Assessing Structural Integrity of Old Bridge Foundations Using Swept Frequency Seismic Imaging vs. Direct Structural Investigation – A Case Study

The nation’s bridge infrastructure is continually in need for rehabilitation to maintain its structural stability for the safety of the traveling public. Bridge foundation reuse can be a viable alternative allowing cutting the duration of bridge rehabilitation and associated traffic congestion, and significantly lowering costs. Guidelines and field assessment of structural integrity for existing substructure are therefore needed to help the practitioner decide if the foundations can be reused. This paper describes the results of a comprehensive seismic imaging study (tomography, reflected waves, seismic profiling) of masonry wall foundations supporting 80-year old highway bridge in the middle of a wide dry wash valley. The seismic measurements were conducted using swept frequency source. The patterns of seismic anomalies detected in the foundations appeared to correlate with known structural features (end-walls, coreholes), and specifically with structural damage identified by visual inspection (coring, and corehole logging data). Hence they seem to properly identify structural changes, in particular zones of structural damage. Considering the age of the bridge, there could be a number of factors which would contribute to the existence of identified weak zones. The evidence of massive flush flooding events from torrential rains upstream from the bridge may offer some clues.

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