Seismic behavior of underground structures and site response

The past 20 years have seen major progress in understanding of seismic response of underground structures. Seismic design loads for underground structures are characterized in terms of deformations and strains imposed by the surrounding ground. In contrast, above-ground structures are designed for inertial forces imposed by ground accelerations. Design methods have been developed to estimate seismic loads on underground structures. These methods include pseudo-static analysis whereby free-field motion is imposed on the underground structure as well as dynamic soil-structure interaction analysis. Free-field ground deformations and velocities due to a seismic event are estimated using site specific response analysis that accounts for local geology. Historically, underground facilities have experienced a lower rate of damage than above-ground structures. However, recent failures of underground structures during recent earthquakes were reminders of the need to include seismic considerations in design details of underground structures.