CHAPTER 2

Literature Review

This literature review covers published material commonly used or prepared by DOTs to apply asset management practices to MSE retaining walls, including inventory, inspection, and assessment of walls, as well as information on rehabilitation and repair of MSE walls in the context of the asset management. The sources used for this investigation were select technical publications, agency asset management plans, asset inspection guidance, and guidance documents provided by the DOTs. The following discussion topics were identified:

• MSE retaining wall design and construction,
• Repair and rehabilitation of MSE retaining walls,
• Inventory of MSE retaining walls,
• Inspection of MSE retaining walls, and
• Asset management of MSE retaining walls.

For the literature review, the authors identified multiple publications from state and federal DOTs on developing an inventory and assessment program, along with recommendations for monitoring MSE wall performance in the future. Published guidance for subsequent incorporation of a wall inventory, condition assessment, and proactive management into a mature asset management program is more limited, reflecting the ongoing development of this area of practice. Available guidance recommends creating an inventory and assessment program, with associated data management, and then using the results of that work to develop performance measures and minimum performance targets, incorporate risk, and analyze asset performance over time. The data from repeated wall inspections may be used over time to develop asset deterioration rates and life-cycle costs (Vessely 2013).

MSE Wall Design and Practice

MSE walls were first developed in the 1970s. Typically, this type of wall is cost-effective to construct and more tolerant of deformation caused by poor foundation conditions or seismic loading prior to failure than other wall types (Berg et al. 2009). The walls rely on metallic or geosynthetic reinforcements to reinforce the soil backfill and ensure overall structural stability. The reinforcement is attached to a facing that prevents the loss of the reinforced backfill. Precast concrete facing panels are the most common in highway applications, but welded wire baskets, blocks, and other materials can also be used for the wall facing (Alzamora and Anderson 2009). The design of MSE walls is well documented at both the state and federal DOT level and so was not the focus of this synthesis and not discussed herein.

Standard design practices for MSE wall construction have changed since the 1970s. The current design guidance published by FHWA uses LRFD design methods (Berg et al. 2009, Taylor et al. 2023). Based on the results of the case example interviews and material provided by survey respondents,

many DOTs have a pre-qualified list of MSE wall suppliers whom contractors can refer to when a project requires an MSE wall. Guidance for specific variation in the management of MSE assets constructed under different design practices was not identified during this literature review.

MSE Wall Performance

Guidance for MSE wall design and construction practices is readily available to state DOTs, but similar guidance documents on maintenance, repair, or rehabilitation of MSE walls were not identified in this literature review. In 2012, NCHRP Synthesis 437 documented the state of the practice for assessing MSE wall performance. Respondents to this 2012 synthesis survey noted the critical role that good construction and good drainage plays in the long-term performance of these retaining walls (Gerber 2012). Although MSE walls are simple to construct compared to other wall types, incorporating a requirement for experienced MSE wall contractors can help ensure that the wall is constructed as designed. Insufficient embedment of the MSE wall’s leveling pads also makes walls more susceptible to scour and loss of backfill, which can be further exacerbated by poor management of surface water around the wall (Alzamora and Anderson 2009). Guidance on MSE wall design and construction to increase resilience to extreme surface water draining around the wall during extreme weather events (e.g., flooding, hurricanes) was not identified in this literature review.

Long-term degradation of the soil reinforcement in the retaining wall also affects the lifespan of an MSE wall. NCHRP Synthesis 437 identified internal corrosion/degradation of the reinforcement as the most critical feature for wall performance (Gerber 2012). The rate of degradation varies based on local conditions and materials used. To monitor the long-term degradation of metallic or geosynthetic reinforcement after MSE wall construction reinforcement, coupons are installed at select wall and depth locations and removed for testing over a pre-programmed time scale (Elias et al. 2009). In applying asset management to this standard monitoring program, initial research has also been conducted on developing a risk-based protocol to identify metallically reinforced MSE walls that are at a high risk of premature corrosion and poor performance. These walls would then be classified at one of five risk levels—from very low to very high, based on the combination of vulnerability and the consequence of failure. This would allow DOTs to prioritize the highest risk walls in inspection and rehabilitation work (Govindasamy et al. 2018). Other research is ongoing to develop an instrumentation strategy that could be implemented to monitor wall performance over time and incorporate wall performance data into the overall risk management framework for retaining wall management (Zekkos et al. 2020).

