Effects Of Wet-Dry Cycles On Erosion Resistance Characteristics Of Different Soil Layers Of Collapse Wall In South Hubei

Collapsing gully is a special soil erosion phenomenon occurring in the granite red soil area of tropical and subtropical zone in southern China.Alternation of rain and drought in disaster-prone areas is frequently,so soil moisture characteristics undergo continuous alternate changes of dryness and wetness.The collapse wall of the collapsing gully is the most active part in the whole process of the evolution of the collapsing gully,and the change of water content in soil of collapse wall is the most dramatic.Therefore,taking the collapse wall of a typical collapsing gully in south of Hubei Province as the research object,this paper collects undisturbed soil samples of A layer?eluvial horizon?,BC layer?transition layer of illuvial horizon and parent material horizon?,C₁ layer?parent material horizon₁?and C₂ layer?parent material horizon₂?of collapse wall,changes its structural characteristics and water and mechanical properties by wet-dry cycle treatments.Considering the typical process of collapsing gully formation,this research conducts three typical erosion tests which are disintegration,splashing and scouring.This article explores the differences of soil structures,water characteristics and mechanical properties of soils as well as their effects on soil erosion resistances in different conditions of different times of wet-dry cycle treatments,which can provide quantitative support and theoretical basis for the comprehensive discussion of the effects of wet-dry cycle treatments on the instability of collapse wall and the process of collapse erosion,and provide scientific basis for the government of collapsing gully erosion.The results are as follows:?1?With the deepening of the profile layer,the soil texture gradually becomes thicker,and soil texture of eluvial horizon is clay soil,transition layer is clay loam soil,parent material horizon₁ and parent material horizon₂ are sandy loam soil;The bulk density of testing soils vary from 1.16 g/cm³ to 1.36 g/cm³,and the bulk densities of different layers of soils show as transition layer>parent material horizon₁>surface soil layer>parent material horizon₂;The clay mineral type of granite weathered soil are mainly kaolinite and illite;The specific gravity of testing soils are mainly in the range of2.54 to 2.57;Water content of liquid limit and plastic limit are both the largest in soil of eluvial horizon and the smallest in soil of parent material horizon₂,while in transition layer,natural water content is the highest;Soil organic matter contents are within2.33-18.55g/kg,and decreases with the deepening of the layer;SiO ₂?60.46-64.93%?and Al₂O₃?25.73-28.54%?are the main oxides in soil,followed by Fe₂O₃,K₂O,CaO,MgO,TiO₂ and so on.?2?In terms of structural characteristics,the proportion of active pore presents the rule of sand soil layer>parent material horizon₁>eluvial horizon>transition layer;With the increase of times of wet-dry cycle treatments,the soil particles in each layer rearrange,the structures are broken,the pore develops and a large number of new pores are produced,the proportions of active pores in soils increase,and the proportions of inactive pores decrease as follows:eluvial horizon>transition layer>parent material horizon₁>parent material horizon₂;Granite residual soil belongs to non-expansive soil with coefficients of linear extensibility lower than 0.03.Wet-dry cycle treatments increase the coefficients of linear extensibility of testing soil,which in soils of parent material horizon₁ and parent material horizon₂ can reach 0.060 and 0.091,respectively.Meanwhile,soil bulk densities decrease,and surface cracks of soils gradually develop and expand.?3?In terms of water characteristics of soils,the saturated hydraulic conductivities show the rule of parent material horizon₂?0.040 cm/min?>eluvial horizon?0.034cm/min?>parent material horizon₁?0.010 cm/min?>transition layer?0.002 cm/min?.The saturated hydraulic conductivity of testing soils increase with the treatments of wet-dry cycles.After 10 times of wet-dry cycle treatments,the saturated hydraulic conductivities show the rule of eluvial horizon?0.071 cm/min?>parent material horizon₂?0.062 cm/min?>parent material horizon₁?0.033 cm/min?>transition layer?0.025cm/min?.The water holding capacities of soils are reduced by wet-dry cycle treatments.In terms of mechanical properties,the internal friction angle of the soil of eluvial horizon is the largest?34.62 degrees?,and the internal friction angles of soils of the other three layers vary from 30.35 to 34.14,which increases with the increase of sand content;The cohesion of soil of upper layer is larger because of the high clay content,while the structure of transition layer is compact and the cohesion of the soil is the largest?14.06kPa?;The treatments of wet-dry cycles result in the trends of decreasing in shear strengths,cohesions and internal friction angles of the testing soils generally,and the attenuation range is the largest after two times of wet-dry cycle treatments.?4?The disintegration of soil of eluvial horizon is very weak,even occurs"negative disintegration".After treatments of wet-dry cycles,the disintegrations of soils of eluvial horizon are slightly improved,but the final disintegrations rate are less than 2%.The disintegration of soil of transition layer is weak,the final disintegration rate is25.31%.After treatments of wet-dry cycles,the disintegration speed and final disintegration rate are significantly increased,and can reach complete disintegration;Soils of parent material horizon₁ and parent material horizon₂ are extremely easy to reach complete disintegration,treatments of wet-dry cycles further accelerate the disintegration speed of soil samples.Soil with low clay content,high sand content,small bulk density,low cohesion,high total porosity and high proportion of active pore has a faster disintegration rate.Mechanical composition and cohesion have a significant impact on the final disintegration rate.Among the pore ratios of different equivalent pore sizes,the ratio of pores of 0.6-29?m equivalent diameter has the greatest influence on the disintegration rate and final disintegration rate,showing a significant positive correlation.?5?The logarithmic decline curve can be used to fit the variation of splash erosion stripping rate with cumulative rainfall energy.The initial peeling rate and trend of peeling rate decreasing with time in splash detachment process are both the largest in soil of sand layer and the smallest in soil of transition layer.Wet-dry cycle treatments accelerate the initial peeling rate and trend of peeling rate of soil in splash detachment process.Soil with loose structure and high content of coarse particles has weak splash detachment resistance,while the splash detachment rate of soil with loose structure,high porosity and strong permeability decreases rapidly with cumulative rainfall energy.Among the pore proportions of different equivalent pore sizes,the pore the ratio of pores of 0.6-10?m equivalent diameter has the greatest influence on the average splash detachment rate of soil.?6?In the process of scouring,the sediment concentration of runoff in soils of all layers decreases obviously with the extension of scouring time,while the scouring coefficient increases obviously.The runoff sediment concentrations increase with the depth of soil layers,and increase with the times of treatments of wet-dry cycles,and increase most significantly after two times of wet-dry cycle treatments.The change rule of scour resistance coefficient is contrary to that of runoff sediment concentration,and the scour resistance coefficient after two times of wet-dry cycle treatments decreases81.67%?eluvial horizon?,89.93%?transition layer?,29.26%?parent material horizon₁?and 54.90%?parent material horizon₂?respectively compared with that of soil samples without wet-dry cycle treatment.According to the average runoff sediment concentration in the first 25 seconds,the soil with fine particle composition,high bulk density,high cohesion,low porosity and low proportion of active pore has better scourability.The average runoff sediment concentration increases exponentially with the increase of the ratio of pores of 0.6-29?m equivalent diameter,while the scour resistance coefficient decreases exponentially.