Research On Mechanical Properties And Testing Technique Of Seasonally Frozen Soils

Seasonally frozen soil distributes over half of China territory.With the economy developing and the proposal of the strategy of “Silk Road Economic Belt and the 21 stCentury Maritime Silk Road”,the construction of transportation system in seasonally frozen region grows rapidly.According to investigation statistics,many defects,such as frost heave,thaw settlement,settlement,frost boiling and slope collapse,appear in seasonally frozen soil subgrade and cause serious economic loss.Therefore,it is important to study the strength-deformation characteristics of seasonally frozen soil based on seasonally frozen soil coverage area,national development strategy and seasonally frozen soil subgrade defects.So far,the study on seasonally frozen soil is still not so well explained.There are still great debates on the change law of shear strength parameters under freezing-thawing cycles and the research of dynamic characteristics of seasonally frozen soil.The source of all those problems is lack of advanced experimental techniques,such as test standard and test instruments.The researches only based on delicate experiments can accurately present the strength-deformation characteristics of seasonally frozen soil.In this dissertation,the main research includes: key problems in the experimental techniques of freezing-thawing cycles soil,the statics constitutive theory of seasonally frozen soil,the strength theory of freezing-thawing cycles soil and the residual strain theory of seasonally frozen soil.The main results encompasses: the new standard for seasonally frozen soil specimen preparation,the stress-strain relationship change law of freezing-thawing cycles soil,the formula of shear strength index,the more reasonable residual strain change law of seasonally frozen soil and the more reasonable vibrated excited subsidence parameters.The main points on the work are listed as follows:(1)The general specimen preparation standards are not suitable for seasonally frozen soil.The freezing-thawing amplifies the discreteness of specimen density,and this is the reason that frozen soil triaxial test results are not stabile.The idea that raising the discreteness standard of specimen density can ensure the disciplinary experiment results is put forward.A new standard for frozen soil specimen preparation is proposed,that its reliability was verified using triaxial tests.(2)According to the new standard,the stress-strain relationship change law and the static strength change law of three kinds of representative soils under freezing-thawing cycles are summarized to further get the confining pressure,the times of freezing-thawing cycles and how the static characteristics of seasonally frozen soil are influenced by the soil variety.(3)The concept of correction coefficients under freezing-thawing cycles is proposed.The law of representative soils shear strength index is obtained.The correction curve and computational formula of cohesion and internal friction angle under freezing-thawing cycles are suggested,and their engineering application methods are illustrated.(4)In the low-temperature dynamic triaxial test,the buried condition,dynamic stress and the frozen process are more reasonable of the seasonally frozen soil.Using the new low-temperature dynamic triaxial apparatus and employing the advanced experimental technique,the residual strain change law and the vibrated excited subsidence model computation parameter of seasonally frozen soil in freezing period are proposed.