Evaluation of Landslide Impacts to the Green River Bridge A Preliminary Geotechnical Assessment

摘要

On October 7, 2006 an earthquake occurred on Mount Rainier, Washington prompting inspection of all nearby bridges by the Washington Department of Transportation. Two days later, an inspection of the SR 169 Bridge over the Green River Gorge revealed that the natural soils and fill near the southeast corner of the bridge abutment had slid, displacing a small crib wall and exposing a significant portion of the abutment and pier. A second investigation was conducted a few days later to determine if the observed slope movement was part of a larger landslide complex called the Green River Landslide. Detailed investigations revealed that the observed slope movement was due to a shallow depth landslide covering approximately 1.75 acres and was not directly related to the larger landslide complex. The lateral scarp of this small landslide exposed a significant portion of the bridge pier, creating concern over the stability of the bridge and the integrity of the pier footing. As-built plans from 1932 revealed that the bridge pier was supported on a spread footing, but it was unclear as to whether the footing was founded on soil or on bedrock. To determine if the bridge was moving, the nature of the foundation material, and the extent and stability of the landslide a geotechnical investigation was initiated. Access difficulties complicated the investigation and a unique approach including geologic mapping, field-developed cross sections, hand auger probing, ground penetrating radar, seismic refraction surveys, pile integrity testing, and the installation of biaxial tilt meters on the bridge piers was used to investigate the problem. Findings to date indicate that the bedrock elevation is approximately 3 feet below the bottom elevation of the bridge footing, the landslide slip plane appears to be at the interface between the overburden soils and the underlying bedrock, movement within the bridge pier has been negligible, and the landslide was likely initiated by surface water runoff traveling down the side of the bridge abutment.