Application of Asset Management to MSE Walls

Many state DOTs are responsible for MSE walls, with new MSE walls being constructed regularly. There is discussion in the available literature of how best to inventory, inspect, and manage them using the asset management principles already applied to bridges and pavements. However, asset management of MSE walls is not currently required by federal regulations, so development of applicable asset management is occurring without a federal directive. This section (1) summarizes the guidance on inventory, assessment, and asset management available to or already used by state DOTs and (2) includes the recommendations from research funded by state DOTs on asset management of MSE walls that has not yet been fully implemented.

Inventory

A DOT maintaining an inventory of only MSE walls was not identified during the literature review or survey. Instead, DOTs identified in the literature review and survey incorporate MSE

walls as one of many wall types in a statewide retaining wall inventory. State DOTs with specific published guidance for developing retaining wall inventories have inclusion requirements based on wall height, function, and location. Based on the sources reviewed, the most common inclusion requirements were that retaining walls must be over 4 feet tall, have a face angle of over 45 degrees, and be located on DOT right of way. However, not all DOTs have requirements in all these categories. Select inventory requirements are presented in Table 1.

The types and/or function of walls included may also vary among agencies. For example, Alaska included culvert headwalls over 6 feet tall in their retaining wall inventory, while Connecticut excludes culvert headwalls from their wall inventory, regardless of height (AKDOT&PF 2013, Connecticut DOT 2023).

Several DOTs catalog select privately owned walls outside of DOT right of way. The inclusion of these walls in the DOT inventory relies on expert judgment from both the inspector and the overall manager for the retaining wall inventory program. However, the intent is to capture walls in noticeably poor condition that could affect the roadway in the event of a failure, so that the DOT can monitor these walls and prepare to notify the property owners if needed (Connecticut DOT 2023).

Inspection and Assessment

The AASHTO Transportation Asset Management Guide provides DOTs with guidance on applying asset management principles to DOT assets (AASHTO 2019). Although retaining walls are not required to be included in DOT TAM plans, FHWA has identified retaining walls as structures that benefit from application of asset management principles (FHWA 2008). For DOTs that are updating or developing retaining wall inventory and assessment programs, TAM guidelines may be used to improve management of all DOT assets. NCHRP Research Report 903: Geotechnical Asset Management for Transportation Agencies, Volume 2: Implementation Manual recommended that a retaining wall asset management program be supported by a complete inventory, with data stored in an enterprise system (or similar) that can centralize, maintain, and share data with all expected agency users. Recommended minimum data includes location; type; geometry; photographs; generalized description of condition, hazard or risk associated with failure; and performance history (Vessely et al. 2019).

The benefits of creating a retaining wall inventory were first presented in a guide for asset management of earth retaining structures developed as part of NCHRP Project 20-07, Task 259,

Table 1. Examples of select requirements for inclusion of a retaining wall in the retaining wall inventory from select agencies with transportation assets.

Note: This list does not include all retaining wall inventory guidance documents or specific requirements related to wall function.

in 2009. This project cataloged wall inventory programs that included regular inspections for five state DOTs, FHWA, two city DOTs, and one Canadian province. The guide outlined a phased approach for DOTs to use in proceeding from implementing an inventory and inspection program for walls to developing a database and applying asset management techniques. In 2012, NCHRP Synthesis 437 identified 15 state DOTs with retaining wall inventories that included MSE walls, a 300% increase. The synthesis also noted that bridge inventories could be a useful data mine for DOTs planning to develop retaining wall inventories, given that MSE walls are frequently associated with bridge abutments (Gerber 2012).

The 2023 revision of the Bridge Inspector’s Reference Manual (BIRM) recommends including assessment of MSE bridge abutment walls and wingwalls in a bridge inspection, even though wingwalls usually do not affect the overall bridge rating (Ryan et al. 2023). If the retaining wall extends a long distance from the abutment, the wingwall is defined as the MSE wall between the first and second joints moving away from the abutment. If joints are not visible, then the wingwall length is considered equal to the exposed height of the abutment (Ryan et al. 2023).

Once the inventory is developed, maintaining the inventory through regular condition assessment of walls is necessary to support a viable asset management program. The National Park Service (NPS) Retaining Wall Inventory was one of the first formally established retaining wall programs and set a reinspection interval of up to 10 years (DeMarco et al. 2010). State and local DOTs that have established a target reinspection interval for walls may also target a shorter inspection interval (City of Cincinnati 2015, Rasdorf et al. 2015, Walters et al. 2016). Variation in the reinspection interval can reflect a combination of the number of walls in the DOT inventory, wall condition, and a desire to reduce loss of institutional knowledge caused by staffing changes over a longer inspection cycle.

