CHAPTER 4

Case Examples

Representatives from five state DOTs provided in-depth interviews following completion of the survey. The interviewees represented the range of DOT groups responsible for managing MSE walls: one was from a DOT Asset Management Division, one from a DOT Bridge group, and three from DOT Geotechnical Divisions. The state DOTs interviewed were selected based on their survey responses. All five DOTs had an asset management program for MSE walls and expressed interest in participating in a case example interview. They were asked to provide insight regarding the following:

• Utilization of MSE walls statewide,
• State DOT guidance for MSE wall design,
• Changes in DOT guidance for MSE wall design over time,
• Experiences with MSE wall failures and repairs,
• Retaining wall inventories and asset management systems,
• Determining retaining wall asset condition,
• Estimating risk and associated costs for retaining walls, and
• Future maintenance or development of existing retaining wall inventories.

The DOTs of Colorado, Connecticut, Indiana, Louisiana and Minnesota provided the case examples. The practices of these selected state DOTs are described in the following sections and with their highlighted aspects summarized in Table 4 at the end of this chapter. Because the different DOTs were at different stages in the development, implementation, and management of their MSE wall asset management programs, different aspects of MSE wall asset management were focused on in the different interviews. The questions shared with DOT interviewees before the case example interview were developed based on the DOT’s survey questionnaire response and are provided in Appendix D which is available on the National Academies Press website (nap.nationalacademies.org) by searching for NCHRP Synthesis 644: Asset Management Practices for Mechanically Stabilized Earth Walls. These questions were used as a starting point in the interview, and the actual interviews covered some topics in more depth and detail than others as the interview progressed.

In general, each interview started with an overview of each DOT’s MSE wall inventory and ended with a brief discussion of what the department planned to do soon to maintain or expand existing inventory. Based on survey responses, the interviews also sought to highlight the different methods agencies may use to advance the same goal. For example, Indiana shared their DOT’s experience with using contracting tools for proactive wall management and Connecticut DOT discussed recent work to develop initial retaining wall replacement costs. Case examples (written from the interviews) follow. Key takeaways are summarized at the end of this chapter.

Colorado DOT

Overview of MSE Walls in Colorado

The Colorado DOT (CDOT) is responsible for maintaining approximately 9,000 paved centerline miles within the state of Colorado. In June 2014, CDOT started developing an inventory and assessment program for all DOT walls (i.e., bridge walls, noise walls, and retaining walls). The inventory is effectively complete, although CDOT estimates that a handful of obscured walls below the roadway may still be unaccounted for. Within that inventory, 49% of walls are MSE walls, for a total count of 630 MSE walls maintained by CDOT. Seventeen of these walls are in Poor condition.

Modifications of MSE Wall Design Based on Observed Performance

When designing MSE walls, CDOT Staff Bridge Department personnel or external consultants are responsible for managing the overall project design. The CDOT geotechnical group assesses global stability of the finished structure, and the designer or supplier performs the internal stability design check. The default designs assume the internal stability is good. CDOT maintains a prequalified list of MSE wall suppliers, but the wall type is typically called out in the project plans.

The Poor condition MSE walls in CDOT’s inventory are MSE block walls, constructed with drycast masonry blocks and geogrid reinforcement. Historically, blocks in many of these walls have been subjected to freezethaw damage from imperfectly drained backfill, leading to deterioration of the block face and eventually, loss of backfill material. In response, CDOT has made changes to the concrete specifications for these blocks to increase durability. Walls constructed under the older standards continue to be monitored, and isolated areas of damage may be repaired with shotcrete. If the damage is larger, the most efficient wall repair option is typically full wall replacement.

Water and drainage are major factors in MSE wall performance as well. Vegetation also correlates with the presence of drainage issues and is removed by Maintenance and Operations staff when requested following inspections. CDOT has developed a quickdesign worksheet to use when developing details for internal drainage designs. The worksheet is conservative by design but has been wellreceived both within the DOT and by their contractors and consultants. Implementation of this quickdesign reference has helped limit MSE wall performance problems caused by poor internal drainage.

