Earthquake Surface Fault Rupture Interaction with Building Foundations
Recent earthquakes have provided numerous examples of the devastating effects of earthquake surface fault rupture on structures. Several major cities are built in areas containing active faults that can break the ground surface (e.g., Los Angeles, Salt Lake City, San Diego, San Francisco, and Seattle). Along with the often spectacular observations of damage, examples of satisfactory performance of structures were also observed. These examples of satisfactory performance indicate that similar to other forms of ground failure, effective design strategies can be developed to address the hazards associated with surface fault rupture. However, at present, no design guidance exists for implementing many of the potential strategies for building near faults.
To address this issue, a comprehensive set of numerical simulations, which were initially validated with centrifuge test results, has been undertaken to analyze potential design strategies for building in the vicinity of active faults. The numerical simulations capture fully this problem as a soil-structure-fault interaction problem where the influence of the structure has been explicitly included. A modified nonlinear, effective stress, elasto-plastic soil constitutive model has been developed and implemented in an explicit finite-difference framework to capture the soil and structural responses to fault rupture.
For the first time, the effects of prior earthquake ruptures through soil (e.g., pre-existing shear bands and the in situ stress state) were investigated. Additionally, the effects of fault type on soil response was analyzed and was found to be primarily a result of varying stress paths in normal, reverse, and vertical faults. The effects of dynamic soil-fault-structure interaction were analyzed in a systematic manner for the first time and procedures to account for these effects were developed.
Hazard mitigation strategies were developed and systematically evaluated and compared. Three main categories of fault-resistant design were identified: (1) spreading fault displacement over a large area, such as with engineered fill; (2) enabling the structure to respond with rigid-body movement, e.g., by using a stiff mat foundation; and (3) diverting fault rupture through soil using stiff structural elements or by strengthening the foundation soil. These strategies were found to be effective at minimizing structural damage during surface fault rupture events. Their use allows for improved designs and retrofits in the active fault regions, which reduce risk while preserving design flexibility.