# Study on the Seismic Effect of the Pebble Soil Site in the Zhongwei Basin
## 4 Results
### The Results of Comparison between the Pebble-Top Interface and Surface  
According to the results of the two previous sections, the overlying silty clay in the Zhongwei Basin has a direct impact on the site PGA amplification coefficient. At the same time, to compare and study the seismic response characteristics between the pebble-top interface and the surface in the basin, this paper set the thickness of the pebble layer to 30 m and the thickness of the overlying silty clay layer to 5 m and established a one-dimensional calculation model of the typical soil layer in Zhongwei Basin, as shown in Table 5.  
As shown in Table 6 and Figure 10, with the increase in the input intensity, the PGA amplification ratio of the pebble-top interface and the site surface showed a downward trend. The PGA amplification ratio of the pebble-top interface is significantly smaller than that of the site surface. Under different intensities of inputs, the PGA amplification ratio of the pebble-top interface was less than 1.1. Under the extremely rare earthquake, the PGA amplification ratio of the pebble-top interface was less than 1. With the increase in the input intensity, the characteristic periods of the pebble-top interface and the site surface show a significant upward trend. Under the same earthquake, the characteristic period of the pebble-top interface was larger than that of the site surface. When the input intensity was greater than the multi-event seismic action (50-63%), the characteristic period of the pebble-top interface was greater than GB 18306-2015. When the input intensity was less than the rare earthquake action (50-2%), the surface characteristic period of the site was less than GB 18306-2015. According to GB 18306-2015, recommended by the response spectrum method for pebble-top calibration interface and the ground surface acceleration response spectrum, as shown in Figure 11, under different input intensities relative to the ground surface, the pebble-top interface of the calibration results showed the phenomenon of \"low-rising value, wide platform\", showing obvious shock absorption and isolation. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Exceeding Probability**} & \multirow{2}{*}{**Bedrock Input/Gal**} & \multicolumn{2}{c}{**Pebble-Top Interface**} & \multicolumn{2}{c}{**Surface**} \\ \cline{3-6}  & & **PGA/Gal** &
\begin{tabular}{c} **Amplification** \\ **Ratio of PGA** \\ \end{tabular} & \multirow{2}{*}{**PGA/Gal**} & **Amplification** \\ \cline{3-6}  & & & **Ratio of PGA** \\ \hline
50–63\% & 60 & 65.4 & 1.09 & 87.6 & 1.46 \\
50–10\% & 213 & 230 & 1.08 & 306 & 1.44 \\
50–2\% & 448 & 473.2 & 1.06 & 603.2 & 1.35 \\
100–1\% & 681 & 602.2 & 0.88 & 769.6 & 1.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison table of amplification ratio of PGA between pebble-top interface and ground surface. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Model** & **Soil Name** & **Layer Depth/m** & **Soil Thickness/m** & \(\overline{Vs}/m\cdot s^{-1}\) & \(\rho/t\cdot m^{-3}\) & **Soil Type Number** \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Typical Soil \\ Model \\ \end{tabular} } & Silty clay & 5 & 5 & 185 & 1.90 & 1 \\  & Pebble & 10 & 5 & 280 & 2.05 & 2 \\  & Pebble & 20 & 10 & 360 & 2.10 & 3 \\  & Pebble & 35 & 15 & 480 & 2.15 & 4 \\  & Sticky sand & 50 & 15 & 450 & 2.20 & 5 \\  & Sticky sand & 65 & 15 & 630 & 2.22 & 5 \\  & Sticky sand & 75 & 10 & 700 & 2.30 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Typical soil models and soil mechanical parameters. Figure 11: Accelerations spectra and standardization spectra of pebble-top interface and ground surface under four seismic inputs. Figure 10: Amplification ratios of ground motion between pebble-top interface and ground surface under four seismic inputs.