| In this study, a typical area in the Chengdao oil field at the old estuary of Yellow River was chosen as the studied area. Based on the real-time wave statistics, the liquefaction probability of the seabed soil and the time it need to reach liquefaction has been caculated. Moreover, different liquefaction parts of the seabed at different water depths are divided according to the liquefaction time. Conclusions are drawn that the seabed between 7m to 8m bathymetric line is the most probable liquefaction area and the liquefaction probability of the seabed soil appeares to decline from inshore area to offshore area. Meanwhile, numerical calculating method is used to simulate the pore pressure accumulating process of the seafloor soil with wave load based on Biot's linearity wave theory and finite cell theory; the subarea results based on dynamic triaxial test is validated by the results obtained from numerical calculating method, and it turns out that the two are compatible. Finally, the author reached conclusions that are more close to the real station that:1. Liquefaction phenomenon spreads widely in Chengdao sea area where the superficial deposits of the seafloor is mainly composed of silt.2. Based on the results of dynamic triaxial test and wave information of the studied area for different water depths and different return periods, the time for seafloor soil to reach liquefaction at different depths was calculated. In case of 50 year return period, the area from 5 meter bathymetric line to 15 meter bathymetric line at 1.5 meter soil depth can be seen as most risk of liquefaction, and special attention should be paid on this area in marine engineering. However, with the soil depth becoming deeper, the liquefaction area of most risk becomes smaller. At the 2.5 meter soil depth, the most risk area reduced. At the 4 meter soil depth, the most risk area not existed, and the second risk area appeared to spread between 7 meter bathymetric line and 13 meter bathymetric line. While the area possible of liquefaction continued to reduce with the soil depth increment. 3. In case of 50 year return period, the soil around 8 meter bathymetric line reached liquefaction in the least time, while in case of 5 year return period, it is the soil around 7 meter bathymetric line that reached liquefaction state with least time. However, the maximum liquefaction depth of each return period has a regular pattern, that is the wather depth where the wave breaks. In case of 50 year return period wave, storm waves make the water depth higher, and thus wave breaks at water depth that is deeper than used. And the most rist of liquefaction water depth removed accordingly.4. Transient liquefaction caused by 1% large wave and effective wave are calculated and analysized, and transient liquefaction limit depth of different return period and water depth are calculated. For 1% large wave, 50 year return period, the liquefaction limit depth is 2.35 meter and for 1% large wave, effective wave , it is 1.9 meter.5. According to the residual liquefaction, the limit liquefaction depth of the soil in the studied area can be reached through real investigation and numerical calculating. The total liquefaction limit of 50 year return period is 6.4 meter, and distributes between 8 meter bathymetric line and 9 meter bathymethic line and it is of low liquefaction risk which would not become liquefaction in 10 hours. The total liquefaction limit of 5 year return period is 4.7 meter, and distributes between 6 meter bathymetric and 7 meter batnymetric line and it is of low liquefaction lever that only liquefact unless 10 hour wave effect.To conclude, different liquefaction zones with wave load were divided based on dynamic trixial tes in this study. The results and compared with numerical calculating results, and also related to real sea condition. Finally the limit liquefaction depth and its existing area was given. The conclusions drawn in this aera may provide some help to marine engineering design , constructure and research in this area.
|
PDF Full Text Request