| Soil erosion in granitic soil areas is the major reason to generate land degradation in South China.Granitic soil profiles with various weathering degrees possess significant anisotropic characteristics,which leads to the unique erosion landscape,namely benggang.Mechanism for the formation and development of benggang can be hardly explored by traditional soil erosion.Few study has been conducted to investigate soil erosion by using the knowledge of Rock-Soil mechanics.The stability of granitic soils has a great significance in soil erosion prediction,risk assessment and erosion conservation in granitic weathering areas.A typical weathering granitic soil profile in Hubei province of South China was selected in this study.The spatial heterogeneity of physiochemical and mechanical properties of granitic soil profile was investigated by determining the soil structure parameters,soil water retention characteristics,deformation and strength for soils from different weathering zones.According to the obtained experimental results,numerical simulation based on finite difference and limit equilibrium method was conducted to analyze the unsaturated seepage and slope stability.The major developments were shown as follows:(1)The major structure parameters and soil water retention characteristics of granitic weatering soils.Kaolinite was the predominant clay mineral of granitic weathering mass;the shape of soil particles were dominated by platy shaped grains;the multifractality of particle size distribution reflected the distinct heterogeneity for one of the structural indexes;soil particle arrangement and structure characteristics showed a significant variability for different weathering zones along the profile;soil bulk density and organic matter content ranged from 1.09~2.2 g/cm~3 and 0.24~36.67 g/kg respectively;the free sesquioxides varied in a range of 4~24 and 1~7 g/kg for Fe2O3 and Al2O3,respectively;the plastic and liquid limit was in a range of 22~30% and 28~52% respectively;clay particle concentration,soil orgain matter and free sesquioxides generally showed a decreasing trend with the increased profile depth;soil porosity was concentrated in a range of 0.01~0.1 μm in the subsurface red soil layers which was smaller than that in the surface layer with its macropores influenced by the bioactivities,while the coarse particle in the sandy and clastic layers generated the higher concentration of macroporosity.The empirical model proposed by Fredlund-Xing with its parameters related significantly with soil particle size distribution and plastic-liquid limit(r>0.98,p<0.01;r>0.8,p<0.05)fitted well with soil water characteristic curves for granitic soils(R2>0.99,SE<0.00,RSS<0.02);the porosity distribution calculated by SWCC method showed a less heterogeneity in comparsion with the mercury intrusion method.(2)The dehydration shrinakge characteristics of granitic granitic weatering soils.Significant soil deformation occurred in the red soil layers with the highest volumetric deformation around 0.2 cm~3/cm~3,while a slight shrinkage capacity occurred for soils in the clastic layers with volumetric deformation less than 0.05 cm~3/ cm~3;proportional shrinkage process playing a dominant role(>52%)during the dehydration,;the anisotropic shrinkage of granitic soils in different weathering layers varied with water content;the volumetric deformation was dominated by radial shrinkage for soils in the surface and sandy layers with geometric factor γ in a range of 1.2~2.2,while axial shrinkage predominated the shrinkage for soils in the subsurface red soil and clastic layers with γ generally greater than 3.0;the shrinkage of granitic soils was mainly influenced by silt contnt,and the variation of shrinkage capacity was contributed by soil porosity(p<0.05).(3)Saturated and unsaturated soil shear strength of granitic weathering soils.There was a great difference in unsaturated shear strength of granitic soils from different weathering zones.Stress softening tendency occurred for soils in the red soil and sandy soil layers with the decreasing water content,and stress hardening for soils in the clastic layer.Soils with cohesive texture showed a greater shear strength especially at a lower water content level.There was a significant nonlinear relationship between shear strength and matric suction of granitic weathering soils which could be described by the unsaturated shear strength equation proposed by Lee et al.(2005).Matric suction and normal stress made contribution on unsaturated shear strength dependently,especially under high matric suction level.Matric suction ranging from100 ~ 1000 k Pa made a great contribution on unsaturated shear strength for soils in red soil layers,but less influence occurred when matric suction beyond this range.The unsaturated shear strength of soils in sandy and clastic layers was lightly influenced by matric suction,particular for soils in clastic layer with shear strength varied in a range of 45~75 k Pa.The unsaturated shear strength of granitic weathering soils showed an apparent linear relationship between saturation degree related parameter(R2>0.6,RMSE<28,AICc >-23),except for soils in the clastic layer.Shear strength and its parameters were relatively larger at soil water content in a range of 14~7%;the distinict anisotropic shear strength occurred at red soil layer with soil water content around 21%.The cohesion and internal friction angle along the weathering profile varied in a range of 0~120 k Pa and 22~52°,respectively,and soil layers,direction and water content made great influences on cohesion(F=9.48,p<0.01),but only water content influenced the internal friction angle significantly(F=17.78,p<0.001).The stress-strain characteristics of granitic weathering soils could be modeled by Duncan-Chang soil constituted model which was also used for calculating the elastic modules E.The value of E vaired in a range of 46~172、66~338 and 61~163 MPa,for soils in the red soil,sandy and clastic soil layers respectively.The saturated cohesion obtained by triaxial test was generally lager than that measured by direct shear test,but was opposite for the saturated internal fraction angel.(4)Unsatured seepage and stability analysis of granitic weathering soils.Soil water was easily accumulated in sandy and clastic layers within the bottom areas of soil profile,which highly consistent with the area where the shear strain increment varied dramatically.The overall stability of granitic soil profile was influenced by the slope,and 45° acted as the critical value under limited equilibrium condition.Shear failure was the predominant soil deformation in granitic soil profile,and an obvious tensile failure occurred in surface and red soil layers.With the increasing of slope angle,the angle of plastic failure surface increased.The movement of potential slope failure was controlled by the slope angle,and slid failure played a dominant role for slope with angle less than 45°,while collapse was the major failure type for soils with slope lager than 65°,which might be the reason why the bottom areas of granitic soil profile were vulnerable to be influenced by erosion. |
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