Experimental Study On Strength Characteristics Of Unsaturated Copper Contaminated Expansive Clay

Expansive clay is a kind of special soil which is widely distributed and often causes engineering disasters.Although the mechanical properties of expansive clay have been deeply investigated,there are few studies on the unsaturated behaviors of heavy metal contaminated expansive soil.Herein,the unsaturated direct shear tests and saturated triaxial tests under different confining pressures,net normal stresses and matric suctions were carried out to address strength characteristics of contaminated expansive soil of various copper ion concentrations.And the quantitative microstructure characteristics on the failure plane were also discussed by means of the scanning electron microscope(SEM)experiments and the image processing technology for the specimens performed after the unsaturated direct shear.Moreover,the mechanism of the effects of copper ion concentration on the microstructure and strength of contaminated expansive clay was further discussed.Conclusions were obtained as following.(1)Results obtained from the consolidated undrained(CU)tests show that measured stress-strain relationship curves all appear strain hardening features,and the failure form of contaminated samples prepared from copper sulfate with various concentrations of 10g/L,20g/L,30g/L,40g/L and 50g/L all show waist drum type.The triaxial shear strength of specimens of different contaminated degree of copper metal performed under the same confining pressure behave the rule of first increasing and then decreasing.The inflection point of peak value of the deviator stress corresponding to specimens of 629 k Pa prepared with the concentration of copper sulfate solution of20g/L is 17% greater than that of the plain expansive soil of 537 k Pa,while specimens prepared with copper sulfate concentration if 50 g/L is the smallest value of 491 k Pa and8.6% lower than that of plain expansive soil.When confining pressure is taken as the only variable,the maximal deviator stress of the specimen increases with increasing confining pressure.With the increase of pollution concentration,the internal friction angle decreases all the time,while the cohesion increases first up to 134.55 k Pa at the solution concentration of 20g/Land then decreases.The change in the strength is cohesion of the copper-contaminated expansive soil.This may be due to that that,the adsorption of copper ions by the montmorillonite in the expansive clay reduces the particle spacing when the concentration of copper pollution is lower than 20 g/L.That may reduce the adsorption of water molecules and promote the macroscopic strength of soil.While the concentration is over 20g/L,the adsorption of copper ions reachessaturation,then copper ions can destroy the cementation between soil particles to enlarger the particle gap.This may change the connection mode between particles,and result in the deterioration of strength in the copper contaminated expansive soil.(2)Results of unsaturated direct shear tests indicate that the shear strength of specimens increases first and then decrease with the increase of the concentration of copper ion.And the maximum of strength is 192 k Pa in the specimens of 20g/L copper ion,and 9.1% higher than the 176 k Pa of plain expansive clay,while the strength of the samples with a concentration of 50g/L is the smallest,value of 159 k Pa and is 9.7%lower than the value of the plain expansive soil.For the specimens with the same solution concentration of 30g/L,the shear strength increases with the increase of the net normal stress and the matrix suction.The unsaturated shear strength parameters of specimens performed on the suction of 200 k Pa are'1c of 114.5k Pa and'? of 18.65°.Meanwhile,for specimens performed under the net normal stress of 200 k Pa,the unsaturated shear strength parameters are'1c of 119 k Pa andb? of 18.55°.(3)Quantitative research on the microstructure on the failure plane in the unsaturated direct shear specimens shows that the plain expansive clay shows a typical flat or slightly curved structure of surface-to-surface.Soil particles are tightly bound and the pores are small in the copper contaminated expansive soil with the low pollution degree,but the face-face structure gradually breaks down with the increment of the coper ion,and changes into point-face and edge-face structures,and the number of larger pores gradually increases.These phenomena are consisted with characteristics of strength variations.(4)It is seen that the number of micro-pores with less than 1 um diameter increases first and then decreases on the failure plane in the unsaturated direct shear specimens prepared with increasing concentration of the copper ion,but the proportion of macro-pores with more than 10?m diameter decreases first to extreme values at 20g/L and then increases.From the parameters of shape and directionality,the roundness and probability entropy of directivity of soil pores and particles gradually decrease first and then increase with the increase of copper pollution degree and reaches 1.85 and 1.32,respectively,in specimens of the coper ion concentration of 20g/L.Their shapes are closest to the circle and the particles show an orderly trend in specimens prepared with the mildly contaminated soil of less than 20g/L,while the arrangement of pores and particles becomes disordered,and the circularity and probability entropy gradually increases in specimens with severely contaminated soil because the cementationbetween particles is destroyed and the shape becomes irregular again when copper ion is over more than 20g/L in soil.