Study On Tests Of Failure Criterion And Dynamic Properties Of Silt Under The Cyclic Loading

Yunnan province is located in the east of collision zone of India tectonic plate and Eurasia plate. It is one of main seismic zones in our country. The seismic design intensity of Yunnan province is above 6 degree. There is 84 percent of Yunnan province's area where the seismic design intensity is 7-9 degree. In addition, the cause of formation, physical and mechanical properties of Yunnan silt is very different from the silt in the other areas. By using the conventionality analysis method and the energy analysis method, the failure criterion and the dynamic properties of Yunnan silt are studied based on laboratory dynamic triaxial tests in this dissertation.The experimental results show that Yunnan silt would liquefy only under isotropic consolidation. The state of limit equilibrium and the state that the strain is 5.0%, which state will be reached first is dependent on the condition of consolidation. If the consolidation ratio is 1.0 or 1.5, the state of limit equilibrium would be reached before the strain reaches 5.0%. If the consolidation ratio is 2.0, the case would reverse.The research on the failure criterion, the dynamic shear strength and the failure stress condition of Yunnan silt using the conventionality analysis method indicate: 1) The initial static stress condition, of Yunnan silt is one of the most important factors that affects its dynamic shear strength. 2) When the consolidation ratio is the same, the effect of failure criterion on the dynamic shear strength would reduce with the failure time of vibration increasing. 3) The failure stress conditions of Yunnan silt on the maximum cyclic shear surface associated with the stain failure criterion and the limit equilibrium failure criterion both can be expressed by the same quadratic formula.The developments of strain and pore water pressure in Yunnan silt during the process of vibration have been studied. The results show the limit pore water pressure is related to the initial static stress condition and the cyclic loading intensity. The relationship of limit pore water pressure, average stress, deviator stress ratio and cyclic axial stress can be expressed by a regression relation. Moreover, the modes of the developments of strain and pore water pressure of Yunnan silt during the process of vibration have been found. And their parameters have been fitted with average stress, deviator stress ratio and cyclic axial stress in order to take the effect of initial static stress condition and cyclic loading intensity into account.From the research on the liquefaction process of Yunnan silt, several conclusions can be reached. 1) The stress-strain hysteresis loop will gradually turn to the shape close to parallelogram, and then the changes become steady. At last, there is only axial strain amplitude increasing. 2) The dilation in Yunnan silt during the process of vibration occurs at the moment when the axial compressive stress acting. This situation is different from the sand's. In this dissertation, it is deduced that it is the different grain composition between Yunnan silt and sand that induced the difference above.In the process of dynamic triaxial tests, the data of stress and strain can be auto-collected by computer. Based on this data, the stress-strain hysteresis loops can be drawn out, andthen their areas can be got, and the accumulate energy dissipation of each time of vibration finally can be got. Based on this, the limit accumulate energy dissipation, the relationship of strain and pore water pressure are discussed in this dissertation. The conclusions show: 1) There is a regression relation between limit accumulate energy dissipation and the failure time of vibration. 2) When the failure criterion is decided, the limit accumulate energy dissipation can be fitted by a formula which includes the average stress, deviator stress ratio and cyclic axial stress. 3) Under isotropic consolidation or anisotropic consolidation, the developments of strain and pore water pressure in Yunnan silt is closely related to accumulate energy dissipated in the soil during the process of vibration. And then the energy modes of the developments of strain and pore water pressure can be found. The modes have taken the effect of initial static stress condition and cyclic axial stress into account.