Study On Pore Structure And Permeability Of Thermally Desorbed Diesel-contaminated Soil

Although thermal desorption is highly effective in removing contaminants from contaminated sites,the additional high-temperature environment changes the material composition of the soil,resulting in changes in the pore structure and permeability of the soil,which has an impact on the slope stability,foundation bearing capacity and settlement of the contaminated site.Therefore,it is of great theoretical significance and practical application to obtain the pore structure and permeability of thermally desorbed contaminated soils for the design and construction of the foundation of thermally desorbed contaminated sites.In this paper,the pore structure and permeability of thermally desorbed contaminated soil are studied,and the main research work and results are as follows.(1)Based on the site geothermal desorption profile,the key elements of the indoor thermal desorption simulation were first identified as the heating method and heating temperature respectively.Secondly,based on the site contaminant characteristics,diesel was identified as the contaminant for the indoor simulation of thermal desorption experiments,and its content range was set.The layered and hydrostatic methods were then found to be more suitable for the preparation of contaminated soil.Finally,an indoor simulated thermal desorption test apparatus was designed and manufactured in-house.(2)The pore structure evolution of the contaminated soil was tested using aging time and diesel content as influencing factors,and the pore structure of the contaminated soil was monitored during the test with the aid of mercury-pressure porosimetry(MIP)and X-ray micro-computed tomography(XRμCT).The results showed that the aging time reduced the porosity and average diameter as well as increased the tortuosity.However,after 30 days of aging time,the pore structure stabilized.The diesel content increased the porosity and average diameter of the contaminated soil and decreased the pore tortuosity.A two-factor analysis revealed the extent to which aging time and diesel content affected the pore structure and a physical model was obtained between them.Mechanistic analysis revealed that inter-particle cohesion,bound water content,and diesel adsorption were the main reasons for the changes in pore structure due to aging time and diesel content.(3)Contaminated soil with a stable pore structure was selected for the study.Indoor simulated thermal desorption experiments were carried out with different heating methods,heating temperatures,and diesel content,and the basic physical properties and pore structure of the thermally desorbed diesel-contaminated soil were measured.The results show that thermal desorption causes the soil to change from cohesive to sandy soil,but the change in pore structure does not correspond to the change in soil properties.It is also found that the heating method controls the direction of change in the pore structure of diesel-contaminated soils,and the heating temperature and diesel content affect the magnitude of change in the pore structure.The mechanism analysis suggests that the heating method influences the mode of matrix suction,resulting in an opposite trend in the pore structure of the soil,with the clay minerals starting to affect the pore structure after a heating temperature of 400 ℃.(4)Tests on the permeability of diesel-contaminated soils before and after thermal desorption revealed that both aging time and diesel content weakened the pore permeability of the contaminated soils.It was also found that pore structure,soil particle hydrophobicity,and pore-free diesel were responsible for the weakening of pore permeability.The permeability coefficients of thermally desorbed diesel-contaminated soils decreased with increasing heating temperature and were all greater than those of diesel-contaminated soils.The difference in pore structure caused by the strength of the potential for water absorption and swelling in the thermally desorbed diesel-contaminated soils and the pore-free diesel are the main reasons for the difference in pore permeability before and after thermal desorption.The fractures in the soils decrease in size or even heal after water absorption and saturation,and the mechanism of their influence on permeability is the same as that of pore space on permeability.