Development of Asset Management Program for Walls and Experience Gained

CDOT’s retaining wall asset management program was initiated at the request of the Chief Engineer and has been the responsibility of CDOT’s Staff Bridge Department since the program’s inception. The Staff Bridge Department includes the Structures Design, Inspection, and Asset Management Units, all of which contributed to development of the retaining wall program. Inventory and assessment work was completed by consultants, who also developed inspection software and a database for the system. CDOT is working on moving that information into an inhouse system (SIMSA) for more consistency between the different types of assets inspected by the DOT.

The retaining wall assessment is intentionally modeled on the National Bridge Inventory (NBI)/National Bridge Element (NBE) system and uses visual inspection methods. Retaining walls receive a score of zero to nine for the overall wall. If the wall foundation can be observed, this also receives a score of zero to nine, but the wall foundations are generally inaccessible for visual inspection. The distinct wall elements are rated on a scale of one to four and combined as in

the NBI/NBE system. In general, using the existing NBI/NBE system as a starting point has made inspector training easier and led to fewer questions and inconsistencies during inspections.

The initial inventory and inspection work took roughly 4 years. The remaining obscured walls are expected to be captured by Maintenance during other work in the next 5 to 10 years. Going forward, CDOT plans to continue inspecting retaining walls on a 4-year interval. Walls in Poor condition may be inspected more frequently, on a 1- to 2-year interval, based on the recommendation of the inspector.

Plans for Further Growth

CDOT is one of only a handful of synthesis respondents with a complete or near-complete inventory of retaining walls. The DOT describes completing the initial inventory and set of inspections as the biggest hurdle to date in developing an asset management program for retaining walls. Now that this hurdle has been overcome, CDOT can transition to maintaining the existing database and starting to mine it for the cost and life-cycle data used to support a mature asset management program.

An unexpected second hurdle has been advocating for funding for a specific wall rehabilitation or repair when the overall condition of inventoried assets is generally good. Currently, the Asset Management Funding budget covers 12 assets, and retaining walls typically receive about 0.9% of that budget. This has been adequate for wall repair and rehabilitation to date but may change as the walls in the inventory grow older. Being able to forecast and better program preservation efforts into wall costs is one of the benefits CDOT expects to see from future routine retaining wall inspections.

The funding received from Asset Management also covers inspections for noise and bridge walls. CDOT inspects only their NBI bridges in-house – all other assets are inspected by consultants, under management of CDOT personnel. The current wall inspection funding pays for roughly 6 to 8 inspectors annually, and their work is managed by the program manager for the Retaining and Noise Wall Inspection and Asset Management Program.

Looking ahead, CDOT has various future projects to improve the management of their existing MSE retaining walls. One goal is to better program retaining wall preservation costs into overall wall costs, to develop an accurate life-cycle cost for both a generic wall asset and specific wall types. CDOT has also started tracking areas of repeated repairs in the asset management program, to support both federal funding after a disaster declaration and more robust rehabilitation/replacement of a specific wall or wall type. However, this is still a very small inventory for walls and no conclusions can be drawn at this time. CDOT uses the Asset Investment Management System (AIMS) to project the future condition of the retaining walls by combining the inspection data and cost inventories for construction, inspection, and repair based on current budget allocations. It is expected that this model will become more accurate as more inspection and project data are collected. The DOT wants to also improve how risk is incorporated into project prioritization and selection and formalize their existing informal decision support tools. In the interview, CDOT reported that developing an inventory of retaining wall inspections was not the end goal, but rather the first step to better managing these assets in the future.

Connecticut DOT

Overview of MSE Walls

The Connecticut DOT (CTDOT) is responsible for maintaining approximately 5,000 paved centerline miles within the state of Connecticut. The Soils and Foundations group within CTDOT

initiated a retaining wall inventory in 2012 and is reinspecting those walls and expanding the inventory where needed. Most of MSE walls managed by CTDOT have been constructed since the 1980s with the majority associated with expressway expansion projects. To date, 910 retaining walls have been inventoried and assessed in CTDOT’s TAMP, of which 31 are MSE walls. All the assessed MSE walls are in Good or Fair condition, as defined by CTDOT’s assessment metric.

