Study On Deterioration Experiment Of Mechanical Behavior And Numerical Simulation Of Undisturbed Loess Under Freeze-thaw Cycle

The effect of freeze-thaw on the mechanical behavior of soil is an important research topic in the field of geotechnical engineering.After freeze-thaw cycle,the soil structure will change with the influence of cryogenesis,which will result in the change of its physical dynamic characters.At the time of basic construction like slope support,cutting excavation and roadbed construction,the change of physical dynamic character of the soil after freeze-thaw cycle must be taken into consideration to analyze the deformation and stability.Therefore,the research on the change of the physical dynamic character of the soil not only has a high research value and also a great practical significance.In light of the current weak research on dynamic characters of the undisturbed soil after freeze-thaw cycle,the thesis has the undisturbed loess in Lanzhou as the research target,and dry density,water content,freeze-thaw cycle and cold side temperature as influence factors under closed test conditions,and has conducted a series of test researches on freeze-thaw cycle.There are mainly following research results by the thesis:?1?Through unconsolidated-undrained tri-axial shear test on the soil samples before and after freeze-thaw cycle,it is possible to get soil sample strength parameters by use of two different methods.The analysis has found that the soil with smaller water content has larger cohesive force.After freeze-thaw cycle,the cohesive force decreases and after the first freeze-thaw cycle,the cohesive force drops to a larger degree.The soil with water content sees a greater drop of cohesive force after freeze-thaw cycle.The soil with low free-thawing temperature can have a bigger change in cohesive force after freeze-thawing.The bigger dry density makes a bigger decrease in cohesive force after freeze-thawing.For internal friction angle,it changes slightly after freeze-thawing;and after freeze-thaw cycle,the internal friction angle increases,but for some soil samples,it is in a decreasing tendency with the changing amplitude between 0° and 2°.?2?Both of the stress-strain curves before and after freeze-thaw cycle have been represented:post-peak softening type and gradual hardening type.With the growing of confining pressure,the stress-strain curve of the samples will turn from the post-peak softening type to stress hardening type;and the freeze-thawing action wil not change the stress-strain curve of the soil.For the deviation stress corresponding to the axial strain with the same confining pressure and the same soil samples,the thawed soil deviation stress value of the soil that is not freeze-thawed is always higher than that of the thawed soil.Under the same freeze-thawing conditions,the soil samples with larger water content has smaller difference value in deviation stress difference under the same axial strain before and after freeze thawing.The soil samples with higher water content have smaller failure strength after freeze-thaw cycle.That suggests freeze-thaw cycle actions have a large influence on the stress-strain curve with low water content,but small influence on the curve with high water content.?3?Based on Duncan-chang model,conduct fitting analysis on the test data to get the initial tangent modulus and the principal stress difference asymptotic value of the soil samples before and after freeze thawing,and then conduct comparative analysis.By use of regression analysis,build Duncan-chang model with initial water content,dry density ?,confining pressure ?₃ as influence factors,to get the parameters a,b,shear strength and stress break ratio.The stress-strain curve from the test and the stress-strain curve from the model are basically identical after comparison.That suggests that the model can better reflect the dynamic characters of the initial water content,dry density,and the thawed soil under confining pressure?₃.?4?Establish a BP soil sample model about the relationships between initial tangent modulus?the principal stress difference asymptotic value?,initial dry density,confining pressure,water content and freeze-thawing frequency,based on the neural network toolbox on Matlab platform.Then after the test with the test samples,small relative errors of predicted values could be got,which can comprehensively reflect the relationship between the soil sample dynamic parameters and all influence factors.?5?Use finite element software ABAQUS for freeze-thaw cycle simulation on the basis of thermal convection,to get different initial conditions and changes of temperature field of the soil samples.Conduct thermo-mechanical coupled analysis on soil samples based on Mohr-Coulomb simulation to get a vertical stress-strain curve.Small relative errors will be found between the shear strength value from the simulation with that from test after comparison.