CHAPTER 5

Summary of Findings

This synthesis documents current DOT asset management practices for MSE walls that are the responsibility of their DOT. As identified in the literature review, various federal, state, and local agencies have expressed a desire to develop inventories of their retaining walls, including MSE walls, to help improve asset performance. In the absence of specific federal guidance, state DOTs have worked to develop formal inventory and assessment programs that best meet their internal needs. Maintenance and repair of MSE walls is performed in response to specific problems, with different repairs being more cost-effective for different DOTs. Proactive mitigation measures or long-term cost and deterioration data associated with mature asset management programs is not the standard of practice among responding DOTs at this time.

A survey was developed and distributed to collect information on the state of the practice regarding asset management of MSE walls. The survey was distributed to asset management personnel in 50 states, as well as Puerto Rico and Washington, DC. Forty-four DOTs responded (an 85% total response rate). The survey covered MSE retaining wall management, MSE wall inventory practices, maintenance and rehabilitation methods, assessment of MSE wall condition, and factoring of long-term deterioration into new construction. The findings of that survey are briefly summarized in the following sections.

MSE Wall Management and Inventory

The survey distributed to state agencies of transportation revealed MSE walls are typically managed without the support of a retaining wall inventory. Out of 41 respondents, 5 (12%) reported having completed an inventory of all DOT retaining walls, and 19 (46%) reported no inventory at all. Design and construction of MSE walls has evolved over time, but to what standards and code an existing wall was designed is not commonly tracked by DOTs. Taken together, these factors suggest the need for more comprehensive strategies to track MSE wall performance.

MSE Wall Inventory Practices

The survey revealed that the group responsible for inventory and management of MSE walls varies among DOTs. Among the responding DOTs, bridge and geotechnical groups are most likely to oversee MSE wall inventories (23 of 39 respondents), followed by asset management, maintenance, or materials groups. Inventory and assessment is most often performed by DOT staff or a mix of staff and consultants (18 of 34 respondents), while 2 of 34 responding DOTs have consultants perform all MSE wall inspections. Within their wall inventories, agencies are most likely to collect information on damage to facing materials, deformation of facing, change in alignment, degradation of the wall foundation, or damage to coping. Inventory work is most

likely to be completed on an ad hoc basis during performance of other work (21 of 35 respondents). For those state DOTs that do maintain a retaining wall inventory, the target inspection interval is less than 10 years.

Methods for Maintenance and Rehabilitation

The survey gathered information on proactive repairs both outside and inside the original MSE wall footprint. The most common response from DOTs was that proactive repairs were not performed for these assets, although state DOTs were more likely to perform work within the wall footprint than outside it (21 of 30 vs. 17 of 32 responding DOTs). For DOTs that have performed proactive repairs outside the original wall footprint, the most common actions reported were improving drainage to direct water from the wall (15 of 32 respondents), repairing erosion or scour below the wall (12 of 32 respondents), and repairing erosion damage above the wall (11 of 32 respondents). Different DOT respondents reported using different proportions of various proactive measures, so these numbers reflect an average across all respondents. For those DOTs that have performed proactive repairs inside the original wall footprint, the most common actions reported were repair of damaged concrete (13 of 30 respondents), repair of missing or damaged facing elements (12 of 30 respondents), filling voids behind a wall facing (12 of 30 respondents), or removal of vegetation from the wall face or top (12 of 30 respondents). A programmatic budget for MSE wall rehabilitation or repair was reported by 3 of 35 respondents, indicating that many DOTs performing proactive maintenance identify the problem and then find the budget to address it.

MSE Wall Condition and Project Prioritization

DOTs with retaining wall asset management programs were asked to provide more information on how their programs defined wall malfunction and service failure. The most common factors included in defining wall malfunction were damage or deformation of facing materials or loss of backfill, with over 90% of DOTs (9 of 10) reporting they use these factors. Service failure was most commonly defined as impact on traffic (90%) but over half of respondents (between 6 and 8 of 10) also reported that excessive loss of retained material, pavement distress above the wall, or impact beyond the DOT ROW would also qualify as a service failure.

Variation in which DOT group started the retaining wall inventory may affect how MSE wall condition is communicated. Inventories started and managed by bridge or structural groups typically follow NBI/NBE rating methods, while inventories started by geotechnical groups typically apply the Good/Fair/Poor methods outlined in NCHRP Research Report 903. Regardless of assessment method, the inspection items used to determine MSE wall condition were consistent among respondents. All respondents reported including damage to facing elements, loss of retained material, and change in facing alignment and spacing in their asset condition rating. When prioritizing spending, out of 10 respondents, 3 reported using wall condition alone, 1 reported using likelihood of failure, and 2 reported using a combination of wall condition and failure consequence.

Determination of Deterioration and Life-Cycle Costs

The survey documents that incorporation of long-term deterioration, MSE life cycle, and associated life-cycle costs is not typical for DOTs at this stage of development of their asset management programs. Of the agencies who responded to the survey, 10 of 37 (approximately 27%) had an asset management program either in place or in development. Of these 10 respondents, half (5) reported incorporating experience gained from wall failures to increase design life of new walls or incorporating lessons from long-term failure into revised construction methods. Construction costs were tracked by 60% (6) of responding DOTs with wall asset management

programs, and inspection costs were tracked by 30% (3). When asked about the development of life-cycle costs for MSE wall assets, 9 out of the 10 respondents reported that their DOT had not yet collected sufficient data for this analysis.

