Experimental Study On Hydrothermal Characteristics And Soil Particle Migration Of Subgrade Replaced By Sand During Freeze-Thaw Process

A lot of engineering practices and subgrade injury investigation in cold region of our country show that it is the main form.As a common method for the prevention and treatment of subgrade frost heave,filling replacement method has many advantages.However,it is found that,under multiple freeze-thaw cycles,the water content of coarse particle filling replacement will also increase significantly,which increases the probability of frost heave.In this paper,coarse sand and fine sand with silty clay and fineness modulus of 3.5 and 2.2 were selected.Through the indoor one-way freeze-thaw test with or without water refill,the variation rule of water and heat at different heights of soil column was obtained,and its influence on the water and heat of the underlying soil sample was analyzed.Through freeze-thaw experiment of soil column with coarse sand on top and silty clay on bottom with or without replenishment,soil particle gradation before and after freeze-thaw at different heights was analyzed to compare the percentage content of soil particles with different particle sizes under different freeze-thaw time and temperature gradient.The following results are obtained:(1)In the study of temperature field changes of soil column,through the cloud map analysis of temperature changes at different depths over time,with 0℃ isotherm as the reference of influence depth,it is found that the influence depth of fine sand and coarse sand on the underlying clay is similar,both being 7cm,under the condition of no water refill,which effectively reduces the depth compared with the silty clay.Under the condition of water replenishment,the depth of fine sand and coarse sand is 6cm and 7cm respectively,both of which are significantly less than that of silty clay,but fine sand has a better temperature inhibition effect.(2)For the study of soil column moisture field change,through the cloud map analysis of moisture content changes over time at different depths,with 10% moisture content contour line as the reference of influence depth,it is found that under the condition of no water filling,the influence depth of fine sand and coarse sand is both 4cm,the influence depth of fine sand has a downward trend while coarse sand is stable at 4cm,and coarse sand has a better inhibition effect on moisture content change.Under the condition of water replenishment,the influence depth of fine sand is stable at 4cm,and that of coarse sand is 7cm and continues to decline.Fine sand can better inhibit the change of water content of the underlying soil sample.(3)The freeze-thaw characteristic curve of the soil sample was studied.By analyzing the influence of different factors on the freeze-thaw characteristic curve,the freeze-thaw characteristic curve at the buried depth of 6cm under the condition of no water refill was selected to fit the silty clay,and the fitting model with Boltzmann equation for both cooling and warming processes was obtained.The curves of the upper filled with different sand were further fitted under the condition of no water refill.The fitting of freezing characteristic curves in the cooling process is Exp Dec1 equation,and the fitting of melting characteristic curves in the heating process is Allometric1 equation,which reflects the influence of different levels of sand on the freeze-thaw characteristics of the underlying soil samples.(4)Research on soil particle migration during freeze-thaw process,based on the above changes in hydrothermal characteristics during freeze-thaw process,through the matching ratio of particle levels before and after freeze-thaw of poorly graded sand soil,it is found that after freeze-thaw cycle,particle content in the range of 1-10 mm particle size decreases,while particle content below 0.5mm particle size increases,showing the migration characteristics of unearthed particles.Further comparing the effects of different freeze-thawing duration and temperature gradient,it is found that with the increase of freeze-thawing duration,the smaller particle size under 1mm increases the particle content.With the increase of temperature gradient,the particle content in the range of 0.25 mm to 2mm decreases,while the particle content below 0.25 mm increases.Due to the difference of hydrothermal characteristics of different soil samples during freeze-thaw process,on the one hand,it has different effects on the change of temperature and water content of the underlying soil samples,on the other hand,it has an impact on the migration of soil particles under the condition of poor gradation.The results of this study have an important guiding role in the mix ratio design of subgrade replacement materials in cold areas.