Q: Our state department of transportation is experiencing problems with settlement of bridge approach slabs. Despite its efforts to properly compact the soil beneath the slabs, they soon settle, creating a bump. Are there any solutions to this problem?

A: We recommend using controlled low-strength material (flowable fill) instead of backfilling with the original soil. Some state DOTs have successfully prevented approach-slab settlement using CLSM.

You suggest using controlled low-strength material, or flowable fill, to prevent the settlement of bridge approach slabs. But you painted the solution using too wide a brush. Settlement of bridge approach slabs is a very complex problem with numerous root causes. While flowable fill may solve the problem in some cases, one should not be let to believe that it will solve all or even many of these situations. In reality, each bridge location is unique, and the cause and mitigation of the "bump at the end of the bridge" depend on site-specific conditions.For more information about this topic, read the Transportation Research Board publication Settlement of Bridge Approaches (The Bump at the End of the Bridge)by Briaud, James and Hoffman (NCHRP Synthesis of Highway Practice 234).--Vernon L. Bump, South Dakota Department of Transportation, Pierre, S.D.

Editor's Note

The publication mentioned above describes current practices for designing, constructing and maintaining bridge approaches to reduce, eliminate or compensate for settlement. It discusses the geotechnical and structural engineering designs and procedural factors that reduce bumps, and includes numerous illustrations.

To order the publication, contact the Transportation Research Board, Box 289, Washington, DC 20055 (phone: 202-334-3214; fax: 202.334.2519). You can also order it online at the TRB publications Web site (www.nas.edu/trb/about/publicat.html).