# Experimental Assessment of the Effect of Vertical Earthquake Motion on Underground Metro Station
## 3 Test Results and Discussion
### Influence of RVH on Dynamic Soil Normal Stress  
Dynamic soil normal stress (DSNS) is defined as the difference between the total stress and initial static stress along the side wall of the model station. As an example, Figure 11 compares the two time histories of the DSNS between RVH = 0 and RVH = 1. It is found that the time histories of the DSNS resemble that of the input motion. The essence of this phenomenon is also revealed by [PERSON], who points out that the input seismic excitation in the form of an acceleration can be converted into a stress wave [35]. With different RVH their phases are basically consistent at all measuring points, though the amplitudes of the DSNS behave differently at different depths of the side wall. For example, the amplitudes are almost the same near the bottom of the side wall (e.g., PR1 and PR2) while the amplitudes under RVH = 1 are always larger than those under RVH = 0 at other depths of the side wall (e.g., PR3-PR11).  
Figure 11: _Cont._  
Figure 10: The difference of horizontal acceleration amplitude with increasing RVH between side wall and adjacent soil at three different depths.  
Figure 11: Cont.  
To figure out the influence of the RVH on the DSNS along the side wall, Figure 12 displays the peak DSNS along the left and right side walls under different RVH.  
As can be seen from Figure 12a, under the input motion of Loma Prieta, peak DSNS along the left side wall follows an approximate L-shaped distribution. With increasing RVH, on the whole, there is no significant change in the amplitude of the peak DSNS. Compared to other RVH, a relatively large increase at the depth of the platform floor can be observed when RVH equals to 1.5.  
With respect to the peak DSNS along the right wall, as shown in Figure 12b, at the depth between the top of the right side wall and the middle of the station hall floor, they all follow an approximately linear distribution. At the depth between the middle of the station hall floor and the bottom of the right side wall, they all follow an approximately \(\sum\)-shaped distribution. It is worth mentioning that the maximum stress occurs at the bottom of the side wall basically. With increasing RVH, peak DSNS along the right wall increase on the whole, except for the bottom of the side wall (base slab level). Compared to the results under only horizontal input motion (RVH = 0), the maximum increase in peak DSNS under RVH = 0.5, 1, and 1.5 reach about 148%, 159%, and 199%, respectively. For the bottom of the side wall (base slab level), with increasing RVH, the peak DSNS has no significant change, which is different  
Figure 11: Comparison of time histories of dynamic soil normal stress (DSNS) along right side wall between RVH = 0 and RVH = 1.  
Figure 12: Distribution of peak DSNS under different RVH along (**a**) left side wall, and (**b**) right side wall.  
from the situation at other position of the side wall and has also been concluded above from Figure 11. It might be attributed to the sharp transition in the corner and a possible stress concentration there.  
When comparing the distribution of the peak DSNS between the left and right side walls, some obvious differences between them can be found. A similar finding was also drawn from a numerical study of a shallow buried rectangular underground structure under earthquake loading with both the horizontal and vertical components [36]. The differences in stress distribution of the peak DSNS between the left and right side walls can be explained by the shadow effect, which is caused by the entrapment of the waves between the ground and the underground structure [37]. With increasing RVH, the above-mentioned differences between the left and right side walls also increase. From the perspective of dynamic soil normal stress, the experimental results have proven that RVH has significant influence on the dynamic soil-structure interaction.