Design Practices for Rock Slopes and Rockfall Management (2022)

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24 Case Examples Following completion of the survey, geotechnical specialists from four state DOTs were selected for in-depth interviews. The states were selected to represent a geographical distribution and to provide insight regarding the following: • Regular involvement with rock slope and rockfall mitigation design projects along highway corridors • A mix of new rock slope design and mitigation of existing rock slopes • Depth of experience with both in-house design and consultant design of rock slopes and rockfall mitigation • Development and application of design guidelines for rock slope and rockfall mitigation projects • Incorporation of additional considerations in project design The four DOT case examples were provided by the state DOTs of Oregon, Tennessee, Utah, and Virginia. Practices of these selected states are described in the following sections, and their highlighted aspects are summarized in Table 1. C H A P T E R   4 Table 1. Summary of DOT practice discussed in case histories. State DOT Topics Discussed Oregon Long-term rockfall evaluation design practice and changes over time, including a high percentage of rockfall mitigation measures no longer functioning as originally intended Perspective on design standards implemented in 2001 Consideration of constructability, long-term maintenance, and funding in rockfall and rock slope design Tennessee Implementation of a management program to help guide site selection for mitigation Challenges in managing rural rock slopes in poor condition Benefits to implementing a standard ditch drawing for rockfall catchment Importance of considering life-cycle costs Benefits gained from internal department outreach Utah Plans to rerate rock slopes and capture changes since initial implementation in 2004 General sole reliance on ditch catchment for rockfall mitigation Design standard adjustments under consideration Design practice between DOT and consultant engineers and incorporating input of operations and maintenance personnel Virginia High rockfall containment goals (100%) in their design standards but rewriting to be less prescriptive Consequences of failure as lead technical consideration for design Working toward inclusion of rock slopes into list of geotechnical assets to facilitate maintenance planning and funding

Case Examples 25   Oregon Department of Transportation The Oregon Department of Transportation (ODOT) pioneered and popularized aspects of rock slope assessment and rockfall mitigation design used by many DOTs. The RHRS and RCAD developed by ODOT in-house have since been adopted by many other state DOTs, with some adjustments to meet local needs (Pierson et al. 2001; Pierson and Van Vickle 1993). ODOT’s adoption of the RHRS spurred numerous rockfall mitigation projects in the 1990s, well before the more purpose-built and robust rockfall mitigation devices in use today. After managing rock slope and rockfall mitigation projects with these systems over multiple decades, ODOT has a unique perspective on long-term performance of their design practices and how these practices have changed over time. ODOT is responsible for 8,033 road miles and has 4,060 slopes (both rock slopes and soil slopes) in its statewide geotechnical asset inventory. The 4,060 slopes include those that are considered unstable and also represent about only 50% of the total highway mileage. Once 100% of the highway miles are inventoried, ODOT expects approximately 8,000 sites in the unstable slope inventory. Since implementation of the RHRS program in 1991, reevaluation and rerating of most rock slopes occurred in the early 2000s. Building on this prior research, ODOT and FHWA recently funded research on predicting seismic-induced rockfall for targeted rockfall mitigation (SPR-809), which was completed in December 2020 (Olsen et al. 2020). This research supports ODOT’s long-term goal of increasing seismic resiliency on critical routes. Rock Slope Condition All inventoried rock slopes in ODOT’s system require maintenance. Ideally, the only required maintenance is routine ditch cleaning on a scheduled interval. However, routine maintenance costs are frequently underreported and not accurately reflected in rock slope costs. Based on survey responses, ODOT reported that a much higher percentage of mitigation measures no longer functioned as designed, estimating that roughly 80% of the mitigation projects statewide are not performing as originally intended. ODOT reported that this is due to multiple factors, including aging of installed systems, an improved understanding of mitigation system design requirements, and original mitigation efforts focused on construction budget limitations. ODOT was an early adopter of many rockfall mitigation methods and approaches; thus, the impact of aging mitigation structures constructed from lower-durability materials is higher in Oregon than many other states. The department intends to fund research in the future on life- cycle costs of mitigation projects, which would help better quantify trade-offs made during design work. Design Standards In 2001, ODOT adopted their current minimum rock slope design standards, which are consistent across all roadway classifications. In general, the target for rockfall containment is 95% of impact and 90% of rollout, although requirements for some projects may be adjusted. ODOT has deliberated internally on policy-driven percentage containment requirements hampering construction of partial mitigation measures, which are frequently more achievable with limited funds. For rockfall mitigation work, the final design requirements in some regions come down to engineering judgment based on route classification, relative risk, and consequence. Currently, the majority of new rock slope and new rockfall mitigation projects are designed in-house.

