Tunnels located close to earthquake faults have been damaged during recent earthquakes, and the damage patterns suggest a complex 3D response of the tunnel in these zones. The domain reduction method (DRM) is a computationally efficient finite element method widely used to analyse the response of finite structures to wavefields generated through earthquake fault rupture simulations. However, this method does not work for simulating the seismic response of infinitely long tunnels. In this work, we modify DRM to accommodate infinitely long tunnels. We verify the accuracy of the modification for a wide range of parameters by comparing the numerical solution with existing semi-analytical solutions for the 3D response of tunnels subject to seismic waves incident on the tunnel from arbitrary directions. The boundary effects resulting from the truncation of the soil domain for numerical modelling are also explored, and recommendations on the numerical domain dimensions to ensure accurate results are presented. This modified DRM would be a powerful tool for analysing the 3D seismic response of tunnels subjected to near-fault wavefields generated through earthquake fault rupture scenarios.

The spatial variability of soil properties is pervasive, and can affect the propagation of seismic waves and the dynamic responses of soil-structure interaction (SSI) systems. This uncertainty is likely to increase the damage state of a structure and its risk of collapse. Additionally, conducting multiscale simulations efficiently in the presence of uncertainties is a pressing concern that must be addressed. In this work, a 3D probabilistic analysis framework for an SSI system considering site effects and spatial variability of soil property (i.e., elastic modulus, E) has been proposed. This framework is based on the random finite element method (RFEM) and domain reduction method (DRM). A multiscale model of a five-story reinforced concrete (RC) frame structure was developed on an ideal 3D slope to verify the effectiveness of the proposed framework. The dynamic responses of the structure were analyzed, and the peak floor acceleration (PFA) and peak interstory drift ratio (PSDR) were selected to estimate the damage state of structures. It was found that the proposed method significantly improves computational efficiency approximately 20 times compared with the direct method. In the regional models, with the increase of the coefficient of variation (COV) of E, the energy of seismic waves becomes more concentrated at the crest and the response spectrum value of medium and long periods increases. In the local SSI model, the soil variability increases the mean value of PSDR, resulting in a more severe damage state compared to the results from the deterministic analysis. Consequently, this study provides some suggestions for engineering practice, and the importance of probabilistic analysis considering spatially variable soils in the SSI problem is highlighted.

For improvement of seismic capacity of NPP, the Non-linear SSI (NLSSI) analysis concepts are studied and applied to practical NPP (Nuclear Power Plant) Structural Analysis. Non-linear SSI analysis requires a different approach from existing linear and frequency domain SSI analysis. In order to consider the non-linear behavior of the ground and structural materials, the analysis must be performed in the time domain, but this is difficult due to a large analysis model and many repetitive analyzes. Accordingly, many researches are being actively conducted to introduce a hybrid method that sets a certain region as the near field, adds boundary elements, and conducts analysis in the frequency domain or time domain. DRM (Domain Reduction Method) is a method proposed by Bielak et al. by developing several theories for calculating effective seismic force, and is the most widely used technique when analyzing NLSSI using the sub-structure method. This method is also provided in LS-DYNA. In this study, the Korean NPP Structures are analyzed with ABAQUS program in the time domain for the NLSSI analysis. The Linear SSI analysis of the NPP Structure is performed with ACS SASSI program in the frequency domain at same time, and LSSI analysis result is compared to NLSSI analysis result. The final produce of NLSSI analysis is the FRS (Floor Response Spectrum) at each story of building. After NLSSI analysis, FRS will be provided for re-calculation of the HCLPF (High Confidence of Low Probability of Failure) of some critical equipment that contribute significantly to the CDF (Core Damage Frequency).