Study On Soil-water Characteristics And Pore Distribution Of Lime-improved Loess Under Freeze-thaw Action

Most of the western region of China is both loess geological region and seasonal frozen region.The influence of collapsibility of loess and freeze-thaw action in seasonal freezing area should be considered in the construction of project in this area.The uneven settlement of foundation or soil landslide often occurs due to the collapsibility of loess.Therefore,lime is often used to improve the properties of loess in engineering.The constant temperature change in the seasonal frozen area causes the soil mass to frost heave in winter and melt sink in summer.The repeated volume change of soil mass is easy to cause settlement deformation of engineering foundation and the weakening of soil mechanical properties,which is of great harm to engineering construction.Therefore,it is necessary to explore the influence mechanism of freeze-thaw action on the lime-improved loess.Lime-improved loess is unsaturated soil.As the basic constitutive model of unsaturated soil,soil-water characteristic curve is of great significance to the mechanical properties of unsaturated soil and is the basis for the study of various properties of unsaturated soil.Therefore,the study of the influence of freeze-thaw action on soil-water characteristic curve of lime-improved loess can better understand the changes of other properties of unsaturated soil under freeze-thaw action,which has certain reference value for the geological engineering construction of loess in the seasonal frozen area.In this paper,the lime improved loess was taken as the research object,and the lime content was set as 6%,8% and 10%.Firstly,freeze-thaw cycles of 0,1,3,5,7 and9 were carried out on samples with different lime contents.The matrix suction of freezethawing samples was tested by filter paper method.The results of filter paper test were fitted and compared using Gardner model,VG model and FX model.After establishing the optimal model,the influence of freeze-thaw action and lime content on soil-water characteristic curve was analyzed.Then mercury injection test was carried out on freeze-thawing samples to measure the pore structure characteristics and porosity.The fractal dimension of pores was calculated based on the thermodynamic relationship model and mercury injection test data.The effects of freeze-thaw cycle,lime content and water content on pore structure characteristics,porosity and fractal dimension were analyzed.Finally,based on the relationship between pore distribution and soil-water characteristic curve,the pore distribution is used to predict soil-water characteristic curve.The soil-water characteristic curve was obtained by measuring matric suction with filter paper method.The influence of freeze-thaw action and lime content on soil-water characteristic curve was analyzed.The results showed that: The air intake value of soilwater characteristic curve decreases gradually due to freeze-thaw action,and the dehumidification rate decreases somewhat.With the increase of freezing-thawing times,the soil-water characteristic curve shifted downward to the left,and the translation amplitude decreased gradually.The freezing-thawing effect mainly had a greater effect on the low suction section of the soil-water characteristic curve,but had a smaller effect on the high suction section.The 10% lime content deteriorate the soil mass.With the increase of freeze-thaw times,it is gradually close to the soil-water characteristic curve with 6% lime content,indicating that freeze-thaw action can enhance the improvement effect of excessive lime on the soil mass to some extent.In the soil-water characteristic curve,8% lime content was the highest,indicating that the improvement effect of 8%lime content is better than that of 6% and 10% lime content.Based on mercury injection test data,the effects of freeze-thaw cycle,lime content and water content on pore structure characteristics,porosity and fractal dimension are analyzed.The results show that: The freeze-thaw effect increases the diameter and number of macropores,resulting in an increase in the total volume of pores.In the pore distribution curve,the curves smaller than 1μm are less affected by freeze-thaw action,and the curves larger than 1μm are more affected by freeze-thaw action,which shows that the peak value and peak width increase.When the number of freeze-thawing reaches a certain level,the changes in the number of four types of pores in the soil are mainly reflected in the mutual transformation of small and medium pores,while the number of micro-pores and large pores is basically unchanged.The difference of lime content will result in the difference of pore distribution structure.For example,the pore distribution curve under 6% lime content mainly shows the increase of dominant pores with the increase of water content,while in the pore distribution curve with 10% lime content,the dominant pores transform to larger pores with the increase of water content,and the increase of dominant pores is small.The effect of water content on pore structure is similar to that of freeze-thaw,but more complex than that of freeze-thaw.On the fractal dimension,the freezing thawing effect makes the fractal dimension decrease first and then increase,and finally increase;the fractal dimensions of 6% lime and 10% lime are similar;while water content makes the fractal dimension first decrease and then increase,and finally decrease.Both freeze-thaw action and hydraulic action increase the porosity.Finally,considering the shortcomings of existing methods to predict soil-water characteristic curves by using pore distribution,combined with the influence of water content on pore distribution characteristics,the improved method based on Fei,Suozhu was used to predict soil-water characteristic curves by averaging the total pore volume after correcting the measured data points of multiple filter paper methods,which has a good prediction effect.