26 Design Practices for Rock Slopes and Rockfall Management Design Practice ODOT considers constructability and long-term maintenance as the most important factors in the design of new cut slopes. For rockfall mitigation projects, constructability and funding sources are the most important. Aesthetics are now incorporated into more than 90% of all rock slope designs due to increased input from other stakeholders and the general public. Although no formal policy on aesthetics has been incorporated into the design process, it now receives more attention during the initial design phase. Tennessee Department of Transportation Rockfall is a significant concern in about one-fourth of the state, largely concentrated in two maintenance regions. In 2001, the Tennessee Department of Transportation (TDOT) imple- mented a version of the RHRS as part of research conducted by the University of Tennessee (Mauldon et al. 2007). The program inventoried and assessed 1,937 rock slopes throughout the state. This initial program was reinvigorated in 2019, and the department also began incorporating landslide locations, which will be inventoried using the Unstable Slope Management Program developed by FHWA’s Western Federal Lands Highway Division (WFLHD) (Beckstrand et al. 2019). These data are currently stored in an internal geographic information system (GIS) data- base. The DOT’s team responsible for inventorying rock slopes recently completed their second season of ratings, using ESRI’s Survey123 application. During the initial implementation, TDOT found that additional training was needed on how to incorporate engineering judgment into the inventory process so that only rock slopes meeting the inventory criteria were added to the database. Rock Slope Condition Along rural interstates, a small number of rock slopes within TDOT’s inventory have been reconstructed or mitigated in the last 25 years. This is a trend that will increase with a dedicated funding source now in place. During that same period, rock slopes along interstates in urban areas have been reconstructed largely as the result of federal funding of capacity improvements. TDOT currently manages only one rock slope where the draped mesh has reached the end of its design life. Securing funding for mitigating rock slopes in remote areas remains challenging. These slopes are typically in worse condition than rock slopes on high-traffic routes. These routes are typically former county roads that were inherited by TDOT and were typically constructed with limited design in geotechnically challenging areas. Recently, TDOT separated the Maintenance Division responsibilities into an Asset Manage- ment Division and an Engineering Bureau Division. The Asset Management Division now focuses on updating the TDOT Asset Management Plan and other related tasks, and the Engi- neering Bureau Division focuses on maintenance contractual responsibilities and other related tasks. As part of that realignment, rockfall and landslide response were assigned separate job codes in their maintenance management system database. Ideally, this will help the department better track maintenance and life-cycle costs of their slope assets. For example, TDOT Engineer- ing Operations has implemented a contract to identify landslide hazards on low-volume roads and perform targeted mitigation work to improve asset performance. Design Standards Standard drawings for rock slope design have been in place in Tennessee for approximately 25 years, with beneficial long-term effects. These drawings incorporate the Ritchie ditch and the

Case Examples 27   RCAD. Design ditch width is clearly tabulated for slopes up to 50 feet tall, which covers most of the rock slopes on TDOT routes. Once clear zone requirements are incorporated, the minimum ditch width is 21 feet. If for some reason the design ditch width cannot be provided, additional rockfall mitigation methods are implemented. This frequently results in ditches wider than the RCAD minimum for TDOT’s design goal of 90% catchment. Design Practice In the design of new slopes, life-cycle costs and long-term maintenance requirements are the most important technical considerations. In rock slope mitigation work, the funding source is the most important, as a significant amount of mitigation work is funded through emergency response or federal disaster relief. TDOT expects that a fuller implementation of their GAM program will make more funding for mitigation projects available through normal planning and budgeting. This will allow Tennessee to identify potential problem areas and address them more proactively, resulting in a more reliable and cost-efficient highway system. As in many other states, most new rock slopes are designed by consultants, whereas the majority of rockfall mitigation work is designed in-house. TDOT is constantly working to improve in-house expertise on rock slope and rockfall mitigation, which in turn has translated into improved consultant management and provided more consistent designs for rock slope projects. Internal Department Outreach In 2016, the geotechnical group within TDOT developed a plan for guiding mitigation work using the GAM program and approached the chief engineer to make a case for funding to support it. This got TDOT leadership more involved in funding and is now considered a positive case study by the chief engineer. The standard drawing previously described and other policy documents were first developed internally by the geotechnical group and then shared with TDOT planning for discussion and approval. Currently, the geotechnical group is work- ing to encourage other department groups to reach out to the geotechnical team earlier in the design process. This will identify potential geotechnical issues and result in fewer costly late design changes. Utah Department of Transportation As a Rocky Mountain state, the Utah Department of Transportation (UDOT) is responsible for numerous rock slopes in a variety of challenging locations. In 2004, the department inven- toried roughly 90% of their routes using a state-specific version of the RHRS and inventoried approximately 1,100 rock slopes. The RHRS inventory has been successfully used to obtain funding for a few mitigation projects but has not been broadly incorporated into planning. UDOT is currently developing a plan to rerate these inventoried slopes and capture changes due to new construction, mitigation work, or continued degradation. Rock Slope Condition Most rock slopes in the department rely entirely on the rockfall ditch to prevent rockfall from reaching the roadway. Similarly, the starting point for many mitigation projects is to expand the rockfall catchment ditch to the greatest extent possible. Based on anecdotal reporting, roughly 80% of UDOT rock slopes require routine maintenance. Exact numbers prove challenging to

