Seismic fragility of approach backfill differential settlement for statewide bridges in California

Abstract

This study develops the first-of-its-kind seismic fragility models for estimation of approach backfill differential settlement for statewide bridges in California. Seismic compression analysis is carried out through a multi-step framework that estimates seismic-induced shear strain profiles for backfills, converts them to volumetric strains, and computes soil settlement through depth integration. As a crucial step, three independent methods are advanced on a uniform basis to estimate soil shear strains under earthquake loading: a simplified one-dimensional (1D) Static approach, a 1D Dynamic method from site response analysis, and a two-dimensional (2D) Dynamic method that accounts for the trapezoidal shape of the bridge embankment. Subsequently, seismic fragility models are derived using the multiple-stripe analysis (MSA) approach that convolves the seismic demands of backfill settlement with capacity models for several broad groupings of approach-slab designs having different abutment and abutment-foundation types, and abutment-connection details. A survey of California's bridge inventory provides stochastic data inputs that quantify various sources of uncertainties in backfill profile, embankment geometry, and both approach slab and abutment designs. Finally, a combined fragility model is developed by considering an equal methodological contribution from each mentioned approach. The final fragility models suggest that the lowest differential-settlement fragility is achieved by combining regular abutments on spread footings with long-length approach slabs connected to the bridge abutment, while the worst case occurs where bridge abutments on deep foundations are not connected with abutting pavements. The proposed seismic fragility models offer a sound basis for first-order estimation of approach-fill differential settlement which may lead to reduced ride quality, traffic speed reductions, and in extreme cases, temporary roadway closure. These represent one of many seismic fragility models for the full range of bridge components needed for prioritizing seismic retrofit measures, facilitating post-hazard inspections and repair actions, as well as assessing the network mobility for resilience quantification.