Experimental Study On Strength Characteristics Of Net-like Red Soil Contaminated By Organophosphorus Pesticides

Net-like red soil is one of the main bearing layers in the middle and lower reaches of the Yangtze River in southern China.With the development of engineering construction,the engineering problem caused by net-like red soil contaminated by organophosphorus pesticides becomes more and more prominent.However,there are few reports on the engineering properties of net-like red soil contaminated by organophosphorus pesticides,it is of great significance to carry out the research on the strength characteristics of it.Based on the artificial production of net-like red soil contaminated by organophosphorus as the research object,through saturated unsaturated direct shear test and triaxial test to study mechanics properties change law,and the corresponding microstructure sem test analysis characteristics,to explore the organophosphorus pesticide pollution net-like red soil performance evolution mechanism,the main conclusions are as follows:(1)According to the unsaturated direct shear test,the unsaturated direct shear strength of net-like red soil is related to the contamination concentration of organophosphorus pesticides,matric suction and net normal stress.When the concentration of organophosphorus pesticides was the only variable,the direct shear strength of soil first increased and then decreased with the increase of the concentration.The plain soil strength was 160k Pa,and when the pollution concentration was 6%,the strength increased to a maximum of 193k Pa,and when the pollution concentration was 12%,the strength decreased to 171k Pa.When the contamination concentration is 6%,the direct shear strength increases with the increase of matric suction and static normal stress.When the net normal stress is200k Pa,the shear strength parameter of soil is c=81.17k Pa,φ=29.78°.When the matric suction is 200k Pa,the shear strength of soil is c=70.33k Pa,φ=33.27°.(2)From the results of undrained triaxial consolidation shear test,it can be concluded that the triaxial specimen is in the shape of"drum"after failure,and the stress-strain curve of the specimen has no obvious peak value,showing strain hardening.When the confining pressure is constant at 300k Pa,the maximum deviatoric stress of plain soil is 606k Pa,which increases first and then decreases with the increase of pollution concentration.When the pollution concentration is 6%,it reaches the maximum of 895k Pa,and when the concentration is 12%,it decreases to596k Pa.The maximum deviatoric stress of soil samples increases with the increase of confining pressure when the pollution concentration is constant.According to the analysis of the shear strength index,the cohesion of soil decreases with the increase of the contamination concentration,and the internal friction Angle increases first and then decreases and reaches the maximum value at the concentration of 6%.(3)According to the qualitative analysis of microstructure,it can be concluded that the soil with low concentration of organic phosphorus pesticide pollution and plain soil and low concentration of organic phosphorus pesticide pollution have smaller pores among the soil particles,and the connection is relatively close,showing the form of dense plane-plane laminate.When the concentration of organophosphorus pesticides was lower than 6%,soil particles became denser with the increase of pesticides.This was because soil colloidal particles absorbed pesticide components,leading to smaller pores among soil particles.When the concentration of organophosphorus pesticides increased to 9%,pesticide components reacted with soil,soil structure was gradually eroded and changed,flocculent matter increased,and the pores between soil particles increased,which verified from the microscopic perspective that a certain concentration of organophosphorus pesticides would lead to the decline of soil shear strength.(4)According to the results of microstructure quantitative analysis,the number of pores in the contaminated soil is increased relative to that in the uncontaminated soil.With the increase of the concentration of organophosphorus pesticides,the proportion of small pores increases first and then decreases,while the proportion of large pores decreases first and then increases.In the uncontaminated soil,the proportion of pores with a pore area of 0～0.2μm~2is 77.6%,the proportion of pores with a pore diameter of 0～0.25μm is 41.13%,and the proportion of pores with a pore perimeter of 0～2μm is 73%.When the contamination concentration is 6%,the proportion of the three increased to 91.6%,45.3%and 83.3%.At 12%pollution concentration,the proportions of the three decreased to 81.6%,39%and 79%.