Iowa LTAP has moved from CTRE to the Institute for Transportation
Identifying bridge approach problems, causes, and possible solutions
Failure of concrete slope protection due to erosion.
Bridge approach settlement and the result-ing formation of "bumps" at the end of bridges is a common problem in Iowa. In addition to annoying motorists, these bumps result in considerable maintenance costs.
David J. White, assistant professor of civil, construction, and environmental engi-neering at ISU, recently led a project that investigated bridge approach problems and recommended improvements to bridge approach design, construction, and main-tenance. The project was sponsored by the Iowa Highway Research Board (TR-481). Following are highlights.
Causes of approach settlement
The project team determined that approach settlement can be caused by a number of factors:
- seasonal temperature changes causing horizontal movements of integral abutments
- loss of backﬁll material by erosion
- poor construction practices
- settlement of the foundation soils
- high trafﬁc loads
Common bridge approach problems
White's team observed the following bridge approach problems at existing bridges with poor performance:
- Backﬁll materials under poorly performing approach slabs are often loose and undercompacted.
- The foundation soil or embankment ﬁll settles.
- Many bridge approach elevation proﬁles have slopes higher than 1/200, which is considered a maximum acceptable gradient for bridge approaches.
- Voids develop under bridge approaches within one year of construction, indicating insufﬁciently compacted and erodible backﬁll material.
- Inadequate drainage is a major bridge approach problem. Many abutment subdrains are dry with no evidence of water, are blocked with soil and debris, or have collapsed.
- Many expansion joints are not sufﬁciently ﬁlled, allowing water to ﬂow into the underlying ﬁll materials.
Recommended design alternatives
The following design alternatives show promise:
- Use a combination of porous backﬁll (preferably crushed material) and geocomposite drainage systems behind the abutment to improve drainage capacity and reduce erosion around the abutment.
- For bridges with soft foundation or embankment soils, improve embankment compaction with moisture control, foundation preloading, ground improvement, soil removal and replacement, or soil reinforcement. This has the potential to reduce time-dependent post-construction settlements.
- Connect the approach slab to the abutment or the deck of the bridge and eliminate the expansion joint at the bridge end of the approach slab. Support the far end of the approach slab on a sleeper beam with a construction joint of two inches and provide an improved joint sealing system.
For more information
See the project report, Identiﬁcation of the Best Practices for Design, Construction, and Repair of Bridge Approaches, and a technology transfer summary on the project web page.
If you have speciﬁc questions, contact David White, 515-294-1463, firstname.lastname@example.org.