## 2 Model Preparation and Testing Procedure
### Experimental Program
#### 2.2.1 Structure and Soil Modeling
In addition, Both S2 and S3 structures were shorter and smaller than the S1 structure. However, the S3 structure had a higher fixed-base frequency while the S2 structure had a lower fixed-base frequency than the S1 building. Therefore, whether height ratio or fixed-base frequency ratio could control SSSI was also determined by comparing the response of S1 structure when it was located adjacent to the S2 or S3 structures. A poorly-graded clean silica sand (\(G_{\rm s}\) = 2.65 and \(D_{50}\) = 0.51 mm) was used in this study, with its index properties are presented in Table 2. An air-pluviation method was adopted to prepare a model ground with targeted relative density (\(D_{r}\)) of 80 %. The ground model (thickness of 27 m at the prototype scale) was prepared with an instrument having an array of eight pairs of in-flight bender elements as shown in Figure 1. Figure 2 shows shear wave velocities (\(V_{\rm s}\)) of the soil measured by in-flight bender element array under centrifuge acceleration. Measured \(\rm V_{\rm s}\) well-matched with \(\rm V_{\rm S}\) estimated by the laboratory test results such as oedometer equipped with bender elements and resonant column (RC) tests performed on the soil at the same relative density. \(\rm FS_{v}\), the vertical factor of safety, was defined with [PERSON]'s equation [38] to determine bearing capacity of the soil-foundation system. The internal friction angle of the sand was evaluated to be 36.6\({}^{\circ}\) by performing triaxial tests at the same relative density of the soil. Calculated \(\rm FS_{v}\) value was 19.6 for S1, 22.5 for S2, and 23.1 for S3, which indicates very high values for all structures. To monitor motions of the soil and foundation/structure simultaneously, a total of 23 accelerometers (PCB353B17, PCB Piezotronics) were installed at the ESB box, in the soil, at the soil surface, or at the structure model as shown in Figure 1. A vertical array of horizontal accelerometers (A2 to A6) in the ground was used to explore ground response in the soil. An accelerometer A23 was located on the ESB box to measure bed-rock input motion accurately. Vertical and horizontal accelerometers (i.e., A7 and A8) were installed to measure the free-field motion (FFM), due to input motion. Note that the locations for these free-field accelerometers were chosen to be far away from the structures and boundary of the ESB box as much as possible, to reduce the structure's motion and boundary effects. Additional accelerometers were attached at foundations, on walls, and at roofs of all structures to monitor foundation input motion (FIM), wall motion (WM), and roof motion (RM), respectively. Six laser sensors (KEYENCE IL-300) were installed at a rigid frame to measure permanent settlement of soil surface (L1) during spinning of the centrifuge and record final settlement and rocking angle of foundations of each structure. \begin{table}
\begin{tabular}{c c} \hline \hline
**Properties** & **Value** \\ \hline Specific gravity, \(G_{\rm s}\) & 2.65 \\ Fine contents passing \#200 (\%) & 0.9 \\ Maximum void ratio, \(\rm e_{max}\) & 1.137 \\ Minimum void ratio, \(\rm e_{min}\) & 0.616 \\ Uniformity coefficient, Cu & 1.6 \\ Soil classification, USCS & SP \\ \hline Relative density, \(\rm D_{r}\) (\(\%\)) & 80 \\ Dry density (t/m\({}^{3}\)) & 1.54 \\ Friction angle, \(\varphi\) (deg.) & 36.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Fundamental characteristics of soil modeling in centrifuge test. Figure 1: Schematic drawing of typical centrifuge models for S1-S2 structures in case of 0.03B distance with installation of in-flight bender elements, accelerometers, and laser sensors. Accelerometers installed in soil are marked in blue while those bonded at structure are indicated as red. Figure 2: Shear-wave velocity profile of prototype soil in comparison with results from Oedometer and RC (Resonant Column) tests.