Case Examples

DOT representatives from Colorado, Connecticut, Indiana, Louisiana, and Minnesota were interviewed to gather insights on various aspects related to asset management of MSE walls. The selected state DOTs represent different geographical regions and climates, along with varying prevalence of MSE walls in their total wall inventory. The interviews covered topics such as DOT guidance for MSE wall design, DOT experience with MSE wall rehabilitation or repair, development of retaining wall inventories, integration of MSE walls into a DOT asset management program, and future goals for asset management of MSE walls owned by the DOT.

Future Research Opportunities

Opportunities for future research on asset management of MSE walls were identified through the literature review, survey, and case example interviews. These opportunities are grouped by research area in the following subsections.

MSE Wall Inventories and Their Applications

Based on the literature review, development of MSE wall inventories has been of interest to transportation agencies since at least the early 2000s, with an increasing number of federal, state, and local agencies developing systems to inventory, assess, and track their retaining walls. At the same time, almost half of the respondents to the survey (19 of 41) reported that their DOT does not have a retaining wall inventory. Research opportunities in this area include

• Research on the technologies and approaches implemented by the DOTs that have inventories. This research could help other state DOTs identify best practices for developing retaining wall inventories of their own.
• Research to identify typical obstacles to developing and implementing an asset management program for MSE walls, to help DOTs increase the likelihood of successful completion and maintenance of a retaining wall asset inventory.
• Research to identify what components of an inventory are most critical in supporting future development of a mature asset management program.
• Research on database systems and database structures for retaining wall asset management. Experience gained from the various data management systems being used by responding DOTs could be used to develop a recommended structure for application across multiple data management systems.
• Research on life-cycle cost analyses for MSE walls.
• Research on estimating the likelihood of failure based on MSE wall condition.
• Research on effective prioritization of repair or rehabilitation projects for MSE walls.

Emerging Technologies for Retaining Wall Inventory and Monitoring

The literature review and case examples revealed that agencies are starting to use emerging technologies for retaining wall inventory and monitoring. Research opportunities in this area include

• Use of remote sensing data (e.g., LiDAR) to help identify retaining walls before starting field inspections or to monitor wall performance over time. Research into this topic could help

• DOTs develop tools to identify walls below the roadway and develop a robust asset inventory more efficiently and cost-effectively than by field work alone.
• Research into new methods for monitoring (e.g., drone data collection with LiDAR or structure-from-motion topographic models) could help monitor MSE wall deformation over time and provide early detection of failures.
• Use of ground-penetrating radar (GPR), falling weight deflectometers (FWDs), and similar non-invasive investigation techniques, to evaluate the condition of wall backfill or buried components of MSE walls.
• Use of total station or surface LiDAR technology to evaluate MSE wall performance over time. This research could also include better quantification of seasonal changes in MSE wall alignment due to temperature variations.

Identification of Cost-Effective Maintenance and Rehabilitation Methods

MSE walls can fail for various reasons, including corrosion of internal reinforcement, loss of backfill through damaged wall facing, or damaged or inadequate internal or external drainage. In addition, MSE walls as a group can be constructed using various types in internal reinforcement or facing, all of which deteriorate at different rates and are more sensitive to different environmental factors. Based on the results of the literature review, the susceptibility of reinforcement to corrosion is the most widely studied of these factors at this time. Research needs for other components of maintenance and rehabilitation include

• Determination of cost-effective maintenance and rehabilitation for damaged wall facing or drainage, with a focus on design of more cost-effective wall rehabilitations.
• Identification of the points in the long-term deterioration of an MSE wall prior to service failure where maintenance and mitigation efforts would be most cost-effective.
• Research on the stressors to which existing different MSE systems are most susceptible to help identify higher risk MSE wall types in a DOT’s inventory based on wall location, age, type, or other inventoried characteristics.
• Development of recommendations for cost-effective additional inspection to identify degradation prior to failure or to prioritize for replacement projects.
• Evaluation of cost-effective methods to increase resilience of MSE walls to extreme weather events.

Standardization of Retaining Wall Inventory and Assessment Methods

The scoring methods used for assessing retaining wall condition varied among agencies based on which DOT group led development of the inventory and assessment program. Agencies where the inventory was developed by bridge groups typically use NBI/NBE-type scoring, while inventories developed by geotechnical personnel tend to follow the Good/Fair/Poor rubric outlined in NCHRP Research Report 903. Research that leads to standard guidance for assessment of MSE walls could help agencies collaborate and share data more efficiently. Opportunities for research in this area include

• Research into how to convert between the different rating systems.
• A shareable format for retaining wall inventory and assessment data.
• Development of a standardized pool of MSE wall performance data that, when combined with available cost data, could support the development of deterioration rates and programmatic life-cycle costs.

• Development of standardized rubrics for rating retaining walls within the assessment methods currently in use.
• Development of standardized methods to quantify the effect of MSE performance on other transportation infrastructure components.

Research in this area would need to recognize the strengths and benefits of the various rating methods currently in use and focus on sharing knowledge among the different groups responsible for designing and maintaining MSE walls.

Factoring Long-Term Deterioration into Design and Construction

One of the objectives of this synthesis was to gather information on the state of the practice for addressing long-term deterioration of MSE walls in the design and construction of new walls. The survey results showed that few DOTs have formal methods to factor long-term deterioration into design and construction. Design and construction at the DOT level is improved in response to specific experience gained from the performance or failure of older MSE walls. Future research needs in this area include

• Determination of deterioration rates of MSE walls to help DOTs better quantify long-term deterioration of their MSE assets. These deterioration rates may vary based on wall type, environment, etc.
• Determination of unit maintenance and rehabilitation costs for different types of MSE walls based on wall condition.
• Development of life-cycle costs for different types of MSE walls. An existing MSE wall-specific life-cycle cost or replacement value was not identified in the literature review or DOT survey responses.