Unlike many other asset types, MSE walls are frequently constructed below the roadway grade and are not visible from the roadway. Several early developers of retaining wall inventories used street-view data, such as Google’s Street-View feature in Google Maps, to create an office-based inventory to jumpstart fieldwork, but this approach typically only collects retaining walls above the roadway (CTC and Associates 2013). Mobile data collection along roadways has also become more cost-effective over time. For example, in developing their asset management program, the North Carolina DOT compared the effectiveness of types of mobile data collection to capture location and basic geometry of assets appurtenance to the roadside. In general, mobile data quality decreased with distance from the road but was still a valuable component of cost-effective data collection, if the limitations are recognized and accounted for (Kim et al. 2009). In the case example interviews in Chapter 4, Connecticut DOT also mentioned that the department plans to use LiDAR collected by other state agencies to help identify walls below the roadway.

As remote-sensing techniques such as LiDAR become more cost-effective, DOTs are looking for ways to use this new technology to help identify retaining walls below the roadway prior to going out in the field. Remote sensing can also help capture retaining wall deformation or settlement, depending on the resolution of the remote-sensing equipment used for data collection (Wolf et al. 2016).

Assessing MSE Wall Condition and Performance

Determining MSE wall condition may incorporate testing of buried coupons, as described previously, but for routine inspection and condition assessment, state DOTs rely on visual, nondestructive methods. In 2012, NCHRP Synthesis 437 summarized the retaining wall features DOTs described as most important for assessing long-term wall performance (Gerber 2012). Components identified as critical over a decade ago, such as alignment, drainage (internal and external), loss of backfill, settlement, joint separation, and panel damage, remain common inspection elements in MSE wall assessments today.

Evaluation systems vary among DOTs, from Good/Fair/Poor to a numerical score and may separately assess wall condition and risk, either qualitatively or quantitatively. The variation in scoring may reflect which group in the DOT was responsible for developing the retaining wall inventory. DOTs where the inventory and assessment programs were developed by personnel from within the Bridge or Structures group frequently use an assessment rubric mimicking NBI assessments. Critical retaining wall elements receive a score of one through four, and the overall wall receives a score between zero and nine, where nine is “like new” condition (Jensen 2009, Walters et al. 2016, Pennsylvania DOT 2022). DOTs where the effort is led by personnel from within the geotechnical group typically apply the methods described in NCHRP Research Report 903: Geotechnical Asset Management for Transportation Agencies (AKDOT&PF 2013, New York State DOT 2018, Connecticut DOT 2023, Mines et al. 2023). In the Geotechnical Asset Management Implementation Manual, geotechnical assets are assigned to Good/Fair/Poor condition categories based on condition and on safety/economic consequence of failure (Vessely et al. 2019).

Methods for Applying Asset Management Principles

Departments or agencies of transportation may incorporate different components of a complete asset management cycle at different times as data becomes available or may intentionally start with costs based on expert elicitation. For example, the City of Cincinnati develops estimated repair costs for all walls with a Poor or Critical rating and prioritizes walls for repair based on the City’s internal decision support metrics. The City’s performance target is to maintain a Satisfactory structural rating for at least 80% of walls, within the limits of available funding. At the same time, a formal retaining wall life-cycle cost has not been developed for their inventory (City of Cincinnati 2015). Alternatively, the Alaska Department of Transportation and Public Facilities (AKDOT&PF) mined data from DOT bid tabulations to develop a rehabilitation and replacement cost for a generic retaining wall based on wall condition. AKDOT&PF then used expert elicitation to develop an initial deterioration rate for a generic retaining wall and combined that with the data mined from the bid tabulations to estimate an initial life-cycle cost and future average asset condition based on a given budget expenditure. These components of the asset management program were developed while the initial retaining wall inventory and assessment work was still in progress (Thompson 2017). The City of Seattle DOT (SDOT) has used wall assessments inspection performed since the 1980s to develop life-cycle expectations for a generic retaining wall asset. Walls in Fair condition have a remaining life expectancy of 15 to 39 years, while walls in Poor condition have a useful life of less than 15 years. Funding to prevent further deterioration by repairing retaining wall damage or defects is not programmatically available, and this type of proactive rehabilitation work is performed on a case-by-case basis instead. A 100-year replacement cycle for existing retaining walls is included in their 2020 Asset Management Plan (SDOT 2020).