Physical-Chemical-Mechanical Properties And Evaluation Methods Of Soil Around Subway Tunnels After Fire

As subway travel gradually becomes integrated into people’s lives,subway safety issues have also become increasingly important.Among many subway safety incidents,the damage caused by subway fires is particularly prominent.Especially in the Shanghai area,many large-scale underground transportation facilities are built on the saturated soft clay layer of the shallow marine deposits.This kind of soft clay has a high organic matter content,and under the influence of high temperature,its physical and mechanical properties will inevitably undergo irreversible changes,which has laid hidden safety hazards for the post-disaster reconstruction of subway tunnels.This thesis takes the fourth layer of gray silty clay as the research object in Shanghai,and studies the thermal reaction temperature range of soft clay and the changes in its physical and chemical properties after high-temperature action through TGA-DSC experiments and geotechnical tests.The static mechanical properties of saturated soft clay after hightemperature action are analyzed through triaxial tests.The dynamic characteristics of saturated soft clay after high-temperature action are studied through graded loading tests and dynamic triaxial tests.Based on the experimental data,the relationship between the pH and static and dynamic mechanical properties of soft clay under different high-temperature actions is analyzed,and a static and dynamic mechanical property evaluation method based on the pH of the soil mass after high-temperature action is proposed.The research results mainly include the following 6 aspects:(1)Through TGA-DSC experiments,it was found that the soft clay is in an endothermic state in the temperature range of normal temperature to 105℃,and in an exothermic state in the temperature range of 105-500℃.The ignition point of organic matter in the soft clay is in the temperature range of 105-500℃.(2)High-temperature action has a significant influence on the physical and chemical properties of soft clay.In the temperature range of normal temperature to200℃,with the temperature increasing,the color of soft clay changes from black to gray-brown,water-holding capacity decreases,liquidity limit and saturation moisture content change,organic matter content gradually decreases,pH gradually increases;In the temperature range of 200-300℃,the physical and chemical properties of soft clay remain unchanged.In the temperature range of 300-500℃,with the temperature increasing,the color of soft clay gradually changes to yellow-brown,water-holding capacity increases,organic matter content gradually decreases,pH gradually increases.(3)In the triaxial test,the change of the static mechanical characteristics of saturated soft clay after high-temperature action can be divided into three stages: in the temperature range of normal temperature to 200℃,with the temperature increasing,the stress-strain curve gradually increases,the excess pore water pressure curve gradually decreases,the effective cohesion gradually decreases,and the effective internal friction angle gradually increases;in the temperature range of 200-300℃,the static mechanical characteristics of saturated soft clay remain unchanged;in the temperature range of300-500℃,with the temperature increasing,the stress-strain curve and the excess pore water pressure curve gradually increase,the effective cohesion and the effective internal friction angle gradually increase.(4)In the graded loading test,the change of the dynamic characteristics of saturated soft clay after high-temperature action can be divided into three stages: in the temperature range of normal temperature to 200℃,with the temperature increasing,the hysteresis loop and skeleton curve gradually decrease,dynamic shear modulus gradually increases within the small strain range(0-0.1%),and gradually decreases within the large strain range(0.1-10%),and the damping ratio gradually decreases;in the temperature range of 200-300℃,the dynamic characteristics of saturated soft clay remain unchanged;in the temperature range of 300-500℃,with the temperature increasing,the hysteresis loop and skeleton curve gradually increase,dynamic shear modulus gradually decreases within the small strain range(0-0.1%),and gradually increases within the large strain range(0.1-10%),and the damping ratio gradually decreases.(5)In the dynamic triaxial test,within the temperature range of normal temperature to 200℃,with the temperature increasing,the strain development curve gradually increases,the excess pore water pressure development curve and dynamic shear modulus development curve gradually decrease.When the temperature is increased from 200℃ to 500℃,the strain development curve and the excess pore water pressure development curve decrease,and dynamic shear modulus development curve increases.At the same time,the degree of influence of loading frequency and dynamic stress amplitude on the strain development curve,the excess pore water pressure development curve,and dynamic shear modulus development curve of saturated soft clay under different high-temperature actions is different.(6)For the fourth layer of gray silty clay in Shanghai,there is a significant linear correlation between the pH and the static and dynamic mechanical properties of soft clay under different high-temperature actions in the range of pH ratio pH/pH0 from1.00 to 1.09(normal temperature to 300℃).The change of the static and dynamic mechanical properties of soft clay after the fire can be preliminarily inferred by measuring the ratio of the pH of the soft clay after the fire to that of the soft clay under normal temperature.This thesis includes 67 figures,7 tables,and 111 references.