The Soils and Foundations group also manages the retaining walls that support the bridges’ approach embankments. The current division of responsibility is that Soils and Foundations is responsible for any walls within the State’s right-of-way that is not part of a bridge abutment. Likewise, if an MSE wall is incorporated into a bridge abutment, the Bridge group is responsible for inspecting and maintaining those walls.

CTDOT Guidance for MSE Wall Design

CTDOT manages the construction of MSE walls under a pair of special provisions:

• Embankment walls are used to support embankment slopes, but do not carry traffic. These walls are limited to a maximum height of 8 feet and may incorporate geogrid reinforcement.
• Retaining walls over 8 feet tall or carrying traffic. This also includes MSE walls that support parking lots. These walls require metal reinforcement and a concrete panel facing. The contractor must select the final wall design from a pre-approved list maintained by CTDOT.

Until 2020, CTDOT’s design manual relied on ASD for retaining wall design. The DOT can track which retaining walls in their inventory were constructed using ASD vs. LRFD design. However, CTDOT expects it to be several decades before differences in performance resulting from the two design methods become apparent as the walls age.

Development of an Asset Management Program for Walls

CTDOT’s wall inventory started with an internally led effort in 2012. Although no official funding was allocated for the effort, the Chief Engineer and Highway Operation Bureau Chief supported the inventory, which allowed the Soils and Foundations group to have Maintenance and Operations personnel collect information on retaining wall condition and location. The long-term goal is to use the retaining wall inventory as a model for other geotechnical asset inventories, such as rock slopes and subgrade improvements.

In 2024, CTDOT is partway through an expansion of the 2012 efforts. The Soils and Foundations group obtained funding through the Jobs Act because the Asset Management group has increasingly recognized the value of the 2012 retaining wall inventory. CTDOT wants to expand CTDOT’s existing asset management program beyond bridges and pavements to include ancillary assets like retaining walls. The current assessment of highway walls is being completed by consultants. The work was divided among three consulting firms, who collectively evaluated approximately 430 walls in 4 months. The Bridge Safety group will also complete inventory and assessment of 50 walls associated with the state’s commuter rail service. The remainder of the highway walls as well as the bus transit walls (approximately 50 walls within the busway system) will be inventoried and assessed by consultants under separate projects.

The retaining wall assessment rubric is designed to be simple. Walls are assessed on a scale of 0 to 6, where Good is 5/6, Fair is 3/4, and Poor is 0/1/2. The deficiencies incorporated in this score are change in vertical or horizontal alignment since construction, loss of backfill or signs of distress, and structural condition of the wall. Visual inspection is the primary method of assessment. The 0 to 6 rating system is different from the 0 to 9 rating scale used in CTDOT bridge assessments. In the current round of assessment and expansion, this difference in rating scales

has necessitated additional training for inspectors who are more familiar with bridge inspections. Based on feedback from these inspectors, the Soils and Foundations group plans to add a “Critical” rating to the original Good/Fair/Poor rubric in the 2024 update. This revision has not yet been officially approved and posted. There are currently no retaining walls inventoried by the state DOT, MSE or otherwise, that would receive a “Critical” rating.

The biggest known data gaps in the current inventory are retaining walls below the roadway. The state of Connecticut is conducting a statewide high-quality LiDAR survey and has offered to process the resulting data to help identify retaining walls within State ROW. CTDOT is looking forward to using these data to help target field inspection work and capture the remainder of DOT retaining walls. Based on CTDOT’s existing requirements for MSE embankment walls described above, these below-roadway walls are expected to include a higher proportion of MSE walls than the current inventory.

Estimating Risk and Costs

CTDOT has programmatic budget set aside for retaining wall maintenance or rehabilitation, but none of that budget is specifically appropriated for MSE walls. The DOT uses a combination age-based and risk-based selection process in determining which walls to repair or rehabilitate. The risk assessment combines likelihood of failure with failure consequence. The qualitative likelihood of failure is a combination of the condition assessment, wall height, and distance from the roadway. The consequence of failure is a combination of replacement cost and the annual daily traffic on the roadway impacted. A method to estimate the cumulative impact of a wall failure that could affect multiple roads (e.g., a retaining wall that supports a frontage road grade-separated from the highway below) is not currently instituted. Instead, the risk is calculated for both roadways and the highest risk score is kept.

