An Experimental Study On Strength Properties Of Net-like Red Soil Polluted By Alkali

Reticulated laterite widely exists in the south of the middle and lower reaches of the Yangtze River in China.With the development of urban construction,there are many examples of the application of Reticulated laterite in engineering,and the corresponding alkali pollution problem is becoming more and more serious.At present,there are few studies on alkali-contaminated soil in China.Therefore,it is of great significance to carry out the strength characteristics test of alkali-contaminated remolded Reticulated laterite.In this paper,the mechanical properties of alkali-contaminated reticulated laterite were analyzed by axial test.The mechanical properties of alkali-contaminated reticulated laterite were verified by scanning electron microscopy and mechanism of action of alkali-contaminated laterite.(1)The results of unsaturated direct shear tests show that the shear strength increases first and then decreases with the increase of NaOH concentration,and eventually tends to be stable.When NaOH concentration is 3.5%,the maximum shear strength is 222 kPa,which is 22.7% higher than that of plain laterite.When NaOH concentration is 21%,the minimum shear strength is 151 kPa,which decreases by 16.6%,brittle failure occurs.The effect of contamination time was similar to that of concentration.The maximum shear strength was 237 kPa after 14 days immersion,an increase of 30.9%.As for the pollution concentration effect of shear strength parameters,the cohesion increases at first and then decreases.The cohesion increases by 92.8% in 3.5% NaOH,decreases by 66.7% in 21% NaOH,and the internal friction angle changes in the opposite direction.(2)The triaxial saturated shear tests show that the shear behavior increases first and then decreases with the increase of NaOH concentration,and finally tends to be stable.When NaOH concentration is less than 7%,the curve shows hardening characteristics.When NaOH concentration is more than 7%,the curve shows softening characteristics.When NaOH concentration is more than 7%,the curve shows brittle failure.When NaOH concentration is 3.5%,the maximum principal stress appears to be 694.79 kPa.Compared with the plain soil sample,the increase was 23.9%.Pore water pressure drops sharply after soaking in NaOH and keeps a low value.The transmission of pore water pressure is obviously hindered.When the NaOH concentration is less than 7%,the stress curve shows the characteristics of right-to-right first,then left-to-right,and finally right-to-right.When the NaOH concentration is more than 7%,the curve shows the typical softening characteristics of right-to-left first.(3)Microscopic characteristics test showed that when NaOH intruded,soil particles appeared adhesion,surface-surface structure was destroyed,and tended to concave-convex or flocculent aggregate structure.After 3.5% NaOH contamination,soil joints became closer,pore volume decreased,pore area larger than 0.2 micron 2 decreased by 9.61%,pore circumference larger than 5 micron decreased by 10.83%;after 21% NaOH contamination,soil mass decreased by 10.83%.The pore size is larger than or equal to 0.2 um 2.The pore size increases by 12.54% and the pore circumference increases by 10.13% when the pore size is larger than or equal to 5 um.(4)The mechanism analysis of alkali-soil interaction shows that six different mechanisms of alkali-soil interaction can be divided into three stages according to the concentration of NaOH.When the concentration of NaOH is less than 7%,it is cementation adsorption stage.The main action of alkali-soil interaction is cementation and adsorption.Colloids and Na+ are adsorbed in the pore,which reduces the pore size and optimizes the physical and mechanical properties of the soil.When the concentration of NaOH is less than 14%,it is material exchange.In the stage of alkaline soil action,cation exchange,erosion and dissolution are the main factors.Soil particles are dissolved,colloidal and salt precipitation in the pore,and the structure of double electric layer is thicker,which makes the pore of soil increase and the physical and mechanical properties deteriorate.When NaOH > 14%,the ion exchange rate reaches equilibrium,and does not change with the increase of NaOH concentration.Soil physical and mechanical properties tend to change.Stable.