Flowfill and MSE Bridge Approaches: Performance, Cost, and Recommendations for Improvements

Construction of a typical Colorado Department of Transportation (CDOT) bridge approach structure includes placement of a high quality backfill material behind the abutment wall, and installation of a concrete approach slab supported by the bridge abutment wall at one end and the sleeper slab foundation at the roadway end. Since 1992, three new alternatives for the abutment backfill have been employed by CDOT: (1) relatively expensive flowfill; (2) lower cost mechanically stabilized earth (MSE) using granular, well-graded Class-1 Backfill, and (3) MSE using free draining Class B Filter soil. However, bridge bump problems at the sleeper slab are still occurring. In the Founders/Meadows bridge structure, both the bridge footings and approaches are supported by geosynthetic-reinforced soil (GRS) walls to minimize the uneven settlements between the bridge and its approaches (called "GRS Abutment"). Since this structure is unique, performance data from gauges embedded in the approaches and from smoothness tests were collected over five years. The objective of this study is to improve CDOT's current practice for bridge approaches (improve performance and reduce costs) based on the following information obtained in this study: (1) comments and suggestions collected from CDOT Staff and reported in the literature; (2) performance and cost-effectiveness of CDOT's MSE and flowfill bridge approaches and performance and design assessment of the Founders/Meadows GRS approaches; and (3) causes and sources of the bridge approach settlement problems observed in some of CDOT's MSE and flowfill bridge approaches.