Investigation of approach slab settlement for highway infrastructure

Approach slabs serve as a transitional system between an approach road and a bridge. Settlement of bridge approach slabs and their supporting backfill has been reported by more than ten Departments of Transportation throughout the United States. According to current Wyoming Department of Transportation inspection reports, bridge approach slab settlements occurred not only on existing bridges but also on newly built bridges that were recently opened to traffic. These settlements typically create voids ranging from 150 mm to 305 mm (6–12 in.) between the base of the approach slab and the geotextile reinforced backfill. This research presents factors causing bridge approach slab settlements and provides appropriate design and construction recommendations.A comprehensive literature review pertaining to approach slabs was performed to examine outcomes of research conducted by 12 states. The current specifications and standards on bridge approach slabs of the corresponding states’ Departments of Transportation were also evaluated. A nationwide survey was conducted to fill in the missing knowledge identified in the literature review. The results of the survey are categorized into three groups based on the percentage of bridges experiencing approach slab settlements in that state. Results of the survey show that 46% of the total 28 states are not satisfied with their current approach slab designs. The survey results revealed that the most common cause of approach slab settlement is poor construction practices. The most important finding of the survey is that performing in-situ tests to control backfill compaction reduces the amount of approach slab settlement. Using the lessons learned from the literature and the results of the survey, recommendations were developed for transportation agencies nationwide.