Experimental Study On Engineering Properties And Micro-Mechanism Of Composite Improved Expansive Soils

Expansive soil is a kind of high plastic special soil with hydrophilic minerals in high proportions,which results in significant volumetric deformation,even shrinkage cracking due to the absorption or loss of water.As a result,the destruction of buildings or structures built on expansive soil foundations is one of the important issues attracted intensive attentions in engineering practices.Therefore,it is necessary to improve the expansive soil to meet the requirements of engineering construction.In this study,based on the engineering properties of the expansive soil,as well as the properties of the iron tailings sand and carbide slag,the composite improved soil by adding the iron tailings sand and carbide slag into the expansive soil is proposed.Systematic researches are performed to investigate the engineering properties and the micro-mechanism of the composite improved soil.The main conclusions are as follows:(1)When the iron tailings sand is used to improve the expansive soil,the content of silt and sand increase and the content of clay decrease of improved soil with the increase of tailings sand content;liquid limit and plasticity index are all decrease;the maximum dry density of the improved soil is higher than that of the soil,and the optimum moisture content is lower than the soil;The unconfined compressive strength increases with the increase of tailings sand content,but it decreases when tailings sand content more than 30%.After immersion,the strength is decrease significantly and the coefficient of water stability is hardly increased.The expansion index of expansive soil,such as free swelling ratio,loaded swelling ratio and swelling pressure are all decrease significantly with the increase of tailings sand content.The microstructure of the improvement soil was observed by SEM technique,results show that the specimen with sand content of 30% owns the most integrate and stable microstructural structure.(2)Iron tailings sand-carbide slag composite improved expansive soil,when the content of iron tailings sand is 30%,the content of clay and silt decrease and the content of sand increase with the increase of carbide slag content;the liquid limit decrease while the plasticity index decrease with the increase of carbide slag content,but it increases when carbide slag content more than 10%;the maximum dry density decrease and the optimum moisture content increase.The unconfined compressive strength increases with the carbide slag content increasing to 10%,after which,the UCS will decrease with the carbide slag content keeps increasing.Free swelling ratio,loaded swelling ratioand swelling pressure are all decrease significantly with the increase of carbide slag content.The water stability coefficient increased gradually with the increase of carbide slag content,and the water stability is improved effectively.The microstructure of the improved soil was observed through the SEM technique,the pore development in the sardstone of expansive soil after composite improvement is significantly weakened,the soil structure is also more compactted,the degree of particles agglomeration of the improved soil is enhanced.The curing time can also plays an important role in development of the strength,expansion and water stability of composite improved expansive soil.(3)In conclusions,the effect of iron tailings sand-carbide slag composite improvement of expansive soil is better than that of the pure iron tailings sand.