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87  Conclusions This synthesis was completed to document the state of the practice for the design, construc- tion, maintenance, and performance of bridge approach systems and their elements. Gaps in knowledge and areas suggested for future research based on the findings are also presented. 5.1 Summary and Conclusions The major findings of the literature review and survey completed in this study are presented holistically in Section 5.1.1 through Section 5.1.3, while lessons learned from case examples are summarized in Section 5.1.4. 5.1.1 Bridge Approach System Design The key components of a bridge approach system are the abutment, foundation, approach slab or pavement, expansion joints, sleeper slab, wingwalls, and backfill. Each component has a variety of different design options, and they may be arranged into a large variety of different configurations to provide a serviceable bridge approach. The design depends heavily on the site constraints and requirements, such as types and thickness of subsoil layers, available right-of- way, and potential for scour. However, survey respondents indicated that integral bridges with the joint between the approach slab and the roadway are generally the most favored system, followed by semi-integral abutments with the joint in the same location. In general, the current practice is to eliminate expansion joints or move them away from the bridge, if possible, but other commonly reported configurations place the joint between the approach and the bridge and use stub abutments. Almost all of the survey respondents reported using RC, CIP approach slabs, which are designed in accordance with AASHTO LRFD 4.6.2.3. These slabs may be omitted and a flexible pavement used instead if the bridge has a low traffic volume, has small thermal movements, or is not expected to experience differential settlement. A sleeper slab may be incorporated to support the roadway end of the approach slab and any expansion joint at this location, but there is no strong consensus on the use of sleeper slabs based on the survey results. Granular or porous granular material is the most common backfill material used behind abutments according to the survey, and the backfill may be wrapped in or reinforced with hori- zontal layers of a geosynthetic, such as a geogrid or geomembrane. In addition to slope stability, geosynthetics may be employed to improve drainage beneath the approach slab and behind the abutment, but only a few agencies incorporate geosynthetics for water management purposes. According to the survey results, the majority of the DOTs capture runoff from the bridge deck and/or the approach slab. Details such as the type of drainage system (open or closed), the C H A P T E R  5
88 Practices for Bridge Approach Systems release location of the runoff, and the type of subdrainage system (weep holes or subdrains) are typically site-specific and may depend on variables such as the abutment dimensions and the proximity of sidewalks. 5.1.2 Bridge Approach System Construction Acceptance or quality testing may be used to ensure construction quality and mitigate the risk of differential settlement and poor ride quality. Specifications for smoothness of the approach riding surface and the compaction and moisture content of backfill at placement are common requirements based on the literature review. However, instead of assessing smoothness of the riding surface, one survey respondent reported measuring joint tolerances to determine if the quality of the approach is acceptable. The as-built smoothness and ride quality may be checked either by hand using a straightedge or by calculating an index from the data collected by an inertial profiler or profilograph. For the latter, the most common metric used is the IRI, based on the survey results. Regarding installation of the backfill, the survey results indicated that 75% of respondents (33 of 44) require compaction of the backfill to between 90% and 100% of the maximum dry density. In situ QC testing of the density is done by a minority of the DOTs, according to literature. Only 34% of the responding DOTs (15 of 44) specify moisture content of the fill at placement. The most common requirement specifies a moisture content within 2% to 4% of the optimum moisture content, but a few DOTs require flooding of the material to prevent bulking and subsequent settlement. 5.1.3 Bridge Approach System Performance and Maintenance Based on the survey results, the median service life for bridge approach systems is 25Â years. The service lives reported depended on which component of the system was assumed to require rehabilitation. Inspections of bridge approach systems tend to coincide with bridge inspections for the NBI system such that the inspection period is every 2Â years. The majority of survey respondents reported that they inspect joints for sealant failure and the system for differential settlement. While differential settlement is monitored, approximately one-third of the survey respondents stated that the severity is not quantified and another third stated that a qualitative description is used. The survey results align with the requirements found in the literature review, which include the measurement of Approach Roadway Alignment, which is an indirect, qualitative indication of ride quality defined in the FHWA Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation's Bridges (FHWA 1995). Bridge approach systems are also subject to element-level inspection, in accordance with the AASHTO MBEI. This type of inspection focuses on material degradation and damage to the individual elements and does not assess the functionality of the entire system. As a result, state DOTs have developed their own inspection guides that address these oversights as required for their region, resulting in wide variation in inspection practice despite the standards set by the AASHTO MBEI. The survey indicates that the performance issues most commonly encountered with bridge approach systems are failure of the sealant in the expansion joint, bump formation at the joints on either side of the approach, and erosion. In general, joint sealant failure requires repair or replacement of the seal or joint. Bump formation may be addressed by joint repair, grinding, patching, wedge paving, HMA application, or jacking of the slab. Some DOTs use asphalt over- lays on top of the approach slab to allow for easy repair of the ride quality by filling any depres- sions that form over time with overlay reapplication. Erosion may also be repaired by void injection or filling, which is done most commonly with a flowable fill.
