Mechanical Behavior And Engineering Application Of Unsaturated Loess-structure Interface Based On Freeze-thaw Action

In the seasonal frozen soil area,periodic freeze-thaw action has continually changed micro-structure and physical-mechanical properties of shallow unsaturated soil,and it has directly affected the interaction between soil and structure.The stress-strain relation and strength characteristics of the interface between soil and structure are the bases and keys to determine the bearing capacity and security of foundation engineering,and to analyze the interaction between structure and soil in seasonal frozen soil regions.In order to serve the actual engineering better and analyze the influence mechanism of freeze-thaw effect on mechanical behavior in the interface between unsaturated soil and structure.The paper study the basic mechanical behaviors in the interface between unsaturated loess and concrete and interface between unsaturated loess and steel under fore-and-aft freezing and different freeze-thaw cycles by indoor direct shear test,respectively.Then,the engineering application of mechanical behavior in the interface between unsaturated loess and structure is studied based on the freeze-thaw action,the model of the retaining wall is established and numerical calculation is carried out by ABAQUS,the finite element software.The main research results are as follows:(1)Based on the direct shear test on the unfreeze-thaw samples,we find that the friction angle and cohesion in the interface show a different variation with the increase of water content,the friction angle in the interface reaches the minimum while the cohesion in the interface reaches the maximum when the water content of samples reaches the marginal moisture.When the water content does not exceed the marginal moisture,the friction angle in the interface increases firstly and then decreases gradually while the cohesion increases steadily with the increase of water content,when the water content exceeds the marginal moisture,cohesion in the interface decreases sharply,friction angle in the interface between unsaturated loess and concrete increases steadily and friction angle in the interface between unsaturated loess and steel increases first and then decreases gradually with the increase of water content.(2)The matrix suction of parallel soil samples is determined by the filter method to investigate the influence of matrix suction on the shear strength of unsaturated soil structure.The shear strength is proportional to action area of matric suction,while is no proportional with the size of the matrix suction,when the matrixsuction is at a maximum the meniscus-shaped water films formed between the soil and the structure will gradually shrink and disintegrate,which resulting in a decrease in the area of the matrix suction,through the first freeze-thaw cycle the number of meniscus-shaped water films at the interface is approximately ideal value due to the migration of water,which maximizes the area of interaction of the matrix suction on the contact surface,at this time,the contribution of matrix suction on the shear strength in the interface reaches the maximum,with the number of freeze-thaw cycles increase water migration intensifies and the action area of matric suction decreases gradually.(3)The damage characteristics of contact surface of the two materials are similar under vertical load,and there are two main types.1.When the vertical load is low(no exceed 200 k Pa),the contact surface is brittle,and the shear stress-displacement curve is softened before freez-thaw and at different freeze-thaw times,the shear strength gradually increases after the contact surface strength peaks;2.When the vertical load is large enough,the contact surface changes from brittle failure to plastic failure,after the shear stress at the interface reaches the peak,it changes from elastic deformation to plastic hardening,the shear stress is released slowly,and the curve does not drop significantly,in addition,the residual strength of interface is very close at different freeze-thaw times but not including the unfrozen-thaw under the same vertical load.(4)The stress and strain curves are established respectively based on the direct shear tests in the interfaces of different materials,according to characteristics of the curves,we choose the respective mathematical model to fit,we choose the Generalized Hyperbolic model for the concrete material while choose the Compertz model for steel material,and the corresponding experimental parameters were obtained,the mechanical models of the shear stress-displacement-freeze-thaw cycles of steel and the unsaturated loess and concrete were established by introducing the freeze-thaw cycle number n.(5)Taking the ABAQUS,a finite element software,as an example,the engineering application research of the mechanical behavior in the interface between unsaturated loess and structure considering freeze-thaw action is carried out.A retaining wall model under freeze-thaw environment is established and numerical calculation is carried out.According to the corresponding test parameters,the freeze-thaw cycles are simulated and the change curves of the side frictional resistance of retaining wall under different freeze-thaw times are obtained.Thedifference is less than 10 percentage between the ultimate friction resistance calculated by the numerical simulation and the ultimate friction resistance calculated by the theoretical method.