Research On Effect Of Grain-size Composition And Cracking State On Collapsing Gully Erosion

Granite residual soil is widely distributed in South China.The region belong to subtropical climate,the corresponding soil with characteristics of loose,large quantity in fractures,easy-disintegration and poor mechanical properties,these characteristics lead to considerable residual erosion,serious soil and water loss and the quality reduction of land in this area.In the process of collapsing erosion development,the effect of crack and gradation of soil on the mechanism of collapsing erosion needs further study.This study takes the granite residual soil sampled from collapse erosion area of Yudu County as the research object.Two soil structure models,namely,different grain composition and shallow fractures were selected.The impact of soil structure on the erosion mechanism of collapse is studied via physical rainfall test,and the research conclusions are drawn as follows.(1)Taking the effect of grain composition on soil desiccation crack into account,the soil infiltration with different grain composition and surface crack are deeply studied through on the laboratory geotechnical test.The results show that residual granite soils with higher content of gravel own the slower cracks evolution,smaller crack density,width and depth,while has better permeability and higher stable infiltration rate.By comparison,crack have a greater impact on soil permeability,When the area ratio of cracking reaches 6.59%,the initial infiltration rate increases by 2 orders of magnitude compared with the stable infiltration rate.The crack has little effect on the stable infiltration rate,which is basically the same due to the healing of cracks.(2)Based on many literatures and the field investigation data,granite residual soils in this region are found great difference in grain composition and large numbers of fractures with different forms occur,and these factors influence the development of collapsing erosion.According to the characteristics of collapse erosion distribution and regional climate,the rainfall intensity and slope angle were determined to be 120 mmh-1 and 20°,respectively.Accordingly,the corresponding rainfall experiments was designed.(3)The rainfall erosion process of slope with different grain composition is analyzed.The results showed that the thin(shallow)gully erosion develop more slowly and lasted for a relatively longer time for the granite residual soils with high content of gravel,the complete gully erosion process is slowly while the final slope erosion is heavier.Meanwhile,the moisture infiltration process is more complex,and the moisture content of deeper soil layers rises slowly,collapse erosion is thus not easy to happen.(4)According to three groups of tests with slope fracture rates of 0,5.23%and 11.7%,the existence of shallow cracks accelerate the development of collapse erosion induced by rainfall.The thin(shallow)gully erosion develops faster and the soil moisture content is higher for slope with cracks,and the entrance into final gully erosion is shorter,which makes it easier to collapse for granite residual soil slope.(5)The physical rainfalls experiments under low temperature are performed,in the process of coupling of moisture and heat migration,the soil moisture content and temperature real-time data are monitored and recorded,and the relevant phenomenon rules are obtained via analysis,The results show that there is a certain correlation between the moisture and temperature,and preferential flow can be effectively tracked by the moisture and temperature sensors,especially for the more sensitive temperature sensors.(6)The hydrologic process of the slope with cracks under rainfall was analyzed and the results are compared with that of numerical simulation,and the calculation results are in good agreement with the measured values.Therefore,the HYDRUS model can well simulate the coupling of moisture and heat transport for soil slopes with or without cracks under rainfall boundary conditions.The results show that the same rule is obtained that the temperature field of matrix soils obviously fall behind that of seepage field,and their distribution characteristics are similar.However,the temperature and moisture of cracks-soil change more rapidly,and the change characteristics of the temperature field and the seepage field are basically the same,and they are correlated.The preferential flow in cracks-soil can be thus well monitored via temperature data.