Research On The Engineering Characteristics Of Lignin Modified Soil Under Freeze-thaw Conditions

The roadbed treatment in seasonal frozen area is a technical problem in road construction in Northeast China.It is urgent to explore an economic and environmental protection roadbed improvement technology.According to relevant reports at home and abroad,lignin can be used for soil improvement of roadbed,with good water stability and erosion resistance.At the same time,these lignins have not been well utilized and are often abandoned as waste materials.Based on the transportation science and technology project of Jilin Province(2018ZDGC-14)and taking the improvement of subgrade soil in seasonally frozen area by lignin as the research object,this paper systematically studies the improvement method through many laboratory tests,which provides a scientific basis for the popularization and application of this method in seasonally frozen area.Its main research contents and achievements are as follows:1.With the addition of lignin,the optimal moisture content of the improved soil decreases.The optimum moisture content curve of lignin improved soil is different from that of lime and cement improved soil.Only 3% and 6% of the light compaction curve and the heavy compaction curve of the lignin improved soil lie above the compaction curve of the lignin improved soil,and both of the compaction curves tend to move to the left.2.The alkalinity of silty sandy soil increases rapidly after adding lignin,and the PH value of improved soil first increases and then decreases with the increase of lignin content.In the same curing period,the PH value of the improved soil with 3%-6% lignin content is relatively high,and the PH value of the soil with 9%-15% lignin content is basically the same.The PH value of each content of lignin improved soil increased with the increase of curing age.Under the same test conditions,the PH value of 5% lime improved soil is 11.73,while that of lignin improved soil is only 10.14.3.In the same curing period,the linear frost heave rate of lignin-improved soil will increase with the increase of lignin content and freeze-thaw cycles;the linear frost heave rate of lignin-improved soil will decrease with the increase of curing age.The correlation coefficient between frost heave rate and the number of freeze-thaw cycles is low,and the fitting effect is not good.4.The cohesion and shear strength of lignin-modified soils decrease with the increase of freeze-thaw cycles,but the internal friction angle increases.The addition of lignin increased the frost resistance of soil,but with the increase of lignin content,the frost resistance decreased.After 15 freeze-thaw cycles,the shear strength and cohesion curves of lignin-modified soils tend to be stable.Comparing shear strength data of different curing periods,it was found that short-term curing had little effect on ligninimproved soil.According to the results of shear strength,linear frost heaving rate and economic factors,the optimum curing age of lignin-modified silty sand is 60 days,and the optimum lignin content ranges from 3% to 6%.5.The CBR value of improved soil increases with the increase of lignin content,while the water absorption of CBR specimen decreases with the increase of lignin content.It is also found that the freeze-thaw attenuation rate of CBR value decreases with the increase of lignin content.The CBR specimens of 9%,12% and 15% lignin improved soil have better water stability and erosion resistance,which explains the reasons for the decrease of water absorption and low freeze-thaw attenuation rate of CBR specimens.6.X-ray diffraction and energy dispersive spectrometer analysis showed that no new mineral components and elements were produced in the lignin-improved soil.In addition,the changes of the microstructure of the lignin-improved soil were quantitatively and qualitatively analyzed by scanning electron microscopy(SEM).The addition of lignin improved soil makes the internal pore of soil smaller and the microstructure more stable.Finally,the relationship between the cohesion,internal friction angle and shear strength of the improved soil and the pore microstructure parameters after 15 freeze-thaw cycles was evaluated by grey correlation method.