Dynamic Behavior Analysis Of Soft Clay Under The Cyclic Loading

China is an active earthquake country.Earthquakes occur frequently these years,which have some characteristics such as great magnitude,low frequency and shallow epicenter and so on. With the development of economy,the concentration of urban population and the development of civil engineering,the importance of anti-earthquake and prevention is increasingly prominent in geotechnical engineering.Taking Shantou soft clay as the research object,a series of static and dynamic triaxial tests are studied.The results show that there exist turning points during the progress of dynamic strain,which can be used to determine the failure criterion of strain;Delay of pore water pressure is the other significant characteristic of dynamic behavior of soft clay.On dynamic research of sand,pore water pressure is used as the criterion of liquefaction;But it is difficult for soft clay to measure the pore water pressure during dynamic triaxial tests.In order to verify the remarkable delay of pore water pressure,a special test method is used to capture the cumulative pore pressure.According to the approximately linear relationship between measured pore pressure and cyclic numbers,the expression of pore pressure is built on the phenomenon of delay of pore pressure.And then,Shantou soft clay sites are studied to analyze the seismic response.It is concluded that soft clay has the ability of either amplification effect or isolation effect,which depends on the seismic waveform,the input acceleration,the appearance time of peak acceleration and the location of soft clay.There exist extreme values while meeting soft clay layers.The soft clay layers filter out a significant portion of the high frequency content of the bedrock acceleration and amplify the portion of the low frequency content.Seismic subsidence of soft clay is another hot research topic.Behavior of static triaxial test after cyclic loading conditions experiences two stages:elastic and strain hardening,when the maximum dynamic axial strain is less than the failure strainε_tp.If over theε_tp,the relationship between deviatoric stress and strain is linear,and the shear strength reduces rapidly.Compared to the static triaxial test after cyclic conditions,conventional static triaxial test experiences three stages:elastic,elastic-plastic and strain hardening.At last,according to the liquefaction potential criterion of sand,a new way of judging seismic subsidence of soft clay is established on the theory of disturbed state concept.Based on the theory of disturbed state concept formed by professor C.S.Desai,a dynamic constitutive model considering structural properties,nonlinear and hysteretic nature of soft clay is built.In the model,two reference states are defined,which are relative intact state(RI) and full adjusted state(FA).Hardening soil model are used for RI state,and the critical state for FA state in this paper.Disturbed function connects the RI state with the FA state to reflect the observed behavior.The microstructure change under the disturbance of applied loading can be considered by macroreaction using the disturbed function.