Study On Mechanical Characteristices Of High Suction Unsaturated Soil With Different Silt Content Under Dryand Wet Cycle Conditionds

The northwest area of Hebei Province is located at the foot of Yanshan Mountains.Under the effect of erosion and accumulation of rivers,a large area of diluvial fan is formed in front of the mountain,and a large amount of silt is deposited,making the local soil powder content more.Affected by local typical continental drought and semi-arid region climate,highway roadbed slope in natural state often high suction unsaturation,once the external environment such as rainfall,drying,sun,freezing and thawing,make soil soil water characteristics change,strength deterioration,easy to cause subgrade collapse and landslide disasters.So in this thesis in the northwest of Hebei province different particle content of subgrade slope soil as the research object,after the dry and wet cycle high suction state of soil-water characteristics,strength characteristics of test research,from water characteristics,shear resistance,tensile strength characteristics,dry shrinkage cracking phenomenon comprehensive analysis of the mechanical characteristics of unsaturated soil.The specific contents are as follows:(1)Soil-water characteristic curves of unsaturated soil samples with different silt content under the influence of the number of wet and dry cycles were obtained by using a tensiometer and the vapor equilibrium method of saturated soil salt solution for suction determination,and the influence of both on the water-holding characteristics of unsaturated soil was analyzed.It was found that the increase of powder content and the number of wet and dry cycles would cause the downward shift of soil-water characteristic curve and the deterioration of soil water-holding characteristics.With the increase of powder content,the percentage of particle groups within the soil changes,resulting in the increase of the maximum pore size of the soil sample and the decrease of the water holding capacity of the soil.The wet and dry cycles increase the pore space between soil particles and deteriorate the internal structure of the soil,resulting in a decrease in the water-holding capacity of the soil.(2)Triaxial compression test and side limit compression test were carried out for high suction unsaturated soil with different particle content,and a series of analysis was carried out for the stress-strain curve and compression curve of unsaturated soil under the effect of high suction.About the strength characteristics of unsaturated soil,the study found that high suction unsaturated soil are brittle failure,with the increase of particle content,high suction unsaturated soil peak strength and residual strength reduction,brittle characteristics,through the cohesion,elastic modulus,peak cutting modulus and quantitative analysis of the above conclusion.For the compression deformation characteristics of unsaturated soil,the study found that the high suction state,suction of the unsaturated soil compression curve less,powder content on the unsaturated soil compression curve,with the pore ratio increases particle content,the growth rate approximate exponential,from the perspective of compression coefficient,that the more particle content,the greater the soil sample compressibility.Change of particle content,introduce fractal dimension,summarizes the strength of unsaturated soil and deformation of variation,the peak strength of unsaturated soil in the fractal dimension between 2.72～2.81 affected by fractal dimension changes significantly,beyond this range,the peak intensity of fractal dimension change sensitivity,the compression of unsaturated soil and fractal dimension meet the exponential relationship,the greater the fractal dimension,the smaller the soil compressibility.Finally,the grey GM(1,1)prediction model was used to predict the shear strength,residual strength and brittle index of high suction unsaturated soil with different particle content,which verified the feasibility of the grey GM(1,1)prediction model in the study of the mechanical characteristics of high suction unsaturated soil.(3)Carry out direct shear test and splitting method tensile test of different silt content under the action of dry and wet cycle.Analysis of dry and wet cycle of high suction unsaturated soil shear displacement-shear stress curve,shear strength and its indices,the decrease in shear strength in wet and dry cycles was found to level off with the increase in the number of wet and dry cycles,and shear strength deterioration by dry and wet cycles increases with increasing silt content.The tensile strength of unsaturated soil was measured indirectly by splitting method,and the dry and moisture circulation degraded the tensile strength of unsaturated soil.To establish the uniform formula of tensile and shear strength of high suction unsaturated soil,the coulomb strength formula is tried to reflect the law of tensile strength,which has a good effect.On this basis,the prediction formula of high suction unsaturated soil with different particle content under the action of dry and wet cycle is established,and the prediction effect is good.(4)The crack expansion depth caused by the powder content,dry and wet cycle times and the influence of suction is calculated,and the order of the influence degree of the crack expansion depth is determined through gray correlation analysis,that is,the particle content of dry and wet cycle number> suction.