Study On The Effects Of Freeze-thaw Cycle And Soil Texture On Soil Detachment Capacity

Soil detachment is the initial process of soil erosion,and its development process is affected by the uncoordinated mechanism between shear strength and flow strength of soil.Although many achievements have been made,its mechanical mechanism is still unclear and less involved in seasonal freezing and thawing.Therefore,this study takes the typical black soil in Northeast China as the research object,combines freeze-thaw simulation,hydraulic scouring and peeling with soil mechanical properties testing to analyze the coupling relationship between soil detachment capacity and flow intensity and soil texture,and constructs a prediction model of soil detachment capacity under non-freeze-thaw conditions.On this basis,the influence mechanism of freeze-thaw on soil mechanical properties was explored,and a comprehensive prediction model of soil detachment capacity under freeze-thaw conditions was established based on soil mechanical characteristics parameters.The research results can provide theoretical basis for prevention and control of regional water and soil loss and sediment disaster.The main conclusions are as follows:（1）The deterioration mechanism of soil mechanical properties with freeze-thaw conditions was explored.The shear strength index of soil deteriorates to some extent with the increase of freeze-thaw cycles and initial water content.The number of cycles for effective deterioration of shear strength index during freeze-thaw cycles is 5,and within the five cycles the friction angle in soil decreases as a whole but not significantly,with an average decrease of 7.2%.However,soil cohesion decreased significantly within 5 cycles,with an average decrease of 37.4%.For low water content conditions of 15%～25%,the deterioration effect of freeze-thaw is small,while for high water content conditions of 30%～40%,the deterioration effect of freeze-thaw is obvious.The deterioration effect of freeze-thaw on soil cohesion was characterized by relative cohesion parameters.A two-parameter mathematical model of the relative cohesion was established,in which the number of freeze-thaw cycles and the initial moisture content were variable（R²=0.850）.（2）A dimensionless prediction model for soil detachment capacity under non-freeze-thaw conditions was established.The coupling relationship between each flow intensity and soil separation capacity was compared and analyzed,and the optimum flow intensity parameter with effective stream power as soil detachment capacity was proposed.Physical properties such as soil water content,median particle size and clay content have significant effects on soil detachment capacity.For black soil,soil water content increases from 15%to 40%,while the corresponding mean value of soil detachment capacity decreases from 0.35 to 0.27 kg s^-1m^-2.For different types of soil,the lower the clay content,the larger the median particle size,and the greater the soil detachment capacity.Based on the elimination of dimension influence by mean particle size,a three-parameter dimensionless comprehensive model with flow intensity,soil clay and water content as variables was established to predict the detachment capacity of different types of soils.The comparative analysis results show that the dimensionless effective stream power comprehensive model achieves the best prediction effect（R²=0.923;NSE=0.921）.（3）The effects of freeze-thaw conditions on soil detachment capacity were explored and a comprehensive model of soil detachment capacity based on soil mechanical properties was established.Freezing-thawing increases soil detachment capacity and coefficient of erodibility and decreases critical flow shear stress.The threshold number of effective effects of freeze-thaw cycles on soil detachment capacity is 5-7 times.The degree of reduction of soil detachment capacity with different water content is different after freezing-thawing.The corresponding decrease of 15%～25%water content is 5.5%～15.9%,while that of 30%～40%water content is 48.0%～80.0%.Water erosion characteristics of soil under freeze-thaw conditions have significant correlation with shear strength index,especially with cohesion.Based on the two-parameter comprehensive prediction model of soil detachment capacity established in this paper,which takes flow intensity and soil cohesion as variables,and further considers the influence of characterization and quantification of freeze-thaw conditions,a three-parameter comprehensive prediction model with the number of freeze-thaw cycles,initial water content and flow intensity as variables is established,and the prediction effect is verified by comparison.The comprehensive prediction model of soil detachment capacity with the best available stream power is obtained（R²=0.935;NSE=0.931）.