28 Design Practices for Rock Slopes and Rockfall Management quantify because there is no specific entry in the maintenance tracking system for rockfall- related ditch cleaning. However, the UDOT risk management group has expressed interest in better tracking of various types of maintenance activities, and more granular data may be available in the future. Where mitigation has been installed, approximately 25% is no longer functioning as intended. This is largely due to deterioration of installed mitigation systems as opposed to insufficient initial design. Design Standards In 2008, UDOT implemented a Geotechnical Manual of Instruction (GMOI) that includes policy documents and design standards for rock slopes and rockfall mitigation projects. Rock slope design standards are identical for newly constructed rock slopes and mitigation of existing rock slopes, but they vary based on route functional classification. These standards are also applied to appurtenant structures, such as pedestrian trails and maintenance facilities. In general, the rockfall containment goal is 98% for interstate routes and 95% for all other routes. The ditch width required to achieve that catchment is estimated from the RCAD and then quantified using rockfall modeling. There is currently internal discussion within the department regarding changes to the GMOI that would relax some of the requirements for 95% and 98% containment for certain projects and incorporate more site-specific modeling and/or drone- based assessment. Design Practice In practice, it is difficult to obtain funding for rock slope mitigation as an end in and of itself, and it is much easier to incorporate that work into existing projects. For example, improvements to the clear zone along a route can also incorporate improvements to the rockfall catchment areas. In construction of both new rock slopes and rockfall mitigation projects, constructability, long-term maintenance, and aesthetics are some of the most important considerations for UDOT. Asset management is also an important consideration in the design of new rock slopes, and funding sources are an important consideration in the design of mitigation. Roughly 80% of rockfall mitigation is designed in-house as part of emergency response work. In contrast, roughly 80% of new rock slopes are designed by consultants because they are included in planned roadway improvement projects. In all design work, UDOT has made a concerted effort to bring maintenance and operations personnel into the design discussions. This helps ensure that long-term maintenance needs can be met with existing personnel and equipment. A sense of ownership on the part of the crew responsible for long-term rock slope maintenance is an added benefit. Rock slope aesthetics are currently considered in 75% to 95% of all rock slope design projects. The perception of mesh as unsightly by some stakeholders has helped fuel a push for larger ditch design as the preferred department strategy for rockfall mitigation. Where mesh is necessary, the least visible option is preferable. Virginia Department of Transportation The Virginia Department of Transportation (VDOT) manages approximately 4,000 rock slopes. A formal statewide inventory has not been conducted, so this number is based on institutional knowledge and correlations to GIS work in areas of concentrated rock slopes. Within a project area, VDOT also uses change in a cut’s orientation around a curve to help

Case Examples 29   define rock slope extents. A change in a cut’s trend of more than 20° delineates a new rock slope within its inventory. The majority of VDOT’s rock slopes are composed of shale and limestone, which are inherently unstable over time. The basalt/metabasalt slopes in the eastern portion of the state account for a minor proportion of rockfall issues. Design Standards VDOT adopted rock slope design guidelines in 2012. Standards are the same for new slopes and mitigation projects but are separated based on highway functional classification. These standards strive for 100% containment by the roadside ditch for interstate and other primary routes, which is generally achievable within the ROW. Although these guidelines have worked well for the department, VDOT is currently in the process of rewriting their rock slope design guidelines to make them simpler and less prescriptive. Design Practice The lead technical consideration for design of both new rock slopes and rockfall mitigation projects is consequences of major slope failures and their potential impact on corridor function. This approach acknowledges risk posed by major failures and considers the consequences of minor slope failures and long-term maintenance requirements but to a somewhat lesser degree for new rock slopes. Both new rock slope construction and rockfall mitigation projects are largely designed by consultants. VDOT is currently managing its second iteration of a rock slope on-call contract with a consultant, and both parties have developed a good working relationship. Approximately 15% of VDOT slopes have mitigation measures installed, which are typically rockfall barriers. VDOT does not currently have an organized system for maintaining these barriers due to the need for a specialty contractor and a project-based funding source. The mitigation measures on roughly one-third of these slopes are not functioning as intended. If a fence is damaged by rockfall, it is replaced in kind. Rock Slope Management Goals VDOT maintains a geotechnical asset inventory that includes soil slopes and embankments but not rock slopes. Because rock slopes are not included, the majority of rock slope work is done as a component of a larger project or on an emergency response basis. The project-based approach reduces the funding available for long-term maintenance of any mitigation compo- nents. The geotechnical group is currently considering adding rock slopes to the department’s list of geotechnical assets in order to establish a funding stream for design and maintenance. In the long term, an asset inventory would also allow VDOT’s rock slopes to be managed accord- ing to a verifiable metric.