CTDOT tracks retaining walls and construction costs, as well as repair costs by retaining wall type. Using this information, CTDOT has estimated a per-square-foot replacement cost for retaining walls of approximately $118 per square foot of wall face for 2024. The replacement cost is the same for all wall types. Using this unit replacement cost, CTDOT estimates that the total asset value of the DOT’s existing retaining walls is $350 to $400 million in 2023 dollars.

Future Database Maintenance and Data Analysis

Looking ahead, CTDOT plans to assess MSE walls on a 2- to 5-year interval, depending on wall condition. All walls in Poor condition where there is an area of concern have already been added to an internal watch list and receive a monthly visual check from the nearest maintenance garage. The current wall assessments are being performed predominantly by civil/structural engineering consultants managed by the geotechnical engineers in the Soils and Foundations group. Looking ahead, the inventory will be maintained by the Soils and Foundations group, but the inspections will be completed by the Bridge Safety and Evaluations group inspectors. The Bridge Safety group is already performing some inspections outside of bridge structures, and the intent is to formalize that arrangement. Through the current contract with consultants, the Soils and Foundations group has been able to determine what type of training and guidance will best help these Bridge Safety group inspectors collect the information geotechnical engineers need.

Currently, less than 10% of retaining walls in the retaining wall inventory are MSE walls and none of the walls are in Poor condition. However, because the existing MSE walls have performed well to date and help support challenging roadway expansions, the DOT expects the proportion of MSE walls in its inventory to grow over the coming years. In the future, as the existing MSE walls age, CTDOT hopes to pull from this larger data set to develop specific life-cycle costs and long-term deterioration rates for the two types of MSE walls constructed by the DOT.

Indiana DOT

Overview of MSE Walls in Indiana

The Indiana DOT (INDOT) is responsible for maintaining approximately 29,800 paved lane miles within the state of Indiana. The Office of Geotechnical Services initiated a retaining wall inventory and assessment program, which was largely completed in 2018. Over 95% of INDOT-owned walls are estimated to be included in this database, although some “hidden” walls may be present below the roadway. The existing database contains roughly 2,500 retaining walls, 90% of which are MSE walls. INDOT estimates that approximately 75% of the walls are associated with bridges. Following the initial inventory and assessment work, INDOT identified 36 poorly performing retaining walls for retrofit work in 2021. Since then, additional locations have been added, and the DOT’s list of walls targeted for repair or rehabilitation now includes 65 walls.

INDOT’s retaining wall asset management program was in transition when this synthesis was being developed. Responsibility for retaining wall assessment and inventory was being shifted from Geotechnical Services to the Bridge Group. This transition, discussed later in this case example, illustrates how an asset management program for retaining walls can be implemented, used, and then modified to better meet department needs.

Development of Asset Management Program for Retaining Walls

INDOT’s wall inventory grew out of a request by the Geotechnical Services to develop and implement a Geotechnical Asset Management (GAM) program. The Asset Management (AM) group provided funding for an initial retaining wall inventory and assessment program that would include all transportation-related retaining walls over 5 feet tall. Culvert headwalls and soil nail walls were excluded from the database. The program started with a list of walls of concern generated by maintenance and operations personnel. Initial inventory and assessment work was completed by consultants in 2018 and shared in the INDOT Collaborative GIS Retaining Walls Map. The data was collected using a form hosted in ESRI’s Collector App and is stored in ESRI’s ArcGIS Online space for use within INDOT.

Visible wall elements were inspected and rated to develop an overall wall condition. Condition scores ranged from 0 (Failed) to 9 (Excellent). Walls with a condition rating of less than 5 were tagged for more frequent inspection and monitoring. In addition, INDOT developed a set of four public safety codes (i.e., Green, Orange, Yellow, and Red), to capture the expected safety impacts of a wall failure and an estimated likelihood of failure based on retaining wall condition. A safety code of Yellow or Red triggers immediate additional inspection by a retaining wall specialist from the Geotechnical Services office.

