## 5 Interpretation and Discussion
### Spatial Effects of the Interchange Station on the Ground Response  
The effects of the single underground structure on the ground acceleration were revealed in a number of experimental and numerical studies [37; 38]. The amplification pattern of the soil near the underground structure depends on several factors, such as the width, depth and relative stiffness of the underground structure. Due to the uniform cross-section, it is understandable that a single tunnel affects the amplification pattern consistently along the longitudinal axis when subjected to transversal excitation. However, in the case of the interchange station, the conjunction brings an abrupt change of the cross-section and, as a result, change to the above-mentioned factors could change the amplification pattern from two-dimensional (horizontal and vertical) to three-dimensional (spatial).  
Figure 14 compares the amplification ratios from the peak acceleration (PA) to the peak base acceleration (PBA) versus the depth along the SA-2-SA-5 and SA-ff soil columns during all of the tested sinusoidal motions. The location of the acceleration arrays in the plane view and the cross-section view is also illustrated in Figure 14. The acceleration from SA-5-1 is not shown in the figure due to damage during the preparation of the model ground. The amplification pattern of the PGA was prominent in the SA-ff soil column. Among the sinusoidal excitations, the largest amplification ratio--to the order of 2.7--was observed for the one with the frequency of 8 Hz. This was because the fundamental  
Figure 13: Comparison of the transfer function during the white noise motions before and after tests, along with the analytical results with 7% damping (**a**) in the accelerometer array out of the single station (SA-ff) and (**b**) in the accelerometer array out of cross interchange station (SA-4).  
frequencies of the ground were around 7.4 Hz, and the resonance-like phenomenon may have been induced under the Sin-8 Hz seismic motion. Overall, SA-2-SA-5 recorded lower PAs at the same depth compared with the SA-ff array, indicating the de-amplification effects of the interchange station on the free field response. A recent numerical study [26] also predicted a similar de-amplification phenomenon and suggested that the reason for this was that the interchange station, to some extent, blocked the propagation of seismic waves. With regard to the four arrays near the interchange station (SA-2-SA-5), the recorded PAs differed in the function of their elevations and relative locations to the cross station structure, indicating the spatial effects of the interchange station on the soil response. Among them, the SA-4 array recorded a more significant amplification and relatively higher PA compared with SA-2 and SA-3. The profile of the soil acceleration in SA-4 was more alike to the free field soil (SA-ff) because SA-4 was the most distant array from the interchange station in both directions in the plane. Correspondingly, the nearest array to the interchange station, SA-3, gave the lowest PGAs and a much lower amplification of soil acceleration from SA-3-3 to SA-3-1. The lower amplification might be attributed to the constraint effects of the stiff conjunction on the near-structure soil. Special consideration needs to be paid to such spatial effects, for example, when considering the interchange station and soil building dynamic interaction in the urban environment.  
Figure 14: Amplification ratio of the peak acceleration (PA) to the peak base acceleration (PBA) as a function of the depth for the accelerometer arrays SA-2–SA-4 and SA-ff during all the tested sinusoidal motions, along with illustrations of the accelerometer locations.