Forensic Investigation of Pavement Distress: Old Airport Road in Bristol, Virginia

About the report:

A few years after Old Airport Road in Bristol, Virginia, was reconstructed, inordinate distortions of remarkable uniformity began to appear in the paved asphalt surface directly above concrete pipe culverts, which were buried beneath and across the road to transport storm runoff. The distortions closely resemble broad speed bumps that extend across all lanes. The growing influence of the distortions on ride quality and subsequent complaints from the traveling public prompted the City of Bristol to request the investigation reported here. A summary of the project's construction background, methods and findings of the investigation, conclusions regarding the cause of distress, and recommendations for remedial action are presented. The portion of Old Airport Road afflicted by the distress is five lanes wide and extends approximately 0.80 mile in length from its intersection with Bonham Road to I-81 toward the north. Of the 20 pipe culverts that were constructed transversely beneath the road, 14 exhibited some sign of surface distortion at the time of this report. The culverts were placed as part of the Old Airport Road widening and reconstruction project in 1994. This forensic investigation, which included visual and video surveys of pavements and culverts, a geotechnical examination of subgrade conditions, a non-destructive pavement deflection analysis, laboratory and microscopic analyses of culvert trench backfill material, and review of a pertinent geotechnical exploration conducted by others, was designed to determine if the distress was the result of (1) settlement between culverts, or (2) heaving of the trenches themselves. Results of this work demonstrate that the cause of the observed distortions is attributed to expansive heave of the black pyritic shale that was used to backfill transverse culvert trenches. In addition, significant structural damage to culvert pipes resulting from excessive heave expansion pressures was documented. The laboratory analysis of shale backfill samples indicates that heave, which results from the oxidation of pyrite and the formation of new minerals, is ongoing. Recommendations for remedial action include removing all shale trench backfill in distorted zones, replacing damaged pipes, and properly backfilling with a flowable fill or a suitable, compactable non-shale granular material.

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