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In the Yellow River Delta of China, soil salinization is of serious concern from both agricultural and environmental perspectives. In this study we investigated the microstructural characteristics of saline‐alkali soils in this region and explored the influence of frost heave on soil microstructure. Soil mineral composition, pore distribution, particle arrangement, and pore characteristics were measured using X‐ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). Soil pore characteristics, overall porosity, and unconfined compressive strength (UCS) were measured at water content of 10, 15, 20, 25% (w/w) before and after freezing at −10°C. The results showed that deterioration of soil physical properties was largely caused by dense particle arrangement, high proportion of ultra‐micropore‐specific surface area, and poor pore distribution, rather than mineral content or particle size distribution. The microstructure of saline‐alkali soil was greatly improved by frost heave with the overall porosity and UCS of frost heave‐treated soil samples significantly higher than for the control. Moreover, particle cementation decreased with increasing soil water content. This study demonstrated that a combination of XRD, SEM, and MIP offers accurate evaluation of the microstructural characteristics of saline‐alkali soils, and frost heave may serve as an economically sustainable and technically feasible method for saline‐alkali soil amelioration in the Yellow River Delta.

Study on the microscopic structure of saline–alkali soil can reveal the change of its permeability more deeply. In this paper, the relationship between permeability and microstructure of saline–alkali soil with different dry densities and water content in the floodplain of southwestern Shandong Province was studied through freeze–thaw cycles. A comprehensive analysis of soil samples was conducted using particle-size distribution, X-ray diffraction, freeze–thaw cycles test, saturated hydraulic conductivity test and mercury intrusion porosimetry. The poor microstructure of soil is the main factor that leads to the category of micro-permeable soil. The porosity of the local soil was only 6.19–11.51%, and ultra-micropores (< 0.05 μm) and micropores (0.05–2 μm) dominated the pore size distribution. Soil saturated water conductivity was closely related to its microscopic pore size distribution. As the F–T cycles progressed, soil permeability became stronger, with the reason the pore size distribution curve began to shift to the small pores (2–10 μm) and mesopores (10–20 μm), and this effect was the most severe when the freeze–thaw cycle was 15 times. High water content could promote the effects of freeze–thaw cycles on soil permeability and pore size distribution, while the increase of dry density could inhibit these effects. The results of this study provide a theoretical basis for the remediation of saline–alkali soil in the flooded area of Southwest Shandong.

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Structural characteristics of local saline-alkali soil in the Yellow River Delta were studied by microscopic test methods of liquid nitrogen vacuum freeze-drying machine, fully automatic mercury intrusion porosimetry, X-ray diffractometer, and high- and low-vacuum scanning electron microscope. Permeability of the saline-alkali soil belongs to two grades of micropermeable water and extremely micropermeable water. Average volume ratio of pores with diameters no more than 2 μm is 86.25%; therefore, the saline-alkali soil may mainly consist of micropores and ultramicropores. Most void ratios of the soil are not beyond 0.5, and its dry densities are all greater than 1.6 g/cm3. Because average proportion of the clay minerals is only 12.24%, they are obviously not the main reason for poor permeability of the local saline-alkali soil. Based on the structural characteristics of compact structure and slightly developed fracture, mechanisms of surface runoff, and water-salt migration of the local saline-alkali soil, a salt-discharging engineering model mainly with surface runoff was established considering auxiliary infiltration and without interflow. Salt content distribution of the local saline-alkali soil is studied experimentally, by which relationship between salt content and conductivity has been fitted as y = 2.74x. The relationships between depth and salt content in the saline-alkali soil region present that the depth of salt-discharging engineering as open ditch should be beyond 60 cm. From the relationships between precipitation and salt content, the effectiveness of engineering measure shown in the salt-discharging model has been verified immediately or indirectly, and the engineering salt-discharging model may be suitable for managing saline-alkali soil in the Yellow River Delta.