Research On Unloading Rheology And Damage Law Of Soft Soil In Shield Tunnel

The rapid development of economics and population in southeastern coastal cities makes the carrying capacity of the ground transportation approach to the limit.Thus,a large number of underground railway project are under construction.However,the marine soft soil with special physical characteristics are widely distributed in coastal areas,which can lead to long-term longitudinal deformation and uneven settlement problems in the construction and operation.Therefore,the study of soft soil unloading and rheological effects during the construction of shield tunnels in marine soft soil areas can support the prevention and solution of shield tunnel deformation and settlement problems theoretically.Our research is based on the underground railway project in Ningbo.The soft soil which comes from the project is used for the unloading rheological tests and microscopic scanning tests.Combined with microscopic parameters,the damage variable equation of soft soil is established.The development law of soft soil damage is explored.The main work of this paper are as follows.(1)Based on the underground railway project of Ningbo,soft soil in the project area is used as samples.Several basic physical parameters of the natural soft soil are obtained through a series of tests.(2)The natural soft soil are used to the triaxial unloading rheological tests.The relationship between strain and time are obtained with different initial stress conditions and unloading paths.The test results demonstrate the effects of the unloading rheological.The laws of unloading rheological is discovered by different conditions of soft soil.(3)Scanning electron microscopy(SEM)is used to microscopically scan the unloaded soft soil specimens and the natural soft soil specimens.Microscopic grayscale images of the cross-section and longitudinal section of the specimens are obtained.Gray image is used for binary processing and microscopic pore recognition statistics by Image Pro Plus software.The microscopic pore quantity,diameter,and area distribution data of the test specimens are obtained.The influence of unloading process on the micro pore change of soft soil is discussed.(4)The damage law of soft soil specimens is analyzed by macroscopic unloading rheology law.The influence of the microscopic pore distribution is also discussed.The soft soil damage variable expressions are derived by the microscopic pore structure change law.The soft soil damage development process is revealed by drawing the soft soil damage law evolution curve.