| Benggang,as the fragmented erosional landform,is the dominant threat to ecosystem and social services in tropical and subtropical China.This erosional landform mainly distributes in granite weathering mantles.The special properties of granite weathered soils,including wide range of soil particles,high content of coarse grains,and loose structure,resulted in a particular erosion landform.The red soil layers with high content of clay particles and strong erosion resistance are critical to initiation and development of the Benggang.Collapsing wall as one of the important components of the Benggang is suffering from severe wetting-drying cycles.The soil particles and pores are broken and rearranged in the microstructure,and the changes in the rock and soil structure directly affect the evolution of the collapsed hills.The systematic study of the pore structure of granite red soil under the action of wetting-drying cycles is the key to the study of changes in soil hydraulic behavior,and it is also the basis for revealing soil deformation and instability.Therefore,this study took the red soil developed by the granite in the collapsed hill area as the test object,and analyzed the pore structure and distribution characteristics of the granite red soil at different distances from the collapse wall through CT scanning and the determination of the soil-water characteristic curve;combined with the Hydrus-1D software,simulation The infiltration process of granite red soil under the action of wetting-drying cycles;based on indoor simulation experiments,the influence of the frequency and intensity of wetting-drying cycles on soil pore structure is further analyzed.The research results are of great significance for revealing the erosion mechanism of gully collapse.The main findings are as follows:(1)Under natural rainfall-evaporation conditions,the soil mass moisture content of the first day of moisture monitoring is used as the reference point.During the monitoring period,the soil mass moisture content of the collapsed wall varies from-1.8%to 2.9%,and the distance from the collapsed wall is 0.5%.The variation range at m is-0.9%～1.7%,the variation range at 1m is-0.4%～0.8%,the variation range at2m is-0.3%～0.3%,the variation range at 2.5m is-0.1%～0.2%,2m After the water content change tends to be stable.CT scan showed that the pore content of>1000μm in the granite red soil decreased with the increase of the distance from the collapse wall,and the content of pores>1000μm at 0m,0.5m,1m,1.5m and 2m from the collapse wall were 12.87%and 8.19%,respectively.,4.77%,4.49%and 3.55%,and the rounding rate of pores increased from 0.64 to 0.84,indicating that the soil macropores are transforming into small pores,and the regular pores are increasing.In addition,the saturated soil moisture content at 0m of the collapsed wall is the lowest at 36%.With the increase of soil matrix suction,the moisture content gradually decreases.The analysis of the granite red loam soil-water characteristic curves at different distances from the collapse wall shows that at 0m,0.5m,1m,1.5m,and 2m away from the collapse wall,the equivalent pore content of>57μm is 25.13%,19.61%,and 14.86%,respectively.,15.12%and 12.60%;and the equivalent pore content of less than 0.2μm is 14.93%,27.57%,27.06%,27.35%and 32.29%,respectively.There is a certain difference between the soil-water characteristic curve and the measurement results of CT scanning technology,but the change trend is the same,and there is a linear correlation between the measurement results of the two(R~2=0.91).(2)Under the conditions of average rainfall and evaporation for many years in the collapsed hillock area,the mass water content of granite red soil varies from15.02%to 42.33%,with dramatic changes in late spring,summer and early autumn.In the process of natural rainfall,the initial infiltration rate of the soil changes drastically,the infiltration rate gradually decreases from 0.15cm/day,and the infiltration rate tends to be stable in the middle of each season,fluctuates up and down at 0.003cm/day,and enters the stable infiltration stage.Summer is hot and rainy,the wetting-drying cycles is intense,and the soil infiltration rate is faster.The cumulative infiltration amount of soil is significantly higher than that of winter with low temperature and low rainfall,indicating that the change in the intensity and frequency of the wetting-drying cycles has a greater impact on soil water infiltration characteristics.(3)With the increase in the number of wetting-drying cycles,the hysteresis phenomenon weakens,and the area of the hysteresis loop is the smallest when the wetting-drying cycles reach 10 times,indicating that within ten times,the influence of hydraulic paths on the soil water holding characteristics increases with the wetting-drying cycles.The increase in frequency decreases;the water holding capacity of the soil decreases with the increase in the intensity and frequency of the wetting-drying cycles,and gradually stabilizes after 5 times of the wetting-drying cycles.When the wetting-drying cycles are 1 and 10 times,the range of mass moisture content in the process of soil matrix suction decreasing from 1500k Pa to0k Pa is 44.9%～13.9%and 38.9%～7.8%,respectively;with the frequency of wetting-drying cycles As the intensity increases,the air intake value and residual water content also show a gradual decrease.The saturated hydraulic conductivity of the soil is the smallest when it has not been treated by the wet-dry cycle,which is only0.00136cm/min.With the increase in the frequency of wetting-drying cycles,the saturated hydraulic conductivity under the three intensities showed a gradually increasing trend,but the degree of increase was different.The high intensity(saturated～10%)increased the fastest,with a growth rate of 6.970%.As the intensity of the wetting-drying cycles increases,the soil water holding capacity gradually decreases.For saturated water content,a,n and saturated hydraulic conductivity,the influence of the intensity of the wetting-drying cycles on the soil water holding characteristics is higher than the frequency of the wetting-drying cycles(p<0.05).(4)Under high-strength(saturated～10%)and medium-strength(saturated～20%)wetting-drying cycles,the frequency of wetting-drying cycles mainly affects the pore content of>57μm,3～10μm,and<0.2μm,with the largest percentage of pores and the change is the most dramatic.With the increase in the frequency of wetting-drying cycles,the pore content of>57μm increases first,and then decreases after seven times of wetting-drying cycles.The range of changes is 34.66%-48.87%and31.05%-38.95%;Under low-intensity(saturated～30%)wetting-drying cycles,the frequency of wetting-drying cycles mainly affects the pore content of>57μm,10-15μm,and<0.2μm,with the largest proportion of pores and the largest variation.The content of>57μm pores first decreases with the increase of the frequency of wetting-drying cycles,and gradually increases after the two wetting-drying cycles,with a range of 22.65%to 34.69%.Large pores(>30μm)are the main channel for soil infiltration and drainage.Under high-strength and medium-strength conditions,it increases with the increase in the frequency of wetting-drying cycles,and decreases after seven times,while low-strength shows a trend of first decreasing and then increasing.When the frequency of wetting-drying cycles is constant,the strength of wetting-drying cycles mainly affects the pore content of>57μm,10～15μm,3～10μm and<0.2μm,with the largest proportion of pores and the most dramatic changes,of which>57μm and 10～15μm pore content All of them decrease with the increase of the wetting-drying cycles strength,and the pore content of<0.2μm increases firstly and then decreases with the increase of the wetting-drying cycles strength.The content of macropores(>30μm)decreases with the decrease of the wetting-drying cycles strength,and the variation ranges are 22.65%～44.95%,26.00～47.55%,31.27%～48.87%and 34.68%～48.17%,respectively.The influence of cyclic intensity was significantly greater than frequency(p<0.05).The contribution rate of the wetting-drying cycles strength to the macropores is 64.1%,which is greater than the 19.4%contribution rate of the wetting-drying cycles frequency to the macropores.In addition,under the action of high-intensity wetting-drying cycles,soil cracks are formed through two wetting-drying cycles,while under low-intensity conditions,five wetting-drying cycles form cracks.The degree of development of soil fissures increases with the increase in the intensity and frequency of wetting-drying cycles. |
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