## 4 NSAS Effectiveness for Underground Structures
### Energy-Based Damage Control  
As a robust evaluation index for this type of seismic excitation [42; 43; 44; 45; 46], energy-dissipation distributions between different components of an uncontrolled underground structure and structures with a CIS or the NSAS isolation system are shown in Figure 15. Since the central column of the Daikai subway station was severely damaged in the uncontrolled structure, after the installation of the CIS and NSAS isolation system the proportion of energy dissipated by the central column was decreased from 20.42% to 3.18% and 1.61%, respectively. At the same time, the dissipated energy by the wall-slab structures (upper slab, bottom slab and sidewall) was increased by a small amount in the acceptable range. Quantitively, the maximal plastic-energy densities (plastic-energy dissipated by the unit volume of the structure) of the different structural components are listed in Table 6 in order to characterize the degree of unwanted damage. The maximal plastic-energy density of the central column was decreased from 2221 (uncontrolled) to 210 (CIS) and 138 J/m\({}^{3}\) (NSAS isolation system). Benefitting from the energy-dissipation mechanism of the NSAS, the proposed NSAS isolation system undertook a heavier energy-dissipation burden to ensure the more effective protection effect of the central column. Particularly, the energy-dissipation proportions of the dashpots of the NSAS isolation system and CIS were, respectively, 4.2% and 1.2%. The NSAS isolation system proposed in this study had a better energy-dissipation effect than that of a conventional seismic-vibration control system. For the wall-slab structures, an increase of about 20% in maximal plastic-energy density was observed, which cannot significantly affect the seismic performance of wall-slab structures [10].  
Moreover, the histograms of plastic-energy proportion for the uncontrolled underground structure and the structures controlled by the CIS and NSAS isolation system under seismic excitation, with increased intensities, are illustrated in Figure 16. Subject to excitation at variable intensities, the CIS and NSAS isolation system exhibited effectiveness in releasing the energy-dissipation burden of the central column. More importantly, the proportion of energy dissipated by the NSAS isolation system was always larger than that of CIS, which further stressed the energy enhancement of the NSAS  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Parts**} & \multicolumn{3}{c}{**Plastic-Energy Density (J/m\({}^{3}\))**} \\ \cline{2-4}  & **Uncontrolled US** & **US-CIS** & **US-NSAS Isolation System** \\ \hline Upper slab & 555 & 678 & 641 \\ Bottom slab & 213 & 294 & 267 \\ Sidewall & 314 & 513 & 483 \\ Central column & 2221 & 210 & 138 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Maximal plastic-energy density for different parts of underground structure (US).  
Figure 15: Energy-dissipation proportion for (**a**) uncontrolled US, (**b**) US-CIS and (**c**) US-NSAS isolation system.  
isolation system. The dashpot hysteresis curves in the CIS and NSAS isolation system are illustrated in Figure 17. It is clear that the amplification of the dashpot deformation in the NSAS isolation system resulted in an obviously improved energy-dissipation capacity using the same damping ratio. In other words, only a lower damping ratio was required in the NSAS isolation system in order to satisfy the same energy-dissipation effect as that of the CIS. These results also demonstrated the robustness of the NSAS isolation system, which guarantees multilevel safety for underground structures.