Research On Mechanical Properties Of Structured Shanghai Clay Under Cyclic Loading

In recent years,the construction of high-speed traffic road networks and marine buildings in the southeastern coastal areas of China has been increasing.The soft clay layers in these areas have formed a strong structure during the deposition process.Under dynamic loads,engineering problems caused by structural clay foundations have become increasingly prominent.Although the study of the dynamic characteristics of soils has been around for a long time,the dynamic response of structural clays under cyclic loading still lacks systematic research.Research on the strength and deformation law of structural clay under cyclic loading not only has practical engineering significance,but also has significant scientific research value.In this paper,the strength,deformation and excess pore pressure characteristics of layer 4 with the lowest engineering properties in Shanghai clay are obtained by dynamic triaxial tests under different dynamic stress amplitude and loading frequency.The effects of structural properties on clay compressibility and particle arrangement are analyzed by comparing the results of the conventional consolidation tests and scanning electron microscopy of undisturbed clay and remolded clay.The effects of structural properties on the strength,deformation and excess pore pressure characteristics are obtained by dynamic triaxial tests of undisturbed clay and remolded clay.By numerically simulating the dynamic triaxial test of Shanghai clay,the effects of initial test conditions,loading conditions and initial structural properties on deformation and excess pore pressure are obtained.The main contents and related conclusions of this study are as follows:(1)The strength,deformation and excess pore pressure characteristics of layer 4 with the lowest engineering properties in Shanghai clay are obtained by dynamic triaxial tests.The increase of the dynamic stress amplitude will enhance the strength of the soil,accelerate the deformation of the soil,and accelerate the development of pore water pressure in the soil.The increase of the loading frequency will enhance the strength of the soil,delay the deformation of the soil,and accelerate the development of pore pressure in the soil.According to the research on the law of dynamic strength,it's preliminarily obtained that the critical cyclic stress ratio of Shanghai undisturbed clay is between 0.15 and 0.2.With the loading frequency increases,the dynamic strength curve of the clay shifts to the right,and the dynamic strength of the soil increases.causes the clay dynamic strength curve to move to the left and.When the loading frequency is low(f=0.02Hz),the damping ratio increases first and then decreases with the increase of axial strain,while the loading frequency is higher(f=0.5Hz and 2Hz),the damping ratio decays only as axial strain increases.In terms of dynamic deformation,under the same dynamic stress amplitude,the higher the loading frequency leads to the smaller the axial strain of the soil.At the same loading frequency,the larger the dynamic stress amplitude leads to the larger the axial strain.In terms of excess pore water pressure development,under the same cycle number and dynamic stress amplitude,the higher the loading frequency makes the larger the oscillating pore pressure and the cumulative pore pressure of the sample.Under the same cycle number and loading frequency,the larger the dynamic stress amplitude leads to the larger the oscillating pore pressure and the cumulative pore pressure of the sample.(2)The effects of structural properties on clay compressibility and particle arrangement are analyzed by comparing the results of the conventional consolidation tests and scanning electron microscopy of undisturbed clay and remolded clay.The structure leads to an increase in the compressibility of the soil and an increase in the porosity between the particles.The effects of structural properties on the strength,deformation and excess pore pressure characteristics are obtained by dynamic triaxial tests of undisturbed clay and remolded clay.The structure will enhance the strength of the soil,slow down the deformation of the soil and inhibit the development of pore pressure in the soil.In terms of dynamic strength analysis,the critical dynamic stress ratio of remolded clay and undisturbed clay is the same.The structure will make the dynamic strength curve of the soil shift to the right,the dynamic strength increases,the damping ratio decays faster,and the average effective stress decreases more quickly.In terms of dynamic deformation,the structure causes greater axial strain and also results in a faster rate of axial strain development.The inflection point position of the strain rate-strain curve is mainly determined by the magnitude of the dynamic stress ratio and is almost not affected by the structure.In terms of excess pore water pressure development,structurally leads to a significant increase in the cumulative pore pressure of the soil.In the initial stage of strain,structural influence on the development of oscillating pore pressure.After that,the oscillating pore pressure of structural soil samples tends to be stable,while the oscillating pore pressure of soil samples without structure continues to increase.Based on the test results,the dynamic strength curve and the cumulative pore pressureaxial strain curve can be used to evaluate the structure of the soil.(3)Based on cyclic triaxial tests results for Shanghai clay,the law of shear modulus degradation under different dynamic stress amplitude and loading frequency is analyzed,and a new shear modulus degradation model is established.The interpretation of experimental data confirms that the shear modulus degradation index has a logarithmic function with the number of cycles;it decreases linearly with the cyclic stress ratio,increases linearly with the loading frequency when plotted on a logarithmic scale.In the new degradation model,the effects of the number of cycles,the cyclic stress ratio,and the loading frequency are implemented explicitly.The applicability of the new model is verified by tests on five soft clays with different plasticity indices under different loading conditions.It should be pointed out that different from previous model,for each type of clay,only one set of parameters is necessary for describing the shear strength degradations under various loading conditions.(4)Finite element simulation results by using the method of complete water-soil coupling and the dynamic constitutive model of elastoplastic soil show that compared with the smooth boundary,the simulation results using the friction boundary are closer to the actual test results;the increase in dynamic stress amplitude and loading frequency leads to an increase in the range of shear bands in the sample;initial structure promotes the formation of the shear band within the specimen.In terms of initial geometric boundary conditions,with a low load frequency(f =0.02Hz)the initial defects of the sample(initial point defects,initial unilateral unevenness,initial bilateral unevenness)have no effect on the overall deformation and pore pressure of the sample,but the pore pressure and vertical strain will have extreme values at the defect.The defect is first destroyed.In terms of test loading conditions,the increase of the dynamic stress ratio will increase the unevenness of the pore pressure distribution in the sample and the width of the shear band in the sample.The increase of the loading frequency(from 0.02 Hz to 2Hz)will increase the difference of the pore pressure between the middle of the sample and the edge of the sample,and the width of the shear band in the sample will increase.Under the same dynamic stress ratio,the change of consolidation confining pressure has no effect on the pore pressure distribution in the sample.As the confining pressure increases,the range of the shear band in the sample gradually decreases,and the dynamic strength of the sample increases.In terms of initial structure,the initial structure leads to uneven distribution of pore pressure in the sample,and promotes the formation of shear bands in the sample,causing local damage to the sample;the pore pressure distribution in the sample without structure is uniform,and no obvious shear band occurs,and the sample is destroyed overall.