Experimental Study On The Characteristics Of Octadecylamine Modified Expansive Soil Under Freeze-thaw Conditions

The overlap between the distribution of expansive soils and the distribution of seasonal permafrost zones in China is large.The high water content of the channel slopes,subject to wet,dry and freeze-thaw action,has led to frequent problems with expansive soil channel slopes in seasonal permafrost zones.Therefore,finding admixtures that limit the expansion and contraction of expansive soils and improve their strength after freezing and thawing is of great importance to ensure the safe operation of hydraulic buildings in seasonal frozen soil areas.Based on this,this paper selects the eco-friendly green material octadecylamine as an admixture to improve the preparation method of improved expansive soils.And the improvement effect of improved expansive soil in terms of water repellency,expansion and contraction characteristics,thermophysical properties,mechanical strength and micro pore changes during freeze-thaw process was studied.The main research results are as follows:(1)The preparation method of the modified expansive soil with octadecylamine was improved,and the particle analysis test showed that the mixture of octadecylamine and soil particles in the modified expansive soil was more uniform,and only the particle size of small particles increased slightly.The water repellency of the improved expanded soil gradually increased with the increase of the dosing amount of octadecylamine.When the dosing amount increased to 0.5% and above,the contact angle of the improved expansive soil was higher than 90°,which was hydrophobic and met the requirement of water repellency improvement.(2)Octadecylamine can effectively limit the swelling and shrinking of expansive soils.The free swelling rate of expansive soils is 50%.The free swelling rate of modified expansive soils gradually decreases with the increase of octadecylamine content.The free swelling rate of modified expansive soils decreases with increasing admixture of 0.5% or more and can be considered as non-expansive soils with a free swelling rate of less than 40%.Octadecylamine significantly reduces the unloaded swelling rate of the amended swelling soil and slows down the reaction rate between water molecules and soil particles,extending the time required for the unloaded swelling rate to stabilise.The development of fissures in modified expansive soils with different doses of octadecylamine during two wet and dry cycles showed that octadecylamine reduced parameters such as fissure size,width and density by limiting the swelling and shrinkage of the expansive soil,with the best effect at 0.8%.(3)Octadecylamine can effectively reduce the thermal conductivity of expansive soils.The thermal conductivity of soils is an important thermal physical parameter that affects the freezing process of soils in seasonal permafrost zones.It was found that the thermal conductivity of the soil increased rapidly during the freezing process,and the thermal conductivity of the soil in the frozen state was greater than that in the thawed state.The thermal conductivity of modified expansive soils gradually decreases with increasing octadecylamine content.(4)Octadecylamine can improve the mechanical strength of modified expansive soils.The shear displacement curves of the expanded soils changed from hardened to softened with the addition of octadecylamine,and the peak value gradually became obvious,and then changed back to hardened when the content of octadecylamine continued to increase.After considering the effect of freeze-thaw,it was found that: the shear displacement curves of the improved expansive soils were all hardened;the number of freeze-thaw cycles required for the shear strength of the improved expansive soils to reach a stable value decreased gradually with the increase of the doping amount of octadecylamine;the shear strength index after freeze-thaw increased firstly and then decreased with the increase of the doping amount of octadecylamine,in which the doping amount of 0.2%～0.5% had a better effect on the mechanical strength of the expansive soils after freeze-thaw cycles.The improvement of mechanical strength after freeze-thaw cycles was better at0.2%～0.5%.(5)Effect of octadecylamine on the microstructure of modified swelling soils.The NMR test on the specimens during freeze-thawing shows that capillary water has a greater influence on the pores in the soil during the freeze-thawing cycle;octadecylamine can effectively limit the conversion of thin film water into capillary water during the freeze-thawing process,thus reducing the tendency of the pores in the soil to become larger during the freeze-thawing process.Combined with the analysis of the soil microstructure by SEM,it can be seen that the 0.5% doping of octadecylamine can reduce the pore related parameters of the modified swelling soil after freeze-thaw cycles.However,when the doping rate increases above 0.8%,the increase of agglomerates in the soil leads to an increase in pore related parameters which are unfavourable to the strength of the soil.