| Water erosion is one of the most serious soil degradation processes.Splash erosion is the initial stage of water erosion,which is caused by the destructive force of raindrops on soil surface aggregates.In addition to rainfall intensity,soil properties and surface conditions are essential for understanding splash erosion.However,at present,there are few studies on the influence of soil water content and gravel factors on splash erosion,especially in the area of granite residual soil in southern Jiangxi where water erosion is frequent in China.In this paper,granite residual soil is taken as the research object.Through indoor rainfall splash erosion test and orthogonal test,the effects of rainfall intensity,gravel content,gravel particle size and early soil water content on soil splash erosion process are studied.At the same time,the surface treatment of granite residual soil was carried out by spraying method and splash erosion test was carried out.Combined with scanning electron microscopy observation technology,the improvement effect of microbially induced calcium carbonate precipitation(MICP)surface solidification treatment on the splash erosion resistance of granite residual soil was studied,and its microscopic mechanism was discussed.The main conclusions are as follows:(1)Through the splash erosion test under different rainfall intensities,it is concluded that the splash erosion rate is positively correlated with rainfall intensity.The relationship between splash erosion and rainfall intensity is exponential function,and the relationship between splash erosion and rainfall duration is linear function.For the splash erosion particles generated by one rainfall and the splash erosion particles during rainfall,the particle size sorting composition is the highest content of silt(0.002～0.02mm)and the lowest content of coarse sand(0.2～2mm).The content of clay(<0.002mm)and silt(0.002～0.02mm)decreased first and then increased with the increase of rainfall intensity,and reached the lowest when the rainfall intensity was120mm/h,while the content of fine sand(0.02～0.2mm)and coarse sand(0.2～2mm)showed the opposite trend.The splash erosion process is a process in which rainfall preferentially separates <0.02 mm particle size particles.The sorting of the rainfall splash erosion process first increases and then fluctuates steadily.With the increase of rainfall intensity,it first decreases and then increases.When the rainfall intensity is120mm/h,the sorting is the weakest;the fragmentation degree of soil particles increased first and then fluctuated steadily with the increase of rainfall duration,and decreased first and then increased with the increase of rainfall intensity.The relationship between each index of splash erosion particle size sorting and rainfall intensity can be described by parabolic equation.(2)Through the splash erosion test under different gravel content and gravel particle size,it is concluded that the smaller the gravel particle size,the more obvious the accumulation of gravel on the surface of the soil layer near the splash wall;the splash erosion rate is negatively correlated with gravel particle size and positively correlated with gravel content.There is a linear function relationship between splash erosion and gravel content.When the gravel particle size is 2～3mm,splash erosion is positively correlated with gravel content.When the gravel particle size is 3～5mm and5～10mm,splash erosion is negatively correlated with gravel content.The raindrop splashing force can not transport 5～10 mm gravel particles;the larger the gravel content or the larger the gravel particle size,the earlier the particle content of each particle size separated by splash erosion reaches the stable state of fluctuation.The splash erosion process is the process of preferential separation of <0.02 mm particles by raindrops.The sorting of splash erosion process first increased,then decreased and then fluctuated steadily with the extension of rainfall duration.The sorting was negatively correlated with gravel content and gravel particle size.The fragmentation degree of soil particles increased first and then fluctuated steadily with the increase of rainfall duration,and decreased with the increase of gravel content and gravel particle size.The relationship between gravel content and particle content and particle enrichment rate of each particle size can be described by power function equation,and the relationship between gravel content and average weight diameter and fractal dimension can be described by linear equation.(3)Through the splash erosion test under different early water content,it is concluded that the higher the early water content,the more uneven the surface of the splashed soil sample,the faster the water accumulation and the thicker the water layer;the splash erosion rate was positively correlated with the early water content,and the splash erosion amount was power function with the early water content.The content of silt(0.002～0.02mm)is the highest in splash erosion sorting,and the content of coarse sand(0.2～2mm)is the least.Under the water content of 3%～10% in the early stage,the content of 0.02 mm particle size in splash erosion sorting showed an opposite trend with the change of rainfall duration and water content in the early stage.The fragmentation degree of soil particles decreased first,then increased and then fluctuated steadily with the increase of rainfall duration,and decreased with the increase of early water content.The sorting of splash erosion process decreases first,then increases and then fluctuates steadily with the extension of rainfall duration.Under the condition of12%～25% water content in the early stage,the change trend of two kinds of particle content and crushing degree is opposite;the sorting of splash erosion process increased first and then fluctuated steadily with the extension of rainfall duration,and decreased first and then increased with the increase of early water content,and the sorting was the weakest when the early water content was12%.In addition,the relationship between the indexes of splash particle size sorting and the early water content can be described by parabolic equation.(4)Based on the orthogonal experiment method,the indoor rainfall splash erosion test was used to study the effects of rainfall intensity,pre-water content,gravel content and gravel particle size on soil splash erosion.The results showed that for the amount of splash erosion,the order of the four factors was rainfall intensity > pre-water content>gravel content>gravel particle size;for the content of clay(<0.002mm)and fine sand(0.02～0.2mm)particles,the average weight diameter of splash sorting,the enrichment rate of clay(<0.002mm)and coarse sand(0.2～2mm)particles and the fractal dimension,the primary and secondary order is rainfall intensity>gravel content>pre-water content>gravel particle size.However,the order of particle content of silt(0.002～0.02mm)and coarse sand(0.2～2mm),particle enrichment rate of silt(0.002～0.02mm)and fine sand(0.02～0.2mm)in splash erosion sorting is rainfall intensity>gravel content>gravel particle size>early water content.(5)The indoor rainfall splash erosion test was carried out after the soil samples were reinforced by microbial solidification technology.The results showed that microorganisms had a repairing effect on soil cracking,and the surface of the treated soil sample had a hard ’ protective layer ’.The surface of the soil sample began to damage from the crack repair site;at each bacterial concentration,the surface damage of soil samples treated with OD600=0.75 was the smallest.The more times of reinforcement,the smaller the degree of surface damage of soil samples;the splash erosion rate of the control group was the highest during the rainfall process.The splash erosion rate decreased first and then increased with the increase of bacterial concentration,and decreased with the increase of reinforcement times.The splash erosion amount and total splash erosion rate decreased first and then increased with the increase of bacterial concentration,and decreased with the increase of reinforcement times.The splash erosion amount has a power function relationship with the bacterial concentration and a linear function relationship with the number of reinforcements.Under the treatment of microbial solidification,the content of silt(0.002～0.02mm)was the highest in the splash separation,and the content of coarse sand(0.2～2mm)was the least.The time for the content of each particle size to reach a stable state of fluctuation in the splash erosion separation of microbial solidified soil samples is earlier than that of the control group.Microbial solidification will reduce the degree of fragmentation of soil particles and aggregates.The degree of fragmentation of soil particles increases first and then decreases with the increase of bacterial concentration,and increases with the increase of reinforcement times.The sorting of splash erosion process increased first and then fluctuated steadily with the extension of rainfall duration.The relationship between each splash sorting index and bacterial concentration can be described by parabolic equation,and the relationship with the number of reinforcement can be described by power function.The surface of the soil sample treated by microbial solidification has a hard shell layer formed by calcium carbonate cementing soil particles,and a large amount of calcium is distributed at the formation of the hard shell layer.The microbial solidification method can effectively improve the splash erosion resistance of granite residual soil. |
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