| According to relevant statistics,the area of saline-alkali soil in China accounts for about 10%of the total land area,especially in the areas formerly flooded by Yellow River of Shandong Province,where soil salinization seriously restricts the agricultural production and development.Soil degradation in this area is serious,with typical characteristics of dense structure,high bulk density and low saturated water conductivity.In addition,the effects of freezing and thawing on soil structure and chemical properties become more frequent and intense,which further affects the efficiency of soil salt removal in frozen regions.Based on these,it’s a great guiding significance to study the basic physical and chemical properties of saline-alkali soil and analyze the influence of freezing-thawing cycle on the hydraulic properties and micro structure of soil in the areas formerly flooded by Yellow River of Shandong Province.In this study,the typical saline-alkaline soil in the areas formerly flooded by Yellow River of Shandong Province was used as the test material.First,the basic physical and chemical properties,particle size distribution and mineral composition of the undisturbed salinealkaline soil were analyzed.Secondly,different water content（13%,16%,19%）and dry density（1.48 g·cm-3,1.53 g·cm-3,1.58 g·cm-3）and the number of freezing-thawing cycles（0,1,5,10,15,20）were studied through laboratory tests.Finally,adopting BP neural network to establish the prediction model of saline-alkali soil permeability.Through the observation of the test process and analysis of the test data,the main results of the study are as follows:（1）The saline-alkali soil in the southwest of Shandong Province is silty clay with a saturated water conductivity of 1.48×10-5cm/s,which belongs to the weak permeable soil,permeability is very poor.Soil Cu=7.14>5,Cc=0.33<1,the soil particle size difference is large,uneven distribution.The particles are mainly small（10）,the soil samples contained more ultra-micro holes（<0.05 μm）and micro holes（0.05-2 μm）.However,the macro holes（≥20 μm）were less.As the freezing-thawing cycle,the peak value of soil pore size distribution curve shifted to the right and increased.After 10 freezing-thawing cycles,the curve presented a bimodal distribution,and the pore size distribution of the second peak was larger than 10 μm,which belonged to the pore size range,and freeze and thaw cycles 15 times had a more severe effect on the pore size distribution.High moisture content can promote the freeze-thaw cycle,while the increase of high dry density can inhibit the effect.With the increase of water content,the dominant pore size changed from 0.05-2 μm micropore to 2-10 μm micropore gradually,and the content increased,and then the porosity increased.With the increase of dry density,the distribution range of the dominant pore size was almost not affected by dry density,but the content of the dominant pore size decreased and porosity decreased with the increase of dry density.（5）By scanning electron microscopy,it is found that the number of freezing-thawing cycles increases the diameter of intergranular pores and broadens the contact form between particles.The original single face-surface contact changes into the coexistence of face-surface contact,point-surface contact and point-point contact.Higher water content can make the soil particles fragmented,produce "slice" structure so that can promote the development of soil pores.In addition,with the dry density increases,soil particles will develop a "plate" structure,which results in a denser soil micro structure.（6）Based on BP neural network,the prediction model of saline-alkali soil permeability in the southwest of Shandong Province was established.The prediction model can be determined that the prediction effect of the neural network prediction model is good.This model can be used to predict the saturated water conductivity of saline-alkali soil in this area. |
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