SUMMARY

Asset Management Practices for Mechanically Stabilized Earth Walls

Mechanically stabilized earth (MSE) walls are structures that rely on reinforced compacted soil backfill for wall performance. These types of walls, introduced in the 1970s, typically are both cost- and time-efficient to construct and can tolerate more horizontal or vertical deformation while maintaining support for the roadway than other types of retaining walls. Most MSE walls managed by DOTs are younger than their design lives. Design guidance, reinforcement materials, and construction protocols for MSE walls have all been updated since the first MSE walls were constructed. No standard guidance document for asset management of MSE walls currently exists.

The objective of this synthesis is to document department of transportation (DOT) practices on asset management of MSE walls, including maintenance and rehabilitation of deteriorated walls, inventory and assessment practices, and development of risk and life-cycle costs. This synthesis consists of a literature review, a summary of responses to a survey on asset management of MSE walls, and a summary of the findings. The literature review summarizes available guidance on application of asset management to MSE walls. A survey was sent to asset management personnel at DOTs and the responses to the survey are discussed, along with case examples from the DOTs of Colorado, Connecticut, Indiana, Louisiana, and Minnesota. These case examples provide additional insights into managing MSE walls and developing retaining wall inventories. Finally, the summary of the findings highlights the current state of the practice and identifies knowledge gaps as well as potential research opportunities that could help improve asset management of MSE walls.

The state of practice in this synthesis was assessed through a survey distributed to lead asset management personnel in the 50 state DOTs, as well as DOTs in Puerto Rico and Washington, DC. The survey focused on the following five categories:

• MSE wall management,
• Inventory,
• Maintenance,
• Condition assessment and project prioritization, and
• Deterioration and life-cycle costs.

Out of the 52 DOTs contacted, 44 responded in full or provided partial responses, resulting in an overall response rate of 85%. Of the 44 responding DOTs, 22 (50%) reported having some form of inventory that includes MSE walls with retaining walls. The median responding DOT estimated being responsible for approximately 375 MSE walls. Inventory and inspection are most often the responsibility of a DOT’s bridge or geotechnical group, and 21 of 34 responding DOTs (62%) reported that inspections were carried out ad hoc in response to reported problems.

The survey revealed that the application of asset management principles to MSE walls is not a standard practice for DOTs. Twenty-one of 35 responding DOTs (58%) reported that no program to manage MSE walls existed in their DOT. At the same time, 17 of 32 responding DOTs (53%) reported taking proactive measures either inside or outside the MSE wall footprint to repair or rehabilitate distressed walls. Drainage improvements and removal of vegetation were the most common proactive measures outside and inside the wall footprint, respectively.

Ten of the 36 responding DOTs shared additional information on their DOT’s retaining wall asset management program. Among these agencies, deformation or damage to facing, loss of backfill, and changes in wall alignment were the most common factors used to define MSE wall malfunction and overall wall condition, and effect on traffic was the most common definition for service failure. Of the agencies that shared information about their asset management programs, 40% were formalizing existing informal guidelines to help prioritize project selection. Roughly one-half of the respondents had incorporated experience gained from the failure or deterioration of older MSE walls to update construction methods. Tracking post-construction costs associated with an MSE wall is uncommon, as is the development of specific life-cycle costs.

Detailed interviews were conducted with five responding DOTs: Colorado, Connecticut, Indiana, Louisiana, and Minnesota. The interviews, used to develop case examples, provided insight into current practices for maintenance and rehabilitation of MSE walls, inventory and assessment of existing MSE walls, and application of asset management tools to their retaining wall assets. Colorado, Indiana, and Louisiana DOTs had experienced more performance issues with MSE walls than Connecticut and Minnesota and described how their DOTs have rehabilitated individual MSE walls in response. The five DOTs interviewed represented different stages in the inventory and assessment process, from still building the initial inventory (Louisiana) to having completed multiple rounds of retaining wall assessments (Colorado). The interviewees also came from different groups within their respective DOTs—Geotech, Bridge, or Asset Management—reflecting (1) the diversity of groups responsible for managing retaining walls within DOTs and (2) the multiple pathways to developing an MSE wall inventory. All interviewees shared plans or goals to improve their existing inventory and assessment programs and develop additional asset management tools, reflecting the evolving nature of this area of practice. The report provides detailed information about each state DOT’s practices, highlighting the key topics discussed.