# Seismic Response Analysis of Buried Nuclear Power Plant Structures in Non-Bedrock Sites
## 2 Seismic Response Analysis of Site-Embedded NPP Structural Systems
### Liquefiable Site
#### 3.3.2 Liquefaction Phenomenon  
Liquefaction is generally considered to begin when the excess pore pressure ratio exceeds 0.8, and the site is regarded as fully liquefied when it reaches 1.0. In this section, the liquefaction characteristics of a fully embedded nuclear island structure under different liquefaction layer positions are studied. The onset time of liquefaction and the final degree of liquefaction are evaluated. Figure 32 shows the excess pore pressure ratio contour maps at the onset of liquefaction, full liquefaction, and the final moment of seismic excitation. Due to the variation in liquefaction locations under different conditions, the regions displayed for the first occurrence of liquefaction differ across cases.  
When the liquefaction layer is located closer to the surface above the structure, liquefaction is more likely to occur. In contrast, when the liquefaction layer is located below the structure, the accumulation of pore pressure in the liquefiable soil layer is significantly smaller than the confining pressure of the soil. This is caused by the self-weight stress of the soil. As a result, the pore pressure ratio remains much lower than the liquefaction threshold. Only partial liquefaction occurs, and the onset of liquefaction is significantly delayed. When the liquefiable layer is sufficiently deep (40.16 m to 53.49 m from the surface in this numerical simulation), no pore pressure accumulation occurs under a 0.5 g base input. Under the same conditions, liquefaction first occurs near the structure. As seismic excitation continues, pore pressure accumulation begins in areas farther from the structure.  
Figure 36 illustrates the timing and liquefaction phenomena under different conditions. As the liquefaction layer deepens, the onset of liquefaction and the occurrence of full liquefaction are delayed. In some cases, liquefaction does not occur. This indicates that, under the same seismic excitation, increasing the depth of the liquefaction layer delays the onset of liquefaction and effectively reduces the likelihood of liquefaction in the liquefiable layer.  
Figure 36: Liquefaction phenomena under varying operational conditions.  
According to Figure 37, it can be observed that liquefaction begins at the top earlier than in the middle and bottom. Additionally, near the structure, pore pressure accumulates more rapidly, leading to a faster occurrence of liquefaction.  
It is observed that the spatial position of the liquefaction layer is found to significantly affect the soil-structure seismic response. Under the same seismic excitation, liquefaction is more likely to occur when the liquefaction layer is closer to the surface. This is caused by the lower initial confining pressure of the soil. As the liquefaction layer moves deeper, liquefaction becomes less likely due to the increased initial confining pressure. When the liquefaction layer is located at the bottom of the structure, the overall acceleration response of the structure is observed to be greater than that in the upper or middle cases. This pattern is consistent with the variation in site response. It is shown that the site response significantly influences the seismic response of embedded nuclear power structures.