Developing Contracting Tools for Proactive Management

Starting in 2016, INDOT implemented Indefinite Deliverable, Indefinite Quantity (IDIQ) contracts to support bridge and pavement rehabilitations. Following completion of the 2018 retaining wall inventory work, the Central Office Asset Management group subsequently identified a need to address retaining walls under this type of contract. The first IDIQ retrofit contract for retaining walls was set up in 2021. Using a combination of wall condition and safety rating, the retrofit contract originally included 36 walls, with 20 locations fixed to date. The contract included expected pay items for bidding and initial expectations for type of repair. Final repairs under the retrofit contract are designed by the contractor. Originally a 3-year contract, it was extended for 1 year and was scheduled to end in 2025.

The most common rehabilitation repairs to date have been related to erosion and loss of backfill. When replacing backfill, INDOT typically relies on using flowable fill to replace lost backfill behind panels. Control of erosion caused by surface water drainage may follow several forms. Common retrofits cited by INDOT include extending curbs on bridge decks to direct water away from wall backfill and plugging gaps at bridge railing or expansion joints that were directing runoff to the top of the MSE wall.

Looking ahead, INDOT has a programmatic budget of $6 million annually set aside for rehabilitation of existing retaining walls, including MSE walls. The initial retrofit contract has been a success, but INDOT plans to make several improvements when letting the next contract. The existing retrofit contract is a single statewide contract. INDOT plans to let the next retrofit contract as three separate contracts, each covering two districts. This will support more district involvement in project selection, which has been identified as a critical component of long-term project success. Splitting the wall retrofit contract will make it more like bridge and pavement retrofit contracts, which are set up by district, and will remove bottlenecks that resulted from having a single contract with a single company. Finally, the existing retrofit contract estimated wall repair costs as a lump sum item, and repair costs were typically higher than estimated. In the next retrofit contracts, INDOT intends to adjust cost estimates and separate costs for items like clearing and mulching from the lump sum cost of wall repair. The Geotechnical Office will continue to provide technical support for all three contracts as needed, because INDOT does not have geotechnical staff at the district level.

Transition to Updated Retaining Wall Asset Management System

Initially, the retaining wall inventory and assessment program was developed and implemented by Geotechnical Services. At the time of the development of this synthesis, INDOT was transferring ownership of the retaining wall inventory and assessment program to the Bridge group. This management change was intended to simplify management and maintenance of the retaining wall data by bringing it into the iTAMS system for easier access. This management change will also bring inspections in-house, where they can be completed by INDOT bridge inspectors. Finally, based on lessons learned from the first retrofit contract, adding the walls to the asset management group managing retrofit contracts for bridges and pavements will improve management of future wall retrofit contracts.

During the transition, the existing 2018 retaining wall data will be transferred from the ESRI ArcGIS Online space to INDOT’s iTAMS, where bridge and pavement data are already stored. Post-2020 retaining wall information, including data on the walls rehabilitated in the retrofit contract, will also be added. All available data on INDOT retaining walls, such as shop drawings, construction records, and subsurface investigations, will be included in iTAMS.

Once this transfer is complete, the Bridge inspectors will be responsible for inspecting all INDOT walls, including those not associated with bridges. Because roughly 75% of INDOT retaining walls are bridge-associated, staffing requirements associated with this change are expected to be minimal. Bridge inspections already include written notes on associated MSE walls, and moving responsibility for retaining wall inspection to the Bridge group will formalize that process. Geotechnical Services has provided input and training for the bridge inspectors to clarify what is needed for the wall inspections. The scoring system will also be adjusted from 0 to 9 to 0 to 4, and the number of data categories in the inspection will be reduced. The geotechnical group will still provide technical support as needed. Retaining walls will be inspected every 5 years, at a minimum. As in the current program, retaining walls may also be tagged for more frequent inspection based on condition.

