Research On Dynamic Characteristics Of Hangzhou Soft Clay Under Intermittent Cyclic Loading On Macro And Micro Scales

In China,most coastal cities are more developed than the inland,and are the region with the highest mobility coefficient of residents and rail transit passenger volume.Also,the coastal areas are widely distributed with marine saturated soft clay.When they are load with long-term cyclic subway train load,they will generate cumulative deformation while their stiffness is softening,resulting in uneven settlement and instability of the track foundation,causing the shield tunnel to be uneven,disjointed,etc.These diseases not only reduce the long-term service performance of rail transit,but also bring huge economic losses.By the way,the subway load is a typical non-continuous dynamic load,which is formed by alternating transient vibration and long intermittency.Therefore,for experiment study of soft clay's dynamic characteristics,intermittent cyclic loading is required to simulate the subway load.In view of the fact that the dynamic characteristics of intermittent cyclic loading are different from continuous vibration,it is urgent to carry out the experimental study on the dynamic characteristics of saturated undisturbed marine sedimentary silty soft clay under intermittent cyclic loading,and provide theoretical basis for laboratory experiments under the matched working conditions for engineering practice.In this study,high-quality in-situ soft clay in west of Hangzhou was obtained by drilling core method.A series of intermittent cyclic loading dynamic characteristics test was carried out by GDS dynamic tri-axial instrument.Scanning electron microscopy and nuclear magnetic resonance is mainly applied in their microstructure characteristics study.Then,the detailed research is as follows:1.The undrained shear tests of isotropic and anisotropic consolidation under different confining pressures were carried out for the undisturbed saturated marine sedimentary silty soft clay in the west of Hangzhou.The stress and strain of the undisturbed saturated silty soft soil under different confining pressures were studied,including the relations of pore pressure and strain,stress and strain,and the development of effective stress path.Based on that,the critical failure line of this soft soil is determined.Via comparison of undrained shear test results on K0 consolidated samples and anisotropic consolidated samples,the structural properties of undisturbed silty soft soil is studied.It is found that K0 consolidation is favorable for shear strength,and has significant influence on stress-strain relationship and effective stress path development.Therefore,the differences in static characteristics of soft soil under two consolidation modes were analyzed.2.According to the discontinuity of subway load,the long-term intermittent cyclic loading experiments on K0 consolidated saturated silty soft clay were carried out,with different vibration duration and intermittency duration in unit cycle.Based on the test results,the pore water pressure,plastic strain and the stiffness softening coefficient under continuous vibration and intermittent vibration were studied.Development,stiffness softening difference,refinement test data in-depth analysis of the impact mechanism of intermittent pressure on pore pressure,stiffness softening,analysis of intermittent effects from the development trend and residual strength,based on the results of different intermittent duration test,further analysis of intermittent duration and The effect of intermittent effect on the macroscopic dynamic characteristics of undisturbed saturated soft clay was studied.3.Under continuous vibration with different relative deviatoric stress levels,the dynamic behavior of undisturbed saturated soft clay consolidated along K0 stress path was studied,i.e.,the development of pore pressure and strain.It is found that the pore pressure and strain growth rate are attenuated during vibration,and the decay process is independent of the relative partial stress level.In this way,an explicit prediction model for cumulative pore pressure and strain under continuous vibration is established.Then based on that,the development law of residual pore water pressure and plastic strain under intermittent cyclic loading was studied.To analyze the effect of the ratio of intermittency and vibration and the vibration duration in unit cycle on the growth rate of pore pressure and strain,the initial growth rate of the pore pressure and strain and the subsequent growth rate decay process was separated.Finally,this paper established an explicit prediction model of the pore pressure and strain under intermittent cyclic loading,which can consider the influence of relative deviatoric water,the ratio of intermittency and the vibration duration in unit cycle.Furthermore,the apparent reason of intermittency effects,the effect of dynamic stress level and initial static deviator stress on the relationship between pore pressure and strain during cyclic loading were studied.4.By mathematical integration method,the hysteresis loop area and damping ratio during cyclic loading are calculated.Afterward the change of stiffness and damping ratio of K0 consolidated saturated soft clay during long-term intermittent cyclic loading were calculated.In the study of stiffness softening prediction,the effects of intermittency and the effect law of intermittency duration on final residual stiffness and stiffness softening rate were discussed.And the a segmentation prediction model of stiffness softening was established to calculate the softening rate,softening duration and residual softening coefficient.Furthermore,based on the hysteresis loop area data,the damping ratio change during cyclic loading is calculated.It is found that the damping ratio is L-shaped attenuation during cyclic loading,and the inhibition effect of intermittency on damping ratio attenuation is analyzed.5.The microscopic characteristics of K0 consolidated undisturbed saturated soft clay before and after tri-axial tests were analyzed.Scanning Electric Microscope method and Image Pro-Plus software were used to analyze the changes of microstructure unit,pore shape and particle contact.Quantitative analysis of microstructure characteristics under continuous vibration and intermittent vibration was performed,including the difference of micro pore shape,distribution of pore diameter and area,pore circularity and orientation distribution,to analyze the different micro-responses of continuous and discontinuous vibration and the influence of intermittency duration.By the way,the correlation analysis between the macroscopic and microscopic mechanical properties was carried out.Via that,the microscopic mechanism that produces the intermittency effect was found.Secondly,combined with nuclear magnetic resonance test and dynamic tri-axial test,the changes of pore water and porosity before and after cyclic loading were studied.It was found that the long-term vibration would cause the chemically bound water in the pore water of the soil to be converted into physical combined water and free water,so that the pore water pressure rises and the stiffness softens.Finally,based on the volumetric flask method and methylene blue adsorption method,the changes of adsorption water before and after three typical loads were studied.The effects of different loading conditions on the pore water and pore of soil were analyzed.