Underground structures are an integral part of civil infrastructure and play an ever-increasing role in a rapidly urbanizing world. Underground structures have generally performed better than aboveground structures during earthquakes when transient motions are considered. Nevertheless, underground structures are vulnerable to ground failure such as fault displacement, liquefaction and slope instability.

This paper reviews past performance of underground structures during earthquakes and available frameworks for seismic analysis. It covers displacement-based design principles using closed form, pseudo-static and dynamic soil-structure interaction (SSI) approaches. These approaches are based on the recognition that system response is primarily driven by the inertia of the surrounding soils and that the contribution of the inertia of the underground structure is limited. In the case of closed form solutions and pseudo-static analyses, structure inertial response is neglected. The paper also discusses approaches to mitigate the impact of ground failure on underground structures.

The paper then describes emerging challenges and opportunities in the seismic evaluation of underground structures including buried reservoirs, transit stations, and the interaction with adjacent tall buildings in urban areas. For these complex structures, the use of pseudo-static approaches is no longer applicable as these approaches cannot account for significant inertial contributions of structural elements nor complex kinematic constraints. For example, in the case of large buried reservoirs, the roof structure is an important driver of forces transmitted into the sidewalls. For underground structures next to tall buildings, portions of the building inertial load in the form of base shear is transmitted to the adjacent underground structure.

The paper then describes advances in nonlinear dynamic soil-structure interaction simulations whereby the soil and structure are equally represented in the numerical model. These dynamic analyses can represent soil and structure details that have an important impact on the system response and cannot be captured in simplified procedures. The use of modern analysis software makes accessible analyses that can take advantage of modern computer hardware and parallelization. It is possible to run multiple ground motions and rapidly process analysis results within a short timeframe. Several research projects are underway by the authors to evaluate the results of numerical models, in a few cases after validation with physical model studies, and to develop reliable analysis protocols. The emergence of these numerical analysis tools allows for an important shift to performance based design of underground structures using a sufficient number of ground motions, while capturing important details of the soil-structure interaction and the underlying uncertainties.

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