Feasibility Study: Alternatives to Prevent Settlements and Bumps at Bridge Approaches in Nebraska

1. Background The ubiquitous bridge approach slab problem, also known as "bump at the end of the bridge", often occurs near the interface of the bridge abutment and the approach, or that of the approach and the roadway pavement. The main cause is a differential settlement between the abutment and the approach {Abu-Hejleh et al., 2008; ). This problem may lead to uncomfortable & dangerous driving conditions, a negative perception in the minds of transportation users, and substantial damages to bridges, which may result in frequent maintenance, an additional budget for the repair, and traffic delay. The "bump" problem has been reported to affect about 25% of entire bridges in the United States {i.e., about 150,000 structures nationwide; Mishra et al., 2010). Moreover, at least $100 million has been spent every year on necessary repairs with this issue (University of New Mexico, 2006). Based on the number of bridges, it can be roughly estimated that the cost in Nebraska could be around $2-3 million every year. In terms of the geotechnical aspect, fundamental causes of the differential settlement and the "bump at the end of the bridge" can be summarized as follows (Wahls, 1990; Briaud et al., 1997; White et al., 2007): • Time-dependent consolidation of the natural soils (embankment foundation), • Compression/consolidation of the approach embankment, • Poor compaction of abutment backfill soils, • Erosion of soils at the abutment face, and • Poor drainage of the embankment and abutment backfill. To minimize the occurrence of the "bump at the end of the bridge" problem due to the differential settlement, Nebraska Department of Transportation (NDOT) has been applying a grade beam policy to new approach slabs (NDOR Bridge Division, 2016). According to the policy manual, grade beams are to be parallel to the abutment and located 20 ft beyond the end of the bridge floor, and be extended to the outside edges of the approach section. The grade beam that typically involves driven piles to reach a bedrock (thus having a full load bearing capacity) is used to support the approach slab, which is enough to prevent any substantial differential settlements. This method has been very successful, resulting in negligible bridge settlement of the bridge approaches. Indeed, a recent nationwide survey showed that Nebraska retains a lower percentage (0-25%) of bridges that have been experiencing the "bump" issues (Yasrobi et al., 2016). Most bridges with the "bump" issues are those that were constructed before the new grade beam policy was at practice. However, the current practice in Nebraska does not provide many details in terms of the design. For example, details about the minimum bearing capacity, allowable settlement, driving length, and cross-sectional area, of the grade beam piles are not given in the BOPP. Such a lack of detailed guideline might lead to an unnecessary increase in the construction cost. In some cases, the current design may result in an overachievement with the pre-assigned length & dimension of the grade beam pile. Moreover, in the current method, considerable settlements were sometimes observed at the interface between the approach slab and the asphalt pavement section where the grade beam is not installed. Accordingly, there is an important research need to provide more details and make necessary revisions to the current grade beam policy to minimize the settlement and bumps at the bridge approaches with less cost and same confidence level as before. Besides, there is a research need to investigate other potential alternatives to prevent such a differential settlement not only at the interface of the bridge abutment and the approach, but also at the interface of the approach and the roadway pavement. One of the feasible alternatives to mitigate such "bump at the end of the bridge" and the different settlement of an approach slab is the application of geosynthetic reinforcement (or geosynthetic reinforced soil, GRS) underneath the approach slab. This method needs to achieve backfill compaction at the optimal moisture content, especially for a coarse-grained backfill material {Abu-Hejleh et al., 2006). According to previous researches (Abu-Hejleh et al., 2006; Helwany et al., 2007; Abu-Farsakh et al., 2013; Bai et al., 2013; Chen and Abu-Farsakh, 2016; Eun et al., 2017), the use of geosynthetic reinforced soil resulted in that the monitored movements of the bridge structure were mitigated compared to those anticipated from the design phase or allowed by the performance requirements. In addition, with the use of GRS systems, post-construction movements can be reduced substantially; thus, the bump problem at the bridge transition is minimized. Furthermore, the obvious advantages of this method are cost-effectiveness ($0.5/yd2 from discussion with industrial engineers), being simple and fast to construct, having a good seismic performance, and that the method can tolerate greater deformation without structural failure. Nonetheless, only a handful of state DOTs, such as LA, CO, WI, TX, and OK DOT, have implemented it in practice probably due to the limited familiarity of this method with the contractors. 2. Objective The proposed research will pursue two principal goals: (1) improve the current design practices of the approach slab foundation in Nebraska, and (2) examine the feasibility of applying geosynthetic reinforcement of soils for preventing the settlement issues at the bridge approaches with less cost. The proposed scope of work is as follows: (1) Extensive survey & review: Survey of current bridge approaches and primary causes of the approach slab settlement for selected bridges in Nebraska. The survey includes investigation of embankment and foundation soil conditions typically encountered in Nebraska. The review of design practices in Nebraska includes a critical review of the design, construction, and post-construction records as well as field investigation at selected bridge sites. (2) Review of causes of approach slab settlement, and solutions to the "bump at the end of the bridge approaches" problems in other states using available literature to determine their relevance to Nebraska conditions and gain insights into possible strategies. (3) In-depth analysis to improve the current design practice of bridge approaches in Nebraska. The improvement of current design practices with the grade beam includes the geometry of piles ( e.g., length and number of piles) as well as geotechnical and structural details of the bridge approach slab. The geotechnical analysis includes the embankment geometry, soil type, compaction requirement, abutment backfill requirement, etc. The approach slab analysis includes structural design, slab support (i.e., pile foundation - dimensions and lengths), drainage layers, and connections at the interface between the bridge and roadway pavement. (4) Experimental and numerical analysis to examine the application of geosynthetic reinforcement for the approach slab foundation. The design and construction recommendations of approach slab foundation associated with geosynthetics will be prepared with the consideration of the type, spacing, location, and the distance from the bridge deck. (5) Cost-effectiveness and constructability analysis on both the design modification and the geosynthetic reinforcement of soils in Nebraska. The optimized design and construction recommendations for the approach slab foundation will be developed to possibly reduce the cost and to enhance the constructability and maintenance. The comprehensive analyses of current design practices and the geosynthetic reinforcement of soils will be implemented with the site-specific soil conditions in Nebraska to suggest which option would perform well for different site-specific Nebraska soil conditions. The geotechnical investigation includes the limit-state analyses and FEM simulations. Details of each task are provided in Section 6 "Tasks".


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    • Status: Completed
    • Funding: $99,469.00
    • Sponsor Organizations:

      Nebraska Department of Transportation

      1500 Nebraska 2
      Lincoln, NE  United States  68502
    • Project Managers:

      Halsey, Lieska

    • Performing Organizations:

      University of Nebraska, Lincoln

      1400 R Street
      Lincoln, NE  United States  68588
    • Principal Investigators:

      Kim, Seunghee

    • Start Date: 20190701
    • Expected Completion Date: 20210531
    • Actual Completion Date: 20210501
    • USDOT Program: Transportation, Planning, Research, and Development

    Subject/Index Terms

    Filing Info

    • Accession Number: 01705859
    • Record Type: Research project
    • Source Agency: Nebraska Department of Transportation
    • Files: RIP, STATEDOT
    • Created Date: May 24 2019 11:50AM