# Seismic Response Analyses of a Large-Span Powerhouse Cavern Considering Rock-Structure Interaction
## 4 Numerical Modeling
### Selection of the Seismic Input  
Selection of input ground motions is a crucial step in the dynamic analysis, because the outcome of the analyses is markedly affected by this choice, thus an accurate estimation of the seismic performance should be carried out based on the seismic hazard at the site where the structures are located. Generally, accelerograms can be defined using different methods as: (1) selecting and scaling real accelerogram (seismic input recorded in real earthquakes), (2) generating artificial records that are compatible with a design response spectrum, and (3) generating synthetic records based on a seismic source model. For the seismic design of some important underground structures such as tunnels and nuclear power plants located in seismically active areas, the real accelerograms, which are recorded in past earthquakes, are still very predominant. The selected accelerograms, according to EUROCODE 8, need to be representative of the seismicity of the site and be compatible with its seismological and geotechnical characteristics. Several criteria have to be taken into account in the selection of accelerograms for the site of interest. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Deformation Modulus (GPa)**} & \multirow{2}{*}{**Possion’s**} & \multirow{2}{*}{**Intact Rock**} & \multicolumn{4}{c}{**Hoek–Brown Model**} \\ \cline{5-8}  & & & **Conpressive** & & **Undisturbed** & & **Disturbed** \\ \cline{1-1} \cline{5-8}
**Undisturbed** & & & **Strength (MPa)** & \(m\) & \(s\) & \(m\) & \(s\) \\ \hline
22 & 14 & 0.25 & 72.3 & 1.18 & 0.012 & 0.28 & 0.002 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Rock mass parameters calibrated from the triaxial tests. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \(\sigma_{x}\) & \(\sigma_{y}\) & \(\sigma_{z}\) & \(\tau_{yz}\) & \(\tau_{xz}\) & \(\tau_{xy}\) & \(\sigma_{1}\) & \(\sigma_{2}\) & \(\sigma_{3}\) \\ \hline
6.11 & 6.05 & 10.58 & 1.38 & \(-1.87\) & \(-1.23\) & 11.8 & 6.4A suite of not fewer than 3 appropriate ground motions shall be used in the analysis. If at least 7 accelerograms are adopted, average values of response quantities can be used for analysis; on the other hand, if the number of accelerograms is less than 7, the most unfavorable value of the response quantity should be used. The average response spectrum should be compatible with the target spectrum. In the range of periods of 0.15 s-0.20 s (and 0.2 T1-2 T1) for horizontal components and the range of 0.15 s-1 s for vertical components, where T1 is the fundamental period of the structure in the direction where the accelerogram will be applied, no value of the mean 5% damping elastic spectrum, calculated from all time histories, should be less than 90% of the corresponding value of the 5% damping elastic response spectrum [21]. The site where the accelerogram station is installed must be classified as stiff soil or rock so that the local amplification effect can be neglected. The selected accelerograms must be in accordance with the seismological parameters, that is, magnitude and distance pairs, which are obtained by the deaggregation process. Seismic hazard analyses should first be performed to determine the seismic parameters, so as to select appropriate real accelerograms. The seismic hazard of the site is characterized by two parameters: the horizontal peak ground acceleration (PGA) and expected macroseismic intensity. The probabilistic seismic hazard maps, generally in terms of these two parameters, can be constructed for areas of different seismogenic regions, showing contours of ground parameter values, to estimate the rate of potential earthquakes across a region and quantify likely ground-shaking levels at a site. The studied cavern is located in a zone characterized by the horizontal PGA in the 0.10 to 0.15 g range and by the site intensity of VI, with a 10% probability of exceedance in 50 years.