| Purple soil is widely distributed in southwest China.It is an important soil for cultivation.Rainfall is abundant in Southwest China,and the sandy clayey purple soil is prone to disintegration and alteration,resulting in landslides,soil erosion and other disasters,which endanger agricultural production and engineering construction.Under natural conditions,most purple soils are in unsaturated state,and their properties are much different from those of saturated soils.It is of great significance to study the hydromechanical properties of unsaturated purple soil for agricultural protection and engineering safety in purple soil area.In this paper,the unsaturated hydro-mechanical properties of purple soil are studied from three aspects: soil-water characteristic curve(SWCC),shear strength and unsaturated permeability.On this basis,ABAQUS finite element is used to simulate rainfall infiltration and slope stability analysis in purple land area.The main research contents and conclusions are as follows:First,this study investigated the influence of geostatic stress on sandy clay purple soil structural evolution and water retention characteristics.Furthermore,the reliability of the generalized soil-water characteristic curve model was validated.The results suggest that:(1)The porosity ratio of the soil sample was changed under the double stress of overburden pressure and suction.In the process of desorption,the inflection point of purple soil’s porosity ratio change appeared later under large overburden pressure.While the porosity ratios were not changed significantly in the process of absorption.(2)During soil drying,the large pores were reduced.And bimodal SWCC was found at high overburden pressure(100,200 k Pa),but no multimodal phenomenon was found under low overburden pressure(0,40 k Pa).During a dry-wet cycle,the SWCC of purple soil has a significant hysteresis.(3)The dual-stress generalized SWCC model was verified with the SWCC of purple soil under different overburden pressures.Combining pore-size density function(PDF)and soil parameter S-index,the formula of the volumetric moisture content suitable for cultivation was proposed,which can be applied to the corresponding agricultural production guidance in purple land area.Secondly,in order to explore the correlation between unsaturated shear strength and soil-water characteristic curve of sandy clayey purple soil,GDS unsaturated triaxial instrument was used to conduct SWCC test of purple soil under different confining pressures(50,100,150 k Pa).The triaxial unsaturated shear strength test of purple soil with consolidation drainage under controlled suction(0,25,50,100 k Pa)was carried out by GDS unsaturated triaxial tester.The analysis results are as follows:(1)SWCC of sandy clay purple soil varies significantly in the range of low suction under different confining pressures.As the suction increases,SWCCs approach and even overlap each other.This indicates that SWCC is more sensitive to different confining pressure environments in the low suction range where macropores dominate.(2)Under a large strain,the stress-strain curve does not show the peak of the shear stress,and the strength increases slowly with the increase of the shear displacement.Under different suction,the cohesion c of sandy clayey purple soil changes greatly.The cohesion increases with the increase of matric suction.The effective internal friction Angle φ’ only changed slightly with the matric suction.φb is related to matric suction and stress states.That is φb has a non-linear relationship with matrix suction,and the extended Mohr-Coulomb envelope surface is a curved surface curved along the suction axis.(3)There is a certain deviation in the prediction of unsaturated shear strength of sandy clay purple soil by using SWCC without considering the influence of confining pressure.The prediction effect is better by using SWCC which takes stress state into account.(4)Through the finite element software ABAQUS,the slope stability analysis is carried out,and the slope stability safety factor of saturated and unsaturated sandy clay purple soil is obtained,which provides a reference for slope protection in the purple soil area.At last,one-dimensional vertical permeability test for sandy clayey purple soil column was carried out at unsaturated condition using the one-dimensional soil instantaneous permeability coefficient measurement system.The results are as follows:(1)In the early stage of rainfall,the matrix suction of the soil column is higher,and the soil-water characteristic curve at this time is closer;as the rainfall time increases,the saturation of the soil column increases,and the soil-water characteristic curve also appear different.(2)The unsaturated permeability coefficient of purple soil is calculated based on the instantaneous profile method.The logarithmic unsaturated permeability coefficient has a linear relationship with the volumetric water content.The empirical formula obtained by linear fitting can be applied to the engineering seepage analysis in the purple soil area.(3)Substituting the relationship between the water seepage coefficient obtained from the infiltration test and the saturation degree into the ABAQUS finite element simulation software,and obtaining the changes in the water content,stress distribution and displacement field of the slope under different rainfall intensities.This paper studies the hydro-mechanical characteristics of purple soil.The research provides a scientific basis for the research of agricultural irrigation engineering and the prevention and control methods of soil erosion and slope instability in purple soil area. |
