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30 Summary of Findings According to input from 49 state DOTs and the 3 regional FHWA FLHD offices, the design of rockfall mitigation and new rock slopes is an important aspect of constructing and main- taining a reliable highway network. Modern design practices, intents, and tools have changed significantly since many highways were constructed in the mid-20th century. Thirty-one state DOTs and all three FLHD offices of FHWA design rock slopes and rockfall mitigation frequently enough to provide thorough questionnaire input. For the purposes of this synthesis, these are considered ârock slopeâ DOTs. This synthesis solicited input regarding the current state of the practice. The responses indicated that design approaches found in the literature review are commonly employed by most of the rock slope DOTs. Many DOTs utilized common design guides, such as the RCAD, that assist with judging catchment efficacy of roadside ditches. Other items, such as rationale regarding selected design goals, development of risk-informed site selection and design criteria, and the relationship between slope performance and highway system reliability, have not been effectively documented. The following are the key findings of this synthesis. Design Goal Utilization Design goals have been formally or informally adopted by 21 rock slope DOTs. Of these, 17 provided information regarding specific design goals. Each of these 17 indicated that the ability of the roadside ditch to contain rockfall is one of the primary methods to achieve the goals. Most evaluate this goal as a percentage of rockfall debris that is contained in the roadside ditch (e.g., 90% of all rocks are stopped in the ditch), whereas a few provide scoring criteria that qualitatively evaluate ditch effectiveness (e.g., âgoodâ ditch effectiveness). The most often used percentage is 90%, with 95%, 98%, and 100% or similar ranges also used by some DOTs. These percentages typically apply to interstate or other route classifications with higher performance expectations. DOTs also communicate or document their design goals in various ways. Some have found success in clear and concise design templates that engineers refer to during roadway design. This method is particularly apt for new highway construction where a wide ditch and modern excavation techniques frequently result in stable slopes with manageable rockfall activity. Other DOTs implement design goals informally within the geotechnical group. Based on case studies and survey responses, DOTs have found success and performance improvements in developing design goals through interaction with planning and management groups and subsequently adhering to them through the design process. C H A P T E R 5
Summary of Findings 31Â Â Site-Specific Goals Some DOTs noted that strict adherence to design goals regardless of site specifics could result in inefficