Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls (2012)

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5 INTRODUCTION As with bridges and pavements, retaining walls are an essen- tial component of our transportation infrastructure. However, unlike pavement and bridges, retaining walls (of which MSE walls are a growing subclass) are often overlooked as an asset. Proper asset management is essential to making informed, cost-effective program decisions and optimizing existing highway resources. The Roadway Data Highway Performance Management System (HPMS) is a national transportation data system that provides detailed data on highway inven- tory, condition, performance, and operations. It describes functional characteristics, traffic levels, and pavement con- ditions for all interstate highway system sections. In addi- tion to the HPMS, at least 36 individual state departments of transportation (DOTs) collect basic pavement inventory data, while more than 41 DOTs collect some type of data relative to pavement fatigue and cracking as part of their pavement management systems (Cambridge Systematics et al. 2009). With respect to bridges, the federal government has man- dated the creation and maintenance of the National Bridge Inventory (NBI), which contains data on all bridges and cul- verts on or over U.S. roads that are greater than 20 ft long. These bridges are also inspected every two years per the National Bridge Inspection Standards (NBIS). In contrast, there is no dedicated management system addressing the whole of the nation’s retaining walls, MSE or otherwise. Indeed, although asset management guidance is provided for highway features such as pavements, bridges, culverts, guardrails, and drainage structures in the Asset Management Data Collec- tion Guide developed in conjunction with AASHTO (2006), retaining walls are not addressed—despite there being an estimated 16.3 million square meters of various types of walls along the nation’s highways with values ranging from approximately $200 to $2,000 per square meter (DiMaggio 2008). With respect to MSE walls specifically, Berg et al. (2009) indicated that an average of 850,000 square meters of MSE wall with precast facing is built each year in the United States, along with an additional 280,000 square meters of modular block wall. Also, according to Berg et al. (2009), typical total costs for permanent transportation-related MSE walls range from $320 to $650 per square meter of wall face, and modular block walls less than 4.5 m high are less expen- sive by 10% or more. Elias et al. (2004) placed the cost of MSE walls in the somewhat lower range of $160 to $300 per square meter. During the preparation of this synthesis, two documents were found to be of particular interest to users of this syn- thesis, thus meriting specific mention. The first document, Guide to Asset Management of Earth Retaining Structures, by Brutus and Tauber (2009), is the product of a study con- ducted for the AASHTO Standing Committee on Highways, with funding provided through NCHRP Project 20-07. This publication presents methodologies and considerations aimed at helping transportation agencies establish asset manage- ment programs for earth retaining structures (of which MSE walls are a component), with particular focus on the devel- opment of inventories and inspection programs. The pub- lication also presents the results of a survey similar to the one performed for this synthesis regarding the inventory, inspection, and asset management activities of transporta- tion agencies concerning their earth retaining structures. The second document is National Park Service Retaining Wall Inventory Program (WIP)—Procedures Manual, by DeMarco et al. (2010b). This document represents the efforts of the FHWA Office of Federal Lands Highway, working with the National Park Service (NPS), to develop and implement a retaining wall inventory and condition assessment pro- gram [collectively referred to as the Wall Inventory Program (WIP)]. The document describes in detail the data collec- tion and management processes, wall attribute and element definitions, and team member responsibilities for conduct- ing retaining wall inventories and condition assessments as derived from experiences involving nearly 3,500 walls. Although MSE walls constitute only a small fraction of the walls involved in the development of the FHWA’s WIP, much of the material in this document is applicable and/or transferable to matters associated with the long-term per- formance of MSE walls. PARTIES WITH RESPONSIBLE CHARGE FOR MECHANICALLY STABILIZED EARTH WALLS MSE walls are multidisciplinary in nature, having both struc- tural and geotechnical components. Once constructed, main- tenance concerns are introduced. To develop and maintain an effective inventory, some party must first take responsibility chapter two STATE OF MECHANICALLY STABILIZED EARTH WALL INVENTORY PRACTICE

6 panel walls, two-stage panel walls, and block walls, respec- tively. The majority of panel walls possess metallic reinforce- ment. Some wall inventories are also maintained by city-level agencies. The cities of Cincinnati, Ohio; New York City, New York; and Seattle, Washington, all maintain retaining wall inventories, including MSE walls. FHWA has developed a wall inventory and database for the National Park Service list- ing more than 3,500 walls, some of which are MSE walls. Although a minority of agencies appear to maintain well- defined MSE wall databases (and fewer still have regular inspections to inform the database beyond the basic identify- ing information), some limited MSE wall inventory and per- formance data are apparently maintained by some agencies. Additionally, some MSE wall inventory and performance data are inherently contained in the NBI and are accessible in software database appli cations such as PONTIS or other agency-maintained databases. These “overlooked” MSE walls would typically be those that serve as bridge abutments or are considered integral to the performance of the bridge structure. These databases contain basic wall information such as spatial dimensions, construction date, and some type of performance rating of bridge support, but greater detail may be lacking. Once recognized, bridge inventory data may be a starting point for developing MSE wall inventories and performance assessments. NATURE AND SCOPE OF INVENTORIES Agencies that have established MSE wall inventories appear to own between 100 and 1,000 MSE walls (with mean and median values of 508 and 400, respectively). However, as explained by Gerber et al. (2008), wall counts can be prob- lematic. Single wall segments at a bridge abutment might be treated as an individual wall, whereas at other times one abut- ment and two adjoining wing-wall segments might be desig- nated as a single wall. Consequently, at a bridge abutment with one MSE wall segment beneath the bridge and two MSE wall segments serving as wing- walls on either side, one could count either one or three walls. If one considers a similar configuration for the other abutment, for the walls. As shown in Table 1, when queried regarding who has responsible charge for MSE walls once the walls are constructed and accepted, 41% of survey respondents noted it was a maintenance engineer at a regional or district level. Those who responded “other” generally indicated a mixed or shared responsibility among the various structural (i.e., “bridge”), geotechnical, and maintenance professionals. Approximately 14% of respondents indicated that no one in their agencies has responsibility for MSE walls after construc- tion and acceptance. AGENCIES HAVING INVENTORIES Several questions of the survey for this synthesis project focused on the nature and extent of transportation agencies’ MSE wall inventories. Thirty (more than two-thirds) of sur- vey respondents indicated that they do not maintain a specific MSE wall inventory. Of the 14 respondents who do have inventories (listed here), 43% reported that the inventory is partial, limited to specific geographic areas, or constrained in some other way. (Although not survey respondents, the states of Ohio, Pennsylvania, and Washington also appear to have at least partial MSE wall inventories. Alberta, Canada, reports “defined problem sites” as a type of wall inventory.) • Alberta, Canada • California • Colorado • Kansas • Minnesota • Missouri • Nebraska • New York • North Carolina • North Dakota • Ontario, Canada • Tennessee • Utah • Wisconsin. In reporting what types of MSE walls are included in their inventories, 100%, 71%, and 86% named one-stage Response Number Percent Structural engineer(s) or similar at an agency-wide level 3 7 Structural engineer(s) or similar at a regional or district level 3 7 Geotechnical engineer or similar at an agency-wide level 3 7 Geotechnical engineer(s) or similar at a regional or district level 0 0 Maintenance engineer or similar at an agency-wide level 4 9 Maintenance engineer(s) or similar at a regional or district level 18 41 No one has this charge 6 14 Other (specify) 7 16 TABLE 1 PARTY HAVING RESPONSIBLE CHARGE FOR MSE WALLS ONCE THE WALLS ARE CONSTRUCTED AND ACCEPTED (most representative response)

7 one could assign one, three, or six wall numbers to the MSE wall segments present at a bridge site. (There could be even more than six if additional walls segments were used to support the exterior sides of ramps.) In the literature, there appears to be little consensus regarding methodologies for individual wall designations. However, sev- eral sources suggest that whatever system is used to identify and count walls, physically tagging the walls with identifiers is a helpful practice. Different agencies use different criteria when determining what MSE walls to count and/or include in their inventory/ database. Brutus and Tauber (2009) provide extended dis- cussion of various possible criteria, which commonly include wall height, proximity to the roadway, batter or face slope, wall ownership, structural type, and proximity to bridges or culverts. The main criteria used by FHWA’s WIP are related to jurisdiction (e.g., is the wall along a qualifying road?), proximity of wall relative to roadway, wall height, wall embedment, and wall face angle. [The WIP uses a wall face angle criterion of 45 degrees or greater so that some rockeries and slope protection buttressing are included in the inven- tory, whereas FHWA (see Berg et al. 2009) typically defines a retaining wall as having an internal face angle greater than or equal to 70 degrees to differentiate walls from reinforced slopes.] The FHWA program also advises that when wall acceptance based on the aforementioned criteria is marginal or difficult to discern, “include the wall in the inventory, particularly where the intent is to support and/or protect the roadway or parking area and where failure would sig- nificantly impact the roadway or parking area and/or require replacement with a similar structure.” Based on synthesis survey results shown in Table 2, most inventories include only those walls owned by the agency. Only 57% include walls not associated with a specific bridge or culvert. When a wall height criterion is used, 1.2 or 2 m are the most frequent threshold values. In evaluating the comprehensiveness of inventory data- bases they currently maintain, transportation agencies report that between 10% and 100% (mean and median of 70% and 78%, respectively) of the walls that satisfy their inclusion criteria are accounted for (Table 11 subsequently shows this information by agency). The particular content contained in each respective database varies and is discussed in the next chapter. As mentioned previously, some MSE wall inventory information and performance data are inherently contained in the NBI. These MSE walls would typically be those that serve as bridge abutments or are considered integral to the performance of the bridge structure. Gener- ally, walls that are not within the vertical projection of the bridge deck and are not constructed integrally with either wing-walls or abutments are not included in bridge assess- ment activities. Table 3 summarizes who in an agency principally manages/ maintains its inventory of MSE walls. Most frequently it is a geotechnical engineer or similar person at an agency-wide level. This may be inconsistent as Table 1 indicates that main- tenance engineers at a regional or district level are the individu- als who have responsibility for MSE walls once they are built. It thus appears that there may be a disconnect between those considered responsible for MSE walls and those actually doing the work of asset management. However, such an arrangement need not be problematic; multiple parties can be involved in MSE wall management provided there is a clear understanding that responsibility for the asset may lie in a place other than the location of the data or even the expertise used to collect and/or evaluate the data. Communication and understanding of individual responsibilities would obviously be essential for an effective inventory and assessment program. Inventories can be maintained in various formats and manipulated using various tools. The current state of prac- tice is summarized in Table 4, which lists the variety of Response Number Percent Wall owned by my agency 14 100 Wall owned by others but adjacent to facilities for which my agency is responsible 4 29 Wall owned by others but may negatively impact adjacent facilities for which my agency is responsible 1 7 Wall is associated with a bridge structure 12 86 Wall is associated with a culvert 7 50 Wall is not associated with a bridge or culvert 8 57 Minimum wall height 6 43 Minimum height of retained earth 2 14 Minimum wall length 1 7 Minimum wall area 0 0 Other (specify) 2 14 TABLE 2 CRITERIA USED TO DETERMINE WHAT MSE WALLS TO INCLUDE IN INVENTORY (multiple responses possible)

8 mation regarding maintenance does not appear to be system- atically maintained by any party. CONSTRAINTS ON INVENTORY DEVELOPMENT AND ASSET MANAGEMENT ACTIVITIES During oral interviews with select survey participants, the participants frequently identified the lack of a government/ legislative directive along with the lack of allocated fund- ing as significant impediments either to initially develop- ing their MSE wall inventory or subsequently populating it with performance data from inspection activities. Although some increasing awareness and impetus toward asset man- agement for retaining walls appears to have existed in the early to mid-2000s (partially characterized by the devel- opment and distribution of informational brochure “Earth Retaining Structures and Asset Management,” developed by FHWA (2008), it appears that the economic down- turn of 2008 through the present has largely halted those efforts. In Colorado, for example, a plan for implementing a state-wide monitoring program for all types of retaining walls and sound walls was developed for the state DOT (Hearn 2003). Although the feasibility report concluded that “no impediment [was] found to full development of standard data and procedures for walls and sound barri- ers,” little progress toward implementation has been made as yet because of funding constraints. DOTs in Oregon (see Turner 2008), Nebraska, Ohio, and Utah have simi- methods used to manage MSE wall inventories, with pref- erences given to simple spreadsheets or MS access-type databases. The potential range of information maintained as part of an MSE wall inventory is broad. Data regarding wall location and geometry are perhaps the most common elements, but depend- ing on the use of the inventory/database, other information might be maintained, including wall features, construction data, and inspection information. Brutus and Tauber (2009) suggest that information such as dates of construction and repairs, geo- metric wall dimensions, wall materials including backfill type, specific element types and manufacturers, as-built and shop drawings, specifications, quality control test data, and inspec- tion reports be included. They also suggest that a wall database should include basic traffic-volume data. Hearn (2003) offers similar suggestions. Table 5 summarizes the frequency at which different types of information is collected and/or maintained by surveyed agencies as part of their wall inventories. The most frequently tracked metrics are wall location by route/milepost and wall type. These metrics are followed by date constructed, rein- forcement type, and shop drawings. Given that degradation and/or corrosion of reinforcement is a primary concern of agencies (as revealed in a subsequent section of this report), it is logical that these two particular and apparently coupled metrics are among the more frequently tracked items. Infor- Response Number Percent Structural engineer(s) or similar at an agency-wide level 4 29 Structural engineer(s) or similar at a regional or district level 0 0 Geotechnical engineer or similar at an agency-wide level 5 36 Geotechnical engineer(s) or similar at a regional or district level 0 0 Maintenance engineer or similar at an agency-wide level 0 0 Maintenance engineer(s) or similar at a regional or district level 3 21 Other (specify) 2 14 TABLE 3 PARTY WHO PRINCIPALLY MANAGES/MAINTAINS INVENTORY OF MSE WALLS (most representative response) Response Number Percent File boxes/cabinets 3 21 Spreadsheet 4 29 MS Access database without GIS support 3 21 Oracle database without GIS support 1 7 PONTIS 1 7 Other non-GIS supported database (specify) 2 14 GIS-based software (specify) 0 0 TABLE 4 PRIMARY TOOL USED AS AN ASSET MANAGEMENT SYSTEM FOR MSE WALL INVENTORY (most representative response)

9 evaluation and inventory of its MSE walls out of corrosion concerns (Wheeler 2002). In follow-up discussions regarding the synthesis survey, many respondents expressed hope that increased awareness and resource allocation could be achieved before any signifi- cant, adverse events such as those that led to the creation and ongoing support of the nation’s bridge inspection and assess- ment programs. larly reported that initially developed and/or implemented plans could not be sustained. In the mid-1980s, Califor- nia’s DOT (Caltrans) established procedures and responsi- bilities for monitoring, sampling, and testing the MSE wall structures; however, in 1997, budgetary constraints elimi- nated the program. Some MSE wall inspections continue, but the process is not systematic. New York State’s DOT is an exception to this trend; its inventory and assessment efforts date to 1985, when the state began an initial field Response Num ber Percent Location by Street Address 3 2 1 Location by Latitude/Longitude 4 2 9 Location by Route, Milepost 7 5 0 Location by State Plane Coordinates 1 7 Wall Type 6 4 3 Wall Function 3 2 1 Wall Geometrics—Maximum Wall Height 4 29 Wall Geom etrics—Average Wall Height 4 29 Wall Geometrics—Wall Length 4 2 9 Wall Geometrics—Slope in Front of Wall 2 14 Wall Geom etrics—Slope Behind Wall 2 14 Wall Geom etrics—Road/Traffic Offset 3 2 1 Date Constructed 5 3 6 Manufacturer 4 29 Contractor/Installer 1 7 Reinforcem ent Type 5 36 Drainage Conditions—Proximity of External Water Sources 0 0 Drainage Conditions—Location and Condition of Drainage Points 2 14 Nature of Adjacent Facilities Owned by Agency 1 7 Nature of Adjacent Facilities or Utilities Owned by Others (e.g., railroad) 0 0 Characterization of Adjacent Roadway Traffic 2 14 Design Data 1 7 Construction Data—Plans 4 29 Construction Data—Specifications 2 14 Construction Data—Shop Drawings/Submittals 5 36 Construction Data—Inspection Documentation 2 1 4 Construction Data—As-Builts 4 29 Post-construction Modifications 1 7 Photographs 4 2 9 Condition of Structure—External Inspection Data 3 21 Condition of Structure—Internal (e .g., corrosion) Inspection Data 0 0 Maintenance Activities 0 0 Other (specify) 1 7 TABLE 5 TYPES OF DATA AGENCIES GENERALLY COLLECTED OR MAINTAINED FOR MSE WALLS (multiple responses possible)