Louisiana DOT and Development

Overview of MSE Walls

The Louisiana DOT and Development (LADOTD) is responsible for roughly 16,000 paved centerline miles of roadway. These roadways are supported in part by roughly 21 miles of retaining walls. Most of these walls are in urban corridors and were built as part of interstate expansions in the 1980s and 1990s. MSE walls are estimated to make up more than 50% of LADOTD’s permanent retaining walls. The total percentage of MSE walls in the DOTD’s portfolio is expected to grow, given that the use of MSE walls in designs is becoming more common. Most of these retaining walls are in Shreveport, in the northeastern part of Louisiana.

The Louisiana Transportation Research Center (LTRC) works closely with LADOTD in conducting research to support LADOTD needs. LTRC led the initial development of the retaining wall office-based inventory, rating application, and data-storage system. A representative of both LADOTD and LTRC participated in the case example interview, to provide input on the various aspects of MSE wall management.

Development of a Retaining Wall Inventory

LTRC started work on a retaining wall inventory in response to NCHRP Synthesis 437 (Gerber 2012). The second question in the synthesis asked if the DOTD maintained an inventory of MSE walls. LADOTD did not have such an inventory at that time, but in reviewing the synthesis, recognized the value of developing one. As described in LTRC Report 664, an LTRC research project was started to create an inventory and data-storage system for a retaining wall inventory. The work started with an in-office study using an assortment of street-view applications, such as Google Maps. An inventory and assessment application was developed within ESRI’s collector system. Wall condition and failure consequence was described using the 1 through 5 scale laid out in NCHRP Research Report 903: Geotechnical Asset Management for Transportation Agencies. The in-office work identified approximately 150 walls. MSE walls were categorized by facing and reinforcement type. Because LADOTD has a limited number of wall vendors on its pre-approved list, reinforcement type can typically be determined from the wall facing.

The intent of the research was to use the in-office study to jumpstart a field inventory and assessment program. The DOTD has not yet identified funding or staffing for implementing the next stage of the process: assessment and management. In part, this is because the DOTD does not have a specific geotechnical maintenance section that would serve as an obvious champion or steward for wall inventory work. LADOTD estimates that the current inventory contains about 20% of retaining walls in the state.

Experience Gained from MSE Wall Design

When developing plans for a highway project, LADOTD typically calls out the need for a wall in each location but leaves the final wall system selection to the contractor. Typically, a wall type is only specified if the DOT wants to match an existing nearby MSE wall visually. Contractors select the wall from LADOTD’s pre-approved list, with the final wall design typically completed by the contractor. LADOTD monitors construction of the wall along with the rest of the project.

LADOTD implemented LRFD design guidance in the late 2000s. The specific design guidance followed in a wall design may be tracked as a note on the plans or in the official report. Geotechnical design reports for MSE walls have become a DOTD standard in the last decade, but do not capture the transition from ASD to LRFD. However, it has been LADOTD’s experience that both wall types perform well if they are designed and maintained properly.

Experience Gained from MSE Wall Maintenance

In Louisiana, the state design office is in Baton Rouge, while most MSE walls are in the northeastern corner of the state, roughly 4 hours away. Given the travel time, these districts typically try to perform emergency maintenance and repairs on their own until the problem becomes too complex for them to handle. Maintenance is also complicated by personnel retention issues at the district level, which leads to a loss of institutional knowledge and complicates asset maintenance that operates on a longer cycle.

In LADOTD’s experience, MSE wall failures start as a minor design issue that is exacerbated by deferred maintenance. Drainage is the leading cause of MSE wall failures, at both the top and bottom of the wall. Sand is the typical backfill material for Louisiana MSE walls and is susceptible to erosion. Expansion joints at the top of MSE walls are frequently not maintained. If these joints separate and are not patched, water can infiltrate behind the wall face, removing backfill and leading to loss of reinforcement support. In response to this observed failure, LADOTD is increasingly avoiding joints where possible in current designs and incorporating a geomembrane into wall design to reduce material transport if drainage does begin to infiltrate into backfill from the top of the wall. Drainage at the base of the wall, particularly scour, has also undermined MSE walls and contributed to the loss of backfill and subsequent damage. The DOTD also responded to an MSE wall failure where UV damage of geosynthetics through the drainage weepholes led to a loss of backfill material through those weepholes and settlement within the wall that translated to the road surface atop the wall.

