| In recent years, more and more offshore engineering has been built. With the engineering becoming deeper and lager, the bearing capability of soft clay foundation must meet higher requirements. Soft clay has large void ratio, high moisture content and low strength. Furthermore, strength reduction may take place for the soft clay under the action of repeated wave cyclic loading. If safety reserve is not enough, the construction will be destroyed by excessive settlement and movement, which causes enormous economic losses. In view of the above problems, the character of undrained shear strength weakening in different confining stress, deviator stress, dynamic stress, cycle times and consolidation degree has been study by means of performing cyclic triaxial test on muddy-silty clay taken from Yantai littoral zone.The main contents are abstracted as follow:1. To study the accumulative pore pressure and accumulative deformation, cyclic triaxial test has been done. According to the results, accumulative pore water pressure variation and accumulative deformation development subjected to different confining stress, deviator stress, dynamic stress and cycle times has been analyzed. Then the modified hyperbolic calculation model of pore water pressure and accumulative deformation was put forward through data fitting.2. To study the undrained shear strength weakening, undrained shear test after cyclic loading has been done. The laws of weakening under different confining stress, deviator stress, dynamic stress and cycle times has been analyzed based on the test. By using the modified theory of quasi-overconsolidation, the pore pressure model and the laws of undrained shear strength were combined. As a result, a strength weakening formula of normally consolidated soil which reflect the whole time history was proposed. And it can be used to the dynamic computing process. In order to meet different engineering needs, the model is divided into two: precise computing model and simplified computing model. And the experiment was done to verified the formula. The scope of the formula was also illustrated.3. The mechanism of strength weakening of the clay has been analyzed. Based on the test, critical dynamic stress ratio of the clay is 0.5. If dynamic stress ratio is less than 0.5, the main reason for strength weakening is the rise of accumulative pore water pressure. If it is lager than 0.5, structural damage is the main reason. And if dynamic stress ratio equals 0.5, the combination of the two leads to strength weakening of the clay.4. To study the effect of consolidation degree, cyclic triaxial test of different consolidation degree has been done. The test shows, with consolidation degree going down, the strength of the clay decreases linearly. And on the basis of the strength weakening model of normally consolidated soil, the effect of consolidation degree was added to the model by using the concept of softening factor. At last, the weakening model considering multiple factors was given.5. Then the model of strength weakening was applied to the dynamic numerical model of breakwater in Yantai Port through the use of ABAQUS. The effect of strength weakening and soil consolidation degree were analyzed based on the results of the numerical model. At last, some effective suggestions were put forward to solve engineering problems. |
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