Nonlinear Strength And Stress Strain Characteristics Of Peat Soil In Excavation Path

Peaty soil is a kind of special soft soil with poor engineering properties,which is composed of humic substances,minerals and organic residues.It is characterized by low natural density,high moisture content,organic matter content and fiber content,poor engineering properties,and large spatial variability.China’s peaty soil is rich in reserves and widely distributed,and peaty soil is a kind of special soil with strong regional characteristics.Specific environment and climate conditions lead to the peaty soil in different regions with unique structures,and the differences in structure will make the strength properties of the soil show significant differences.Taking the lacustrine deposit peaty so il deposited in Kunming area as the research object,the analysis of a large number of measured data of foundation pit engineering found that the deformation and design value of the soil and supporting structure around the foundation pit are quite differen t,especially on the saturated peaty soil site.The reason is that the design parameters of foundation pit support generally do not consider the influence of excavation unloading and resulting in unreasonable design.In this paper,combined with the actual characteristics of foundation pit excavation,laboratory soil test,high pressure consolidation test and triaxial test with different excavation paths were carried out by taking thin-walled undisturbed soil samples on site,and related research was carried out on the stress-strain relationship and deformation characteristics of different excavation paths under K₀consolidation.The main research contents and achievements are as follows:（1）（1）Using secant modulus method to calculate foundation settlemen t has gradually become familiar to scholars due to its unique advantages,and a large number of studies have found that the calculation results are small when using this method to calculate foundation settlement.The traditional secant modulus method regards the relationship between vertical stress and vertical strain as hyperbolic form and ignores the reference configuration,which limits the application range and affects the calculation accuracy of the method.A large number of high pressure consolidation tests have been carried out on peaty soil and silty soil in Kunming and the stress-strain curves of the two soils have been fitted with various functions.The experimental results show that,compared with the hypothesis of hyperbolic model,the Gunary model can better characterize the stress-strain curves of Kunming’s peaty soil and silty soil under the condition of lateral confinement.Based on the modified secant modulus method with the introduction of correction coefficientγ,the foundation deformation formula considering the change of overburden load is established.（2）In order to explore the influence of peaty soil’s characteristic structure on engineering properties,the progressive evolution process of peaty soil strength was considered,the exert ed cohesion and internal friction angle of the original and reconstructed peaty soil samples were calculated,and the change law of exerted cohesion and internal friction angle was analyzed,so as to explain the evolution process of peaty soil strength.In this paper,triaxial consolidation undrained shear tests were carried out on the undisturbed and remolded peaty soil samples under different consolidation confining pressures in the fifth floor of Kunming National Convention Center.According to the experimental results,the influences of burial depth and confining pressure on the mechanical properties of the original and reshaped peaty soil were analyzed from the aspects of stress-strain curve,residual strength,secant modulus,effective stress path and allowable limit strain.The results show that the strength parameters of original peaty soil and remodeled peaty soil are affected by different factors.（3）Considering the real stress path during excavation in foundation pit engineering,a triaxial excav ation unloading path test of undisturbed peaty soil samples from Qinglanjun project site in Kunming City was carried out by using TSZ-1S stress control triaxial apparatus.According to the unloading path during excavation,the tests are divided into latera l unloading test,axial unloading test,axial lateral unloading test,conventional triaxial loading,loading and unloading failure comparison test under K₀ consolidation.The stress-strain relationship,the variation of pore water pressure and the variation of shear strength were compared.It is found that mechanical parameters of peaty soil are obviously different under different stress paths.For peaty soil,the strength of CU test is greater than that of K₀ consolidation test under low confining pressure,and the strength difference decreases with the increase of confining pressure.The mechanical properties of soil change under different unloading paths,and the changes are different under different stress paths.Due to the small confining pressure reduc tion and the different ways of shear stress generation,the curve shape,Mohr circle development direction,pore pressure development and pore pressure coefficient Af of lateral unloaded soil are significantly different from that of the loading test.（4）The derivation process of Duncan-Zhang nonlinear elastic model is simulated,and the applicability of peaty soil with different excavation unloading stress paths to the constitutive model is verified by reference to the stress-strain curve under soil unloading,and the initial tangential modulus under loading and unloading conditions is given.Considering different normalization factors,the normalization equation of Kunming peaty soil with fixed mathematical structure was constructed,and different normalization factors were selected for different consolidation modes to normalize stress-strain curves of different paths.It is proved that the results are reliable in a certain range.