When MSE walls are damaged past the point of maintenance repair, LADOTD typically encapsulates the failed wall in a new wall or provides other external support (depending on ROW limitations). These repairs are more likely to allow traffic to be maintained during repair work and are easier to design in an emergency repair situation. Pulling from a series of examples of failed MSE walls in Shreveport, LADOTD has built a timber lagging wall in front of one MSE wall, buried another failing wall in an expanded embankment, and constructed a berm at the toe of a third to restore support. Construction of the berm also included drainage improvements in the retained soil at the base of the wall. All these repairs successfully restored support to the roadway.

Future Implementation Goals

Although the retaining wall inventory has not officially been implemented because of budget and staffing issues, support for inventory and management continues to grow within the DOTD. For new construction and repairs, LADOTD is adding wall data to the DOTD’s OpenGround database. The goal is to have the data available for future reference and maintenance and to avoid “forgotten” walls, especially when emergency repair of a failed wall makes the original wall inaccessible for visual inspection. LADOTD Operation and Maintenance has started work on inventorying ancillary assets such as signs, and there are discussions ongoing about incorporating retaining walls into that work. Finally, LTRC is drafting a research proposal to build on previous GAM efforts. This second phase will focus on inventory and assessment of culverts with a follow-up on full implementation of retaining wall inventory and assessment. LADOTD’s long-term goal remains the implementation of a geotechnical asset management program, even if the highway to that destination is a winding one.

Minnesota DOT

Overview of MSE Walls

The Minnesota DOT (MnDOT) is responsible for approximately 12,000 centerline miles of state and federal highways. MnDOT also helps with an additional 35,000 miles of county and

municipal roads. MSE walls make up slightly more than 10% of MnDOT’s retaining wall inventory and are generally in Good or Fair condition. Within the existing inventory, no MSE walls are identified as being in Poor or Critical condition. Most known MSE walls are in the Metro District, which includes Minneapolis and St. Paul.

MnDOT Guidance for MSE Wall Design and Maintenance

The Standards Group within the Bridge Office is responsible for the design of non-bridge concrete structures and has historically designed many of MnDOT’s MSE walls. MSE wall designs are completed by contractors, then reviewed by the Geotechnical and Structural groups for potential failure modes. As more MSE walls are built in Minnesota, the Geotechnical group is expected to perform the design review on a larger proportion of the MSE walls, coordinating with the Structural group as needed based on the wall’s location. The state’s structural wall engineer is still working to identify the optimal MSE wall type for Minnesota, both in terms of wall facing and reinforcement, but this work is not being undertaken in response to a specific MSE wall failure.

According to MnDOT, the most common cause of the failure of MSE walls and other retaining walls in Minnesota is water. Restoring seals, clearing drainage channels, and improving drainage above or below the wall have been the most effective methods for MnDOT to address signs of distress in an MSE wall and for all other wall types in their inventory as well. Other items considered in the inspection of MSE walls are panel deformation, potential loss of backfill, and widening or narrowing of joints.

Development of an Asset Management Program for Retaining Walls

MnDOT was a pilot state for Transportation Asset Management (TAM) and the Asset Management group wanted to go beyond bridges and pavements from the beginning of program implementation. MnDOT identified retaining walls as a key ancillary asset. Such walls were also identified as an “orphan asset” without a clear owner or champion within the DOT. MnDOT is the only survey respondent where the Asset Management group is directly responsible for inventory and assessment of any retaining walls. In part, this is because the Metro District Asset Management team is housed within the Maintenance department, so the Asset Management team could draw on Maintenance support for the initial 2016/2017 inventory and inspection of retaining walls in the Metro District.

This pilot inventory started with the assumption that the Metro District (Minneapolis – St. Paul) had approximately 300 retaining walls. Inspectors identified roughly 850. Since completion of the pilot project, MnDOT has “crowd-sourced” retaining wall information from other districts. About 1,400 retaining walls are in the inventory.

