Research On Earthquake Induced Liquefaction And Lateral Displacement Of Submarine Soil Layer In Near Fault Area

Liquefaction and lateral displacement of submarine soil layer damage offshore platform and submarine pipeline due to earthquake, which emerged as an important and difficult problem. At the meantime, near the seismic fault, most earthquake damages have been found and submarine slope has more possibility to produce lateral displacement. So, in this dissertation, the two relatively independent researches, which have related on the other hand, have been done. One is near-fault peak ground motion attenuation; the other is earthquake induced liquefaction and lateral displacement of submarine slope. In the former engineering seismic study, an attenuation model is proposed for estimating near-fault strong earthquake peak ground motion, regression analysis have been done with all the near-fault earthquake data. Compared with attenuation relations derived from middle and far-field, the proposed relationships have more coincidental with ground motion near the fault. The results of this study can provide input parameters for the latter one, and that can be applied to parameters estimation of earthquake ground motion and earthquake resistant design in active fault area. Based on the mechanics analysis, in the latter research, numerical methods have been proposed for calculating earthquake liquefaction, liquefaction-induced lateral sliding and lateral displacement due to the whole large deformation on submarine soil layer. Compared with actual observation and the results of related studies, the validity of the new methods are verified. The numerical studies can offer technical supports for site selection and earthquake resistant design of ocean engineering. The major contributions of the work presented in this dissertation are listed as follows:(1) An attenuation model is proposed for near-fault strong earthquake ground motion. The definitions of fault distance are obtained from statistical analysis, which are carried out using selected data grouped by magnitude and fault distance. Considered site conditions, attenuation relations are attained for peak ground motion in near fault area using least square one-step method. 3D regression curves have been drawn, which are changed with magnitude, fault distance and focal depth. Compared with the relations of other researches, the near-fault attenuation laws are more suitable to earthquake ground motion in near fault area. So it is suggested that the near-fault ground motion attenuation relations should be considered firstly when seismic risk evaluation and determining criterion of earthquake resistant design for important engineering structures in active fault area.(2) A new numerical method is proposed for assessing liquefaction on submarine soil layer due to earthquake. In the new method, pore water pressure building-up are calculated by using effective stress non-linear dynamic finite element method. Most importantly, the effect of wave loads is considered as initial excess pore water pressure and static loads on the seabed. Numerical simulation has been performed with survey data of Chengbei area in Bohai Sea. Compared with engineering evaluation, the new method is more adequate with wave loads taken into account.(3) Based on the above liquefaction analysis, a numerical method is presented for calculating lateral sliding induced by earthquake liquefaction of surface non-liquefied soil layer on submarine gentle slope. Newmark's rigid sliding block model is introduced into the method for calculating lateral slide. The effects of wave loads on lateral displacement are considered. The effects of seabed inclination and seawater depth and earthquake ground motion on lateral slide are studied by some examples, and the numerical results are compared with the results of other related researches. It is shown that this method is available for seismic liquefaction-induced lateral sliding assessment of submarine gentle slope which the inclination is not more than 5°.(4) A practical numerical method for large deformation induced lateral displacement of submarine soil layer due to earthquake is proposed. Softened modulus of soil element in each time division is calculated with 2D non-linear dynamic finite element method, where module soften theory is included. At the same time, pore water pressure is calculated, too. If soil liquefied, numerical analysis will continue with liquefied modulus derived from simplified formulas. The effects of wave loads are considered, too. Compared with other test and numerical researches, it is shown that the proposed method gives reasonable results for the conditions indicated. The important impacts on lateral displacement are studied by some numerical examples. The studies shown that the new method is applicable to simulate large deformation of submarine soil layer due to earthquake, simultaneously, wave loads are considered.