Mechanical Properties Study Of Lignin-improved Silty Railway Subgrade Soil

The silty soil belongs to fine-grained soil.Not only it is widely distributed throughout the country,the Yellow River alluvial silty soil is also found in many parts of Shandong province and the area is about 34%of the total area in Shandong province.The Yellow River alluvial silty soil mostly belongs to C group railway embankment filler.According to the code for design of railway subgrade in operation,it must be improved before being used.In this paper,lignin was used as the main curing agent to improve the low-liquid limit Yellow River alluvial silty soil so as to meet the requirements of railway embankment filler.Through laboratory tests including the unconfined compression strength test,the basic physical and mechanical parameters were contrasted of pure soil,lime improved silty soil,lignin improved silty soil and lignin-lime improved silty soil.The reasonable modifier and the optimum mixing ratio were obtained and the improvement effect and influence law of the modifier were studied.The change of the microstructure of soil samples were observed by using of scanning electron microscope(SEM),and the mechanism of improved silty soil was analyzed.The dynamic characteristics of lignin-lime improved silty soil were studied by dynamic tri-axial test.The main conclusions of the study are as follows:(1)The improvement effect of lignin-lime modifier is better than that of lime or lignin.When the lignin content is more than 4%,under different mixture ratio,the maximum dry density of the improved silty soil with lignin-lime is larger than that of pure soil and the optimum moisture content is less than that of pure soil.The addition of lignin-lime improved the compactability of the Yellow River alluvial silty soil.(2)When the content of lime is no less than 3%and the ratio of lignin to lime is controlled within the range of 2:1~3:1,the improved silty soil with lignin-lime can meet the engineering requirements of railway embankment filler.From the perspective of saving natural resources,it is suggested that the lime content should be controlled at 3%and the lignin content should be controlled at 6%~9%.(3)Less amount of lignin-lime modifier can get the same effect as fly ash-lime modifier.When the content of lignin-lime modifier is 6%+3%,the best effect can be obtained.Compared with pure soil,the cohesion and internal friction angle of lignin-lime improved silty soil increased significantly.The compressibility decreased and the water stability improved.The microstructure of improved soil became more stable.The overall engineering properties of the Yellow River alluvial silty soil with low liquid limit have been effectively improved.(4)Compared with pure soil,higher dynamic elastic modulus,dynamic shear modulus and dynamic strength can be obtained from lignin-lime improved silty soil.The dynamic elastic modulus E_d is approximately hyperbolic with the dynamic strain?_d.There is a linear relationship between the dynamic shear strength?_d and the logarithmic failure time lgN_f.