A challenge for municipal authorities of growing urban areas is to provide larger and faster transportation and utility networks that are safe and resilient to significant disruptions after an earthquake event and other disasters. Urban regions, like the San Francisco Bay or the Los Angeles area, are situated in seismically active regions. In these areas, underground structures, such as water ducts or metro tunnels, must cross active faults to ensure population sustainability.\
It is commonly known that earthquakes with magnitudes greater than M6.0 can cause significant damage to tunnels in seismically active areas. In particular, large strains due to fault offsets and ground shaking can lead to severe damage in the tunnel lining, (e.g., concrete spalling), which can lead to potential closure and disruptions to the transportation network.\
Examining the behavior of concrete lined transportation tunnels built through active fault zones is critical to ensure resilient design and safe operations. Understanding the response of tunnels crossing active faults will further inform rehabilitation and maintenance measures and support tunnel safety. A 2D model of a circular reinforced concrete tunnel crossing an active fault is developed within the finite element framework OpenSees. A parametric study with varying structural and ground properties is performed. The effects of earthquake magnitude, geology, fault zone width, and structural properties of the tunnel are studied and assessed to develop novel tunnel design strategies to accommodate large fault motions and to minimize tunnel service disruptions. \
The research is based on three main questions. (1) Can we give guidance when it is acceptable for a tunnel to cross an active fault? (2) What is the influence of faulting on the circular tunnel lining in the cross-section, but also along the longitudinal tunnel axis? and (3) What consequences and generalizations can be drawn to support serviceability of the tunnel after an event? \
Guidance to engineers on these research questions include a possibly reduced length of retrofit measures along the longitudinal tunnel axis where the tunnel crosses the active fault zone. This assessment is based on localized strains and stresses in the concrete lining. The flexural displacement capacity of the tunnel beam ranges between 0.2~\% and 2~\% of the inner tunnel diameter. A corresponding earthquake magnitude threshold for reaching peak compression concrete strain can be as low as M5.5 at active fault crossings. A generalization that can be drawn is the interaction between the tunnel diameter and the fault zone width, where a fault zone width of less than 1 to 2 times the tunnel diameter might be a larger concern for the tunnel lining due to abrupt shearing. \
The applicability of such research to existing tunnels is assessed for the Berkeley Hills tunnels crossing the active Hayward Fault. The ultimate threshold displacements of the tunnel beam model are correlated to earthquake magnitudes. An evaluation with charts results in similar earthquake magnitude thresholds for concrete failure compared to a specific 2D numerical analysis. This shows that a simplified chart assessment that predicts an approximate threshold of an earthquake magnitude the concrete lining is able to withstand, might be applicable for early stage projects, e.g. during a feasibility assessment.
Presented at the following conferences: 11NCEE - National Conference on Earthquake Engineering, L.A., CA, 2018. NAT2018 - North American Tunnel Conference, Washington, D.C., 2018. 16th D-A-CH - Conference on Earthquake Engineering and Dynamics, Innsbruck, Austria, 2019.