4.2.3. An Example on Site

In this section, we enter a concrete example in reference to the field of wind turbines (large-scale prestressed concrete bucket foundation in Qidong Sea), in order to make readers understand the issues discussed in this article more clearly.

Qidong Sea is located in Jiangsu Province, China, near the border between the East China Sea and the Yellow Sea. In October 2010, the first large-scale prestressed concrete bucket foundation (diameter 30 m, buried depth 7 m) was constructed in this area. As shown in Figure 2, the ground conditions China's four major marine areas are soft and layered. In this wind farm, the geological survey showed that the soil properties from 0 to 33.5 m below the seabed are mainly silty sand and sandy silt, and the soil properties change into dense silty fine sand with the buried depth greater than 33.5 m [83]. These soils are liable to liquefy under strong seismic motion. Therefore, the effect of soil liquefaction needs to be considered in the design process of the wind turbines.

According to the detailed geological surveys and the seismic fortification intensity of this site (7 degrees), Zhang et al. used the ADINA program to analyze the liquefaction-resistance ability of soils below and inside this foundation and showed improvements due to the overburden pressure of the foundation and the constraint effect of the skirt [76,77]. They found that the concrete bucket foundation could still work after soil liquefaction. However, they only added the design ultimate wind loads to the structure, and the dynamic effects of seismic waves combined with the winds were not considered.

Many other new structures have not been constructed in real engineering, but some related research works are also based on site geological conditions. For example, the model tests of modified suction buckets with honeycomb compartment were also carried out in Jiangsu Province [84], and the umbrella suction anchor foundation has been designed for the Yellow River Delta area in the future [64].

In addition, although there are no actual engineering cases of new measures, many scholars have studied earthquake-induced liquefaction in Taiwan, Mexico and other sites. Kuo et al. focused on Changbin offshore wind farm in Taiwan Strait, and evaluated the liquefaction potential based on the typical ground profile of this site [85]. Mardfekri et al. proposed a probabilistic framework to evaluate the vulnerability of wind turbines in the Gulf of Mexico [86]. Martín del Campo et al. used numerical methods to analyze a wind turbine in Mexico under combined loads of earthquakes and winds, and made the fragility analysis [87].

In the above research, we can see that there are not many examples of new liquefaction-resistant structures that have been built. Works of this topic are still dominated by model tests and numerical simulations. The advanced numerical models are generally consistent with the results of the dynamic centrifugal tests. Numerical calculation has the advantages of being efficient and low cost while being able to evaluate many parameters and provide insight into the entire process of liquefaction-induced failure of structures. However, in view of the complexity of the marine environment, pore-pressure models and soil-structure interactions need to be further studied. Thus, numerical analysis of seismic seabed liquefaction will still be a focus of future research.