Conclusions 89  The decision on whether or not to repair these issues is generally made on a case-by- case basis. Regarding ride quality, user complaints, safety concerns, and recommendations from inspectors or inspection data may be used to initiate an investigation to determine if a repair is necessary. Erosion may not be addressed unless the situation is severe resulting from the design redundancy of a robust approach slab that is designed to consider loss of support. Similarly, other minor maintenance may be deferred until a more significant repair is required. In general, repairs are completed on an as-needed basis rather than a predetermined schedule. 5.1.4 Lessons Learned from Case Examples The Colorado, Iowa, Massachusetts, Texas, and West Virginia DOTs were selected as in-depth case examples to discuss their unique policies and demonstrate the variation in bridge approach system design, construction, and management. Information about the typical design and construction practices; inspection and maintenance; and performance issues and mitigation practices was compiled and summarized from the documents found in the literature review and the survey responses. The DOTs were then contacted for further discussion on lessons learned and their suggested practices or revisions to existing practices. Key points about their unique practices include the following: ⢠The Colorado DOT always uses HMA on approach slabs as a design practice to limit bump formation. The DOT also typically uses a structural backfill with geotextile reinforcement, and this choice of backfill reportedly provides good quality compaction and favorable composite performance of the geotextile and soil. ⢠The Iowa DOT uses a three-approach slabs detail which includes double-reinforced, single- reinforced, and unreinforced slabs as typical practice. The DOT places backfill under a flooded condition to avoid bulking and volumetric issues with the backfill. While the Iowa DOT does not use geosynthetics for soil reinforcement purposes as the Colorado and West Virginia DOT do, geosynthetics are used to contain the flooded backfill at placement. ⢠The Massachusetts DOT has a long history of using buried approach slabs, which decrease the live load requirements on the abutment and mitigate bump formation. ⢠The Texas DOT specifies cement-stabilized backfill as one of their approved backfill materials. Reportedly, the cement-stabilized backfill prevents issues associated with compaction and poor moisture control, but this material has a risk of piping. ⢠The West Virginia DOT typically uses a reinforced soil mass backfill that relies on geotextile reinforcing layers and attributes decreased settlement in approach fills to this geosynthetically reinforced soil system. Finally, the case example DOTs were asked about which divisions are responsible for the bridge approach system and how these responsibilities and roles change through the design, construction, and operation and maintenance phases of the structure. The responsible party changed throughout the life of the structure, and the DOTs indicated that a wide variety of disciplines, including bridge and structure groups, geotechnical groups, highway design and maintenance groups, and bridge crews, play important roles in the successful management of the bridge approach system. 5.2 Knowledge Gaps and Suggested Future Research In general, the information collected in this synthesis shows that design considerations for bridge approach components vary across the DOTs. This is illustrated by the differences observed in the reviewed DOT standard specifications as well as the survey responses. As a
90 Practices for Bridge Approach Systems result of this synthesis, the following areas have been identified as knowledge gaps that could be addressed through future research: ⢠Identification and characterization of correlations between design choices and performance metrics, such as long-term ride quality, for bridge approach systems. The overall goal would be to identify the optimum design configurations for bridge approach systems, which requires a multidisciplinary approach to cover all the components of such systems. ⢠Determine the types and frequency of QC testing that are most beneficial to long-term serviceability of bridge approaches. ⢠Assess the effectiveness of unique design details used by some of the state DOTs that have achieved good performance for potential wide implementation across other DOTs. ⢠Develop and evaluate innovative solutions to improve the long-term serviceability of bridge approaches. ⢠Set criteria and metrics for which repair of joints and ride quality is required and below which deferred repair is appropriate.