The initial inventory and inspection program was completed by a consultant using the bridge rating scale as guidance for both critical elements and the overall asset rating. Wall assessments rely on a visual, non-destructive inspection that requires up to 2 hours. MSE wall elements are described using a Condition State rating of 1 to 4, and the overall wall rating scale is 1 to 9. Application of the same type of numerical scoring used for bridge inspections has helped reduce potential confusion or miscommunication when describing wall condition.

To create their retaining wall inventory, the Asset Management group worked within their existing Agile Assets system, so that the retaining wall inspection data remains readily accessible to as many interested users as possible. As the retaining wall inventory has been used over the last 5 years, the Asset Management group has identified potential improvements. Work is underway on an improved user manual and storage location in Agile Assets that will make it easier to sort and filter by different wall components and ratings. In part, this is the result of the recent failure of a

cantilever concrete wall. The wall had been inspected, but key observations pointing to an increased failure potential were not readily accessible to end-users given how the originally designed tables were nested. The MnDOT interviewee noted that keeping users and stakeholders informed and involved was key to being able to maintain a developing inventory system in use and maintaining the support needed to resolve issues and make improvements as needed.

Next Steps for Asset Management of MSE Walls

MnDOT currently treats MSE walls like all other retaining walls in project prioritization and plans to continue to do so for the foreseeable future. In prioritizing projects, planners look at a combination of the probability and consequence of failure. The failure probability is largely controlled by wall condition, but the wall’s size and structure type are also included. The incorporation of structure type helps capture systems like MSE walls, which can deform significantly before total failure, if backfill support for the reinforcement is maintained. MnDOT defines “consequence of failure” as a combination of wall height, proximity to the roadway, and average daily traffic.

MnDOT’s next major goal for its retaining wall asset management program is to develop an “asset folio” for retaining walls that will include the life cycle of a generic asset, including life-cycle costs. The DOT already tracks inspection and maintenance costs, but determining the life cycle of a retaining wall is more challenging. In MnDOT’s experience, a construction defect or serious design failure will become evident rapidly, but a well-constructed wall lasts long enough that the controlling factors for degradation are difficult to determine. MnDOT is planning to use expert elicitation to develop an initial generic life cycle for any retaining wall asset and then gather information over the coming years to support developing life cycles and life-cycle costs for specific wall types.

Planning for Flexibility

In implementing asset management of retaining walls statewide, MnDOT has recognized the value of flexibility, particularly when staffing for implementation. In the Metro District, the relatively high number of walls means that retaining wall assessments will likely be collected by consultants. In greater Minnesota, which has proportionately fewer walls, inventory and assessment work will be completed by MnDOT staff as part of other duties. The asset management team is also working to identify both key personnel and backups to help prepare for the turnover that exists in any agency.

MnDOT’s original internal drive to go beyond the minimum in TAM and add ancillary assets is now further supported by a legislative directive to add geotechnical assets to MnDOT’s asset management system. MnDOT describes their asset management for retaining walls as in development, but the DOT has already recognized benefits from the program and is looking forward to expanding the program and drawing on the experience of other DOTs, like Colorado, which are farther along in the process.

Key Asset Management Findings from the Case Examples

The five DOTs interviewed use different tools to manage their MSE wall assets and are at different stages in developing asset management programs for these walls. In addition, the interviews reflect the variation in DOT asset portfolios, with MSE walls ranging from less than 5% to 90% of DOT wall assets. The reason for this variation was not a focus of this synthesis but could be a subject of future research.

Table 4 summarizes key findings from the case interviews on how different responding DOTs developed, implemented, and use their MSE wall asset management programs. Discussions of specific wall maintenance and rehabilitation practices are not included in this table but are available in the individual case examples. The variation among asset management programs reflects the absence of a national mandate to include retaining walls in transportation asset management programs. All the DOTs interviewed for this synthesis described their programs as still being improved to meet needs identified by the department.

Table 4. Summary of the areas of DOT MSE wall asset management practice discussed in case example interviews with each DOT.