Study On Freeze-thaw Cycle Resistance Of Clay Modified By Colloidal Silica

Freeze-thaw cycle damage of seasonal frozen soil is one of the main problems in civil engineering in high latitude and altitude areas of China.In spite of various measures such as cement and lime grouting,frost deformation of highway and railway roadbed in seasonal frozen soil area of China is still widespread.With the concept of sustainable development put forward,colloidal silica(CS)as a new environmental protection material has been gradually applied in civil engineering.In this paper,the applicability of CS in clay soil was studied by threedimensional freezing-thawing cycle deformation test and unconfined compressive strength test.The effects of saturation,pore ratio and CS concentration were discussed.The main research contents and conclusions are as follows:(1)The freeze-thaw cycle tests were carried out on pure clay with different saturation and porosity ratios.The results show that the volume of high saturation soil expands during freezing and shrinks during thawing,while the volume of low saturation soil shrinks during freezing and expands during thawing,and the critical saturation of the test clay is in the range of 0.8～0.95.Besides,the high saturation clay soil with large porosity tends to be compact while the high saturation clay with small porosity tends to be loose in the process of freeze-thaw cycle.The pore ratio difference has no obvious effect on the cumulative volume change of low-saturation clay and the volume of all samples expands by 1%～2% relative to the initial after the freezethaw cycle.(2)Freeze-thaw cycle tests were carried out on the improved soils with different CS concentrations.After the addition of 5% and 10% CS respectively,the volume change of the improved soil decreases during the single freezing and thawing stage,and the cumulative volume change after the freeze-thaw cycle also decreases to a certain extent..Higher CS concentration produced more obvious inhibition effect.However,the improvement effect of CS is affected by saturation and porosity ratio.In this study,when the saturation is 0.95 and the porosity ratio is 1.0,the improvement effect of silica sol is the most obvious.(3)The unconfined compressive strength of each sample before and after the freeze-thaw cycle deformation test was measured by unconfined compression test.The results show that the unconfined compressive strength increases with the decrease of saturation,and decreases with the increase of porosity ratio.Adding 10% silica sol can increase the unconfined compressive strength of the improved soil by 40%～90%.After six cycles of freeze-thaw,the unconfined compressive strength of soil will decrease by about 50%～80%,but the change rule with saturation and porosity ratio is basically unchanged,and the improvement effect of silica sol on the unconfined compressive strength is weakened.(4)The influence of saturation and silica sol concentration on the test results was evaluated by range analysis.The results shows that the cumulative deformation of the improved soil after freeze-thaw cycle is same affected by saturation and CS concentration.The unconfined compressive strength is mainly affected by saturation while less affected by silica sol concentration.In engineering application,when the main task is to limit the accumulated deformation development of improved soil after freeze-thaw cycle,it can properly increase the concentration of silica sol or reduce the moisture content of the improved soil.When the purpose is to enhance the strength of the improved soil before and after the freeze-thaw cycle,the amount of silica sol can be saved,and the actual demand can be met by reducing the saturation of the improved soil.