Study On Mechanical Properties Of Modified Subgrade Silty Soil In Seasonal Frozen Area

Three Provinces in Northeast China,Inner Mongolia,Gansu,Ningxia,the northern part of Xinjiang,and Qinghai,deep seasonal permafrost with a depth of more than 1m is distributed,accounting for about one third of the country's land area.The seasonal permafrost is frozen in winter and thawed in summer,the moisture in the soil is constantly moving and changing under the action of freezing and thawing,and the bearing capacity of roadbed is destroyed.Under the action of capillarity,the moisture migration and accumulation of silty soil is more significant,and the frost heaving and muddying of subgrade are more obvious.Therefore,there is an urgent need to conduct special research on silty soil in the seasonally frozen area to improve the strength and durability of silty soil roadbeds,provide a reference for silty soil improvement technology,and expand the application range of silty soil improvement technology in the seasonally frozen area.In this paper,the mechanical properties of the improved soils under repeated freezing-thawing were studied by means of clay,polypropylene fiber and lime.The optimum moisture content and maximum dry density of the modified soil were obtained by compaction test,and the triaxial test was carried out under the condition of optimum moisture content.According to the experimental results:1.Compaction test: the change of the optimal water content of the modified silty soil is more obvious than the change of the maximum dry density.The improvement of clay and lime increases the optimal water content of the soil and reduces the maximum dry density.The optimum moisture content and maximum dry density of soil after polypropylene fiber improvement showed a decreasing trend.2.Triaxial test: in the unconsolidated and undrained triaxial test,the stress-strain relationship curve of silty soil shows a softening pattern,and the peak strength appears at 3%-5% strain.The freezing-thawing effect can reduce the peak strength of the curve,but cannot change the curve shape.3.The cohesion,internal friction Angle and shear strength of silty soil increased to different degrees after being improved by clay,fiber and lime.The recommended dosage for the three improvement methods is: clay content above 25%?30%,lime content 6% and polypropylene fiber content 0.3%.4.After clay improvement,the ultimate strength,cohesion and shear strength of silty soil all decreased to different degrees under the action of freezing-thawing cycle,and gradually became stable after the sixth freezing-thawing cycle.The internal friction Angle fluctuated under the action of freezing-thawing,with no obvious rule.5.Under the action of freeze-thaw cycles,the polypropylene fiber modified silty soil will first decrease,stabilize,and then increase or stabilize with the increase in the number of freeze-thaw cycles.The internal friction angle fluctuates as the number of freeze-thaw cycles increases,and there are no obvious rules to follow.6.Under the action of freeze-thaw cycles,the lime-modified silty soil will decrease in cohesion first and then increase as the number of freeze-thaw cycles increases.The internal friction angle decreased significantly after the first freeze-thaw,and showed an upward trend with the increase in the number of freeze-thaw cycles,and was basically stable after the end of the sixth freeze-thaw cycle;the shear strength gradually increased with the increase in the number of freeze-thaw cycles,and the later strength was high.