Practices for Bridge Approach Systems (2021)

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4 Introduction 1.1 Background The purpose of a bridge approach system is to provide a smooth transition between a high- way pavement and a bridge. However, as a result of several factors, differential settlement or bumps are often present at the end of bridges (Briaud et al. 1997). Bridge approach systems are comprised of a number of components. The key components are the abutment, which supports the end span of the bridge and retains the approach’s embankment fill; the approach slab or pavement, which provides the riding surface; the backfill and sleeper slab (if used), which support the approach slab or pavement; and the expansion joints, which accommodate thermal expansion and contraction of the bridge superstructure and approach slab. In conventional systems, the expansion joint is often located between the abutment and the bridge superstructure and bridge expansion and contraction are accommodated using underlying bearings. However, because this design is susceptible to joint leakage and corrosion and subsequent deterioration of the girders, bearings, and abutment, there is a general preference to eliminate expansion joints at the bridge abutment. As a result, integral systems are generally favored. Integral systems eliminate the need for an expansion joint between the abutment and the bridge deck by incor- porating a rigid connection between the bridge superstructure and the abutment. The expansion joint may then be located between the abutment and the approach slab, but joint leakage in this location often causes erosion of the backfill resulting in higher loads on the abutment and loss of support to the approach slab or pavement. Therefore, many departments of transportation (DOTs) prefer to rigidly tie the approach slab to the abutment as well and transition the expan- sion joint to the roadway end of the approach slab where the consequences of joint leakage are not as severe. The choice between using an integral or jointless system and using a nonintegral system is a major design decision and both affects and is dependent on the design of other components, such as foundations and wingwalls. The drainage system at the bridge end and the approach is generally considered separately from the bridge approach system, but it can significantly affect bridge approach performance. The bridge approach drainage system consists of both surface elements, such as gutters, curbs, and inlets, and subsurface elements, such as free-draining backfill, weep holes, and underdrains. Both systems are used to prevent erosion, and the surface system captures runoff to prevent leakage onto underlying elements and fill while the subsurface system prevents hydrostatic pressures on the abutment and retaining walls. Runoff flow between the approach and the bridge may be minimized to prevent the bridge drainage system from being overwhelmed and to protect any joints present and minimize leakage. Alternatively, bridge runoff may be channeled to the approach drainage system to decrease the risk of deterioration of the bridge structure. At the approaches, open surface systems or closed systems may be used to release the runoff safely down the embankment slope. C H A P T E R   1

Introduction 5   Quality control (QC) measures incorporated during construction often include QC test- ing of the compaction of the backfill and smoothness assessment of the riding surface of the approach. Tests on the backfill are typically in situ measurements of density using nuclear gages. The smoothness of the riding surface may be assessed by hand using a straightedge or by using an inertial profiler to automatically capture the surface profile and calculate an index to represent the surface roughness and ride quality. Ride-quality measurement methods are often used to assess the need for repair later in the life of the system. Ride quality of a bridge approach system is often compromised as a result of loss of support beneath the approach slab or pavement, either by differential settlement, erosion, or failure of the paving corbel supporting the slab or pavement at the abutment. Other issues such as joint leakage can cause or exacerbate these problems. As a result, a bump may form either at the bridge end or the roadway end of the approach slabs or the approach may experience cracking. It is relatively difficult and can be expensive to provide a lasting repair to address these issues and, therefore, they are combatted using preventive and mitigation strategies during bridge approach design. Based on literature review information provided in Chapter 2, strategies to address the following performance issues or degradation mechanisms of bridge approach systems include • Differential Settlement. Specifications for the type of backfill and its properties, such as compaction and moisture content requirements for the backfill, at placement and QC test- ing are used to mitigate settlement of the backfill. Ground improvement methods such as temporary surcharge loads may also be applied to eliminate short-term consolidation of the underlying soils and decrease long-term consolidation. • Erosion. Limits on the amount of fines and permissible gradations for the backfill material are commonly specified to prevent erosion. This additionally helps prevent infiltration and blockage of subdrains. • Approach Slab Rotation. Inadequate support at the roadway end of the approach may cause the slab to rotate and a bump to form at the bridge end of the approach. Foundation (sleeper) slabs may be used to provide better support at the roadway end of the approach. • Joint Leakage. The most effective method to avoid joint leakage is to eliminate joints by using integral connections as much as possible. However, integral connections between all of the approach elements are not always feasible because of structural loading or site conditions. • Slab or Pavement Cracking. Cracking is generally handled by using more robust slab and pavement designs. This may entail using thicker, doubly reinforced slabs or fiber-reinforced concrete, or by adding tensile reinforcement to pavement. • Failure of the Paving Corbel. The paving corbel may fail as a result of inadequate design or poor-quality construction (White et al. 2005). Options for addressing this issue include reevaluating the loads and necessary steel reinforcement and using a paving seat with a simpler cross section such as a paving notch. 1.2 Synthesis Objectives The objective of this synthesis was to document current state-of-the-practice for the design, construction, and maintenance of bridge approach systems. Information about the following topics was collected: • Design criteria and standards for approach slabs, approach pavement, and supporting elements such as sleeper slabs and backfill materials; • Performance criteria for bridge approach systems such as surface smoothness and joint performance; • Practices related to the connection between the approach slab and the bridge abutment;

6 Practices for Bridge Approach Systems • Geotechnical considerations for settlement and compaction of backfill material; • Water management practices and mitigation strategies to address drainage issues; • Prioritization of major performance issues; and • Practices deemed effective by DOTs. 1.3 Synthesis Scope and Approach This synthesis includes a literature review, a survey of DOT practices, and case examples to detail and highlight the practices used by five state DOTs. In the literature review, design guidance for bridge approach systems and selection between the different types of components and standard specifications related to the construction of the system components are presented. Research studies on the causes of approach performance issues such as poor ride quality and erosion—and preventive and mitigation strategies and repairs to address these issues—are summarized. A survey of state practice was distributed to the voting members of the AASHTO Committee on Bridges and Structures to augment the information gathered during the literature review. Questions were divided into four topics: (1) design and construction of bridge approach systems, (2) water and joint management, (3) operation and maintenance, and (4) performance issues and mitigation practices. The survey particularly inquired about DOT design preferences, construction and inspection requirements, experience with performance issues, expected service life of the bridge approach systems and their components, and improvements DOTs have made to their specifications. A total of 44 (out of 51) DOTs responded, an 86% response rate. Based on their willingness to share additional information and unique practices, Colorado, Iowa, Massachusetts, Texas, and West Virginia were selected for more in-depth interviews to discuss their experiences with bridge-approach system design and construction and their effects on the systems’ performance and to identify which practices they found effective. 1.4 Organization of the Synthesis Report This synthesis report has five chapters: • Chapter 1 (this chapter) introduces the synthesis topic by providing background information and identifies the objectives, scope, and organization of this synthesis. • Chapter 2 presents the literature review of DOT practices for the design and construction of bridge approach systems and strategies to address major performance issues. • Chapter 3 presents the results of the survey of DOT practices regarding the design, construc- tion, inspection, maintenance, and performance of their bridge approach systems. • Chapter 4 reports the case examples conducted for select state DOTs. • Chapter 5 provides a summary of the collected information and concludes the synthesis by identifying key observations and trends as well as suggested areas for further research. In addition to the references, the report includes three appendices. Appendix A contains references for the 33 state DOT specifications and standards used in Chapter 2. Appendix B contains the survey questionnaire distributed to the voting members of the AASHTO Committee on Bridges and Structures for each state department of transportation. Appendix C is a tabulated summary of the individual survey responses that were received.