Experimental Study On Engineering Properties Of Modified Expansive Soils

Expansive soil has strong swelling and shrinkage characteristics,which is harmful to light-weight overburden buildings and difficult to be directly applied to engineering.In this paper,Handan strong expansive soil is used as raw material,in which fly ash,lime and fly ash activator are added.The swelling and shrinkage characteristics,compression characteristics,water stability and strength characteristics of the modified expansive soil(Modified Soil)are systematically studied by single mixing and mixed mixing method,which can provide reference for the field of expansive soil modification.The main research work and brief conclusions of this paper are as follows:(1)Based on the experiments of free expansion rate,unloaded expansion rate,loaded expansion rate,direct shear and compression,the modification effects of fly ash,lime,lime-fly ash and composite modifiers on expansive soils were preliminarily studied by means of step-by-step in-depth research methods.The results show that the expansibility,strength and compression resistance of expansive soil modified by fly ash are poor,while the lime modification of expansive soil is better.When the content of lime is 6%,the unloaded expansion rate is less than 1%,when the content of lime reaches 12%,it becomes non-expansive soil.The expansibility and compressibility of lime-fly ash modified soil are similar to those of lime modified soil,but the strength of lime-fly ash modified soil is obviously improved.Compared with lime-fly ash soil,the compressibility of composite modified soil has little change,the expansibility slightly increases,the shear strength increases obviously,even surpasses that of high-content lime modified soil with 9%and 12%lime content.With the increase of age,the strength of composite modified soil increases and the expansibility decreases.(2)Through shrinkage and water stability tests of plain soil and modified soil,the effects of moisture content and dry density on the shrinkage of plain soil were studied,and the shrinkage and water stability characteristics of modified soil were further studied.The results show that water content promotes shrinkage,while dry density has little effect on shrinkage rate.There is no obvious correlation between shrinkage coefficient and shrinkage rate.On the whole,the shrinkage rate of saturated soil is larger than that of compacted soil,while the shrinkage coefficient of compacted soil is larger than that of saturated soil.The shrinkage of the modified soil is obviously less than that of the plain soil,and its shrinkage is hardly affected by the initial moisture content.The shrinkage inhibition effect of lime-fly ash modifier is better than that of lime modifier,and the composite modifier has the best shrinkage inhibition effect.Lime modified soil,lime-fly ash modified soil and composite modified soil all have better water stability.(3)The unsaturated shear strength of expansive soils and modified soils with different moisture content and matrix suction was studied by means of filter paper method and direct shear test.For plain soil,the effects of compaction method,saturated dehydration method and natural dry-wet method on the strength of unsaturated soil are also studied.The analysis shows that the saturated sample has the highest strength among three different samples of plain soil.The shear strength of plain soil decreases with the increase of moisture content,and increases with the increase of matrix suction,while the shear strength of Lime-modified soil has little change with moisture content and matrix suction.φ~b of compacted plain soil has a non-linear relationship with matrix suction.With the increase of matrix suction,the contribution of matrix suction to the improvement of shear strength decreases.