Experimental Study On Unsaturated Strength Characteristics Of Expansive Soil Improved By Straw Ash

Expansive soil is a kind of cohesive soil widely distributed all over the world.It has the engineering characteristics of water swelling and water shrinkage.With the rapid development of urban construction,the damage caused by expansive soil is becoming more and more serious.In this paper,the wheat straw ash is used to reinforce the expansive soil to meet the basic requirements of engineering construction.Firstly,triaxial test and one-dimensional unsaturated direct shear test were carried out on expansive soil before and after wheat straw ash improvement.Study on the relationship between soil strength and ash content,confining pressure and curing time of wheat straw.Secondly,the expansive soils with different ash content were tested by SEM to understand the effect of wheat straw ash on soil microstructure.The main conclusions are as follows.(1)The results one-dimensional unsaturated direct shear test show that that the shear strength increases first and then decreases with the increase of wheat straw ash content when other experimental conditions remain unchanged.When the wheat straw ash content is 9%,the shear strength reaches its maximum value,which increases by 24%compared with plain expansive soil.Soil samples with9%ash content were maintained for 0 days,7 days,14 days and 28 days respectively,and other test conditions remained unchanged.Compared with the specimen cured for 0 days,the shear strength of the specimens cured for 7 days increased significantly,but with the increase of curing time,the shear strength tended to be stable.(2)The results triaxial consolidation undrained shear test show that the triaxial shear failure form of the expansive soil is plastic failure,and the damaged sample exhibits a"waist drum type".When wheat straw ash is added,the failure mode of expansive soil becomes typical brittle failure,and there are obvious shear planes in the direction of 45 to 60.With the constant confining pressure,the shear strength of the sample increases with the increase of straw ash content.Under different confining pressures,the peak strength of soils increases by 150%,154%,152%and 188%respectively.The shear strength of expansive soil increases with the increase of confining pressure while keeping the ash content of wheat straw unchanged.Compared with plain expansive soil,the modified expansive soil has reduced cohesion and increased internal friction angle.(3)The qualitative study results of soil microstructure show that the microstructure of the expansive soil exhibits a surface-to-surface structure,and there are many large-diameter pores with irregular distribution between the soil particles.After adding wheat straw ash,the surface-surface structure in the soil is destroyed,the arrangement of soil particles is not obvious,and the number of macropores is significantly reduced.(4)Quantitative study results of soil microstructure show that he total number of pores in the sample after adding wheat straw ash increased,but the total pore area decreased,the proportion of small pores increased significantly,and the proportion of large pores decreased.With the increase of straw ash content,the average area,average diameter and average perimeter of the pores in the soil decrease as a whole.When the ash content was 9%,all of them reached the minimum,which were 1.84?m~2,1.1?m and 5.48?m,respectively.With the increase of ash content in the soil,the mass percentage of Ca and Mg elements in the sample increased significantly,the mass percentage of Al and Fe elements decreased,and the mass percentage of O and Si elements changed little.