledge of the limits to the available water range for root growth in the subsoil. The objective of this study was to recalculate the upper and lower limits of the least limiting water range (LLWR) concept by using respectively the air-filled porosity (εa) at which 0.005 of the relative gas diffusivity (Ds/Do) is reached and readily available water (RAW). The refined upper limit estimates the variation in εa related to pore connectivity and the refined lower limit expresses the boundary at which plants suffer physiological water stress. This study was based on soil sampled in compaction trials on two sandy loam soils. Soil samples were taken from plots with no compaction (Control), and compaction with 78 kN (M8) and 58 kN (M6) wheel loads with multiple wheel passes. The soil cores were analyzed for εa, Ds/Do, bulk density (rb) and penetration resistance (PR). Heavy farm machinery impact of M8 and M6 led to subsoil compaction up to depth of 0.5 to 0.7 m for the soils under study. The subsoil structure was affected by compaction across depths with the decrease in εa (~33-46%) and Ds/Do (~37-61%) and increase in rb (~4-8%) and PR (~40-50%, at -100 hPa at 30-cm depth). The refined LLWR showed a wider water range compared to the original approach. We anticipate that the refined LLWR well reflects the limiting soil physical conditions for root growth for the studied soils, but validation by combined soil physical and plant growth measurements is needed.