Study On The Mechanical Properties And Structure Changes In Microscale And Mesoscale Of Loess In Heifangtai Under Freeze-thaw Action

Heifangtai is a typical loess platform and belongs to the seasonal frozen soil region.Seasonal freeze-thaw action plays an important role in the physical and mechanical properties of loess;meanwhile,the properties are related to the stability of the slopes and the safety of the irrigation canal and other irrigation works at loess area.The study on the mechanical characteristics and microstructural changes of the loess under the freeze-thawing effect has a theoretical and practical significance for the stability analysis of the slope,the prevention of the landslides and the protection of the irrigation and drainage engineering during the freezing and thawing period in Heifangtai.Through the methods of laboratory soil test,CT scanning,scanning electron microscopy(SEM)and image analysis,this paper has studied the physical properties of loess in Heifangtai,the strength characteristics of the frozen loess,the strength characteristics of the loess before and after freeze-thaw cycles,frost heaving behaviors,and microstructural changes and their relationship with strength under freeze-thaw cycles.The main results obtained are as follows:(1)The results of conventional geotechnical tests show that the internal friction angle of unsaturated loess decreases linearly with increasing water content and the cohesion force decreases as a power function curve under the conditions of non-freezing and thawing;internal friction angle and cohesion force would to be bigger while increasing dry density;the unconfined compressive strength(UCS)of undisturbed loess shows a negative logarithmic function curve as the increasing of water content.(2)The results of freeze-thaw cycles and triaxial shear tests show that the freeze-thaw cycle has a little effect on the moisture content of the loess samples in the closed system;freeze-thaw cycles have a significant weakening effect on the cohesion of loess with high saturation,and have little effect on the low-saturation loess' cohesion and internal friction angle;the effect of freeze-thaw cycles on the cohesion is greater than that of internal friction angle.Saturation has an important effect on the effect of freezing and thawing.(3)The study of the uniaxial compressive strength of loess in frozen state shows that the uniaxial compressive strength of frozen loess between-10°C and-15°C is about 10 times bigger than that of unfrozen loess;dry density,initial moisture content and low temperature have different degrees of strengthening effect on frozen loess.(4)The results of the experiments on frozen-heaving ratio and frozen-heave force show that the loess with high saturation is more prone to frost heaving deformation;when initial moisture content of loess is higher than 19%,growth rate of frost heaving ratio will increase;the greater the moisture content is,the greater the frozen-heaving stress is,and the frost heaving stress at high water content is greater than the UCS of intact loess or remolded loess.The maximum linear frost heaving ratio has a quadratic function relationship with the frost heaving stress under elastic constraints,and they are positively correlated.Moisture migration and soil frost-heaving damage are the influencing factors of the loess landslide during the freeze-thaw period.(5)The study of meso-porosity characteristics based on CT shows that the parameters such as porosity,number of pores,average pore size,roundness of porosity,anisotropy of pores and fractal dimension of pore area under freeze-thaw action are changed and generally not conducive to structural strength,on the contrary,the pore parameters change in the direction that favors the structural strength during freezing;forceful correlation between different parameters exist.(6)Quantitative analysis of microstructure based on SEM shows that the variation trends of particle parameters between undisturbed soil and remolded soil are not completely same;under the freeze-thaw action,the particles of remolded soil tend to be uniform and simplification,which is not conducive to the interparticle occlusal force and resistance.The micro pore morphology under freezing and thawing condition is more complex.The Grey Relational Analysis results show that the parameters of pore and particle size have greater influence on the cohesion,and the parameters of pore and particle shape have greater influence on the internal friction angle.