Study On Strength Characteristics And Constitutive Model Of Frozen Silty Clay

In recent years,the artificial freezing method has been widely used in the subway construc-tion of Nanjing area,Permafrost scholar mastered the strength characteristics of typical frozen soil,however,the creep unloading,constitutive model and different consolidation and the influence of the stress path on the triaxial strength is very little.Therefore,it is necessary to understand the strength characteristics of the permafrost in the region by indoor experiment,and provide a reliable basis for the design and construction of the artificial freezing method in the future.In this paper,the strength characteristics of artificial frozen soils were studied by using the(3)-2b2-3 silty clay in Nanjing subway line 5,and the difference of strength characteristics between frozen and silty clay was studied,the strength of the frozen soil,the long-term strength,the resilience modulus and creep unloading of the remolded silty clay,and the constitutive model of the frozen silty clay considering the confining pressure based on the Duncan-Zhang model.The main conclusions are as follows:(1)The uniaxial compressive strength and elastic modulus of remolded frozen silty clay are 12% and 7% higher than that of the original frozen silty clay respectively;The uniaxial compressive strength increases linearly with the increase of water content,and decreases slightly when the soil is saturated,and the elastic modulus is related to the increase of water content;The uniaxial compressive strength first has a linear relationship with the strain rate(less than 10%/min)and then has a logarithmic function with the strain rate(greater than 10%/min),while the elastic modulus increases exponentially with the increase of the strain rate increase.(2)The modulus of elasticity of remolded frozen silty clay is hyperbolic with the number of loading and unloading of the specimen.When the number is more than 7 times,the modulus of elasticity tends to be stable,and the elastic modulus is about the same condition of the elastic modulus of 12 times,which is due to the plastic deformation of the total deformation ratio is rapidly reduced.(3)The cohesion and internal friction angle of the remolded frozen silty clay are 2% and 5% larger than that of the original frozen silty clay respectively;The maximum axial deviatoric stress and internal friction angle under different consolidation modes: isotropic drainage consolidation> isotropic non-drainage consolidation> first freeze after consolidate,where the latter two parameters are closer,and cohesion: first freeze after consolidate > isobaric non-drainage consolidation> isobaric drainage consolidation,but but the three values are similar;The maximum deviatoric stress under the conventional triaxial compression is 1.1 times of that of the reduced triaxial compression,while the elastic modulus under the conventional triaxial compression is 1.12 times of that of the reduced triaxial compression,which indicates that the stress path is different and the strength characteristics of the permafrost are different.Special attention should be paid to the project.(4)Based on the Duncan-Chang model,the constitutive model of frozen silty clay consider-ing the influence of confining pressure is established.The reliability of the model is verified by the comparison of the experimental value and the calculated value.The damage ratio of The original silty clay and remodeled silty clay(isotropic drainage consolidation,isotropic non-drainage consolidation and first freeze after consolidate)are 0.87,0.89,0.93 and 0.87 respectively.(5)At the same constant load level,the creep deformation of the remolded frozen silty clay is smaller than that of the original frozen silty clay,and the mathematical model is used to fit.The parameters show that the effect of the remolded frozen silty clay on the axial stress is greater than that of the original frozen silty clay,and the influence of the time is opposite;The long-term strength limits of the original and remolded frozen silty clay were 1.81 MPa and 2.31 MPa respectively(the long-term strength coefficients were 2.6 and 2.3 respectively),indicating that the ability of the remolded frozen silty clay to resist the long-term load was destroyed.(6)Under the three stress levels(0.3?,0.4? and 0.5?),the total rebound returns were 0.32%,0.72% and 0.96% respectively,and the instantaneous rebound strain averaged only 58% of the total rebound deformation,indicating that the results show that the elastic response and the total rebound strain are logarithmically increasing with the stress loading coefficient and the creep strain at 10h;Remolded silty clay unloading rebound curve can be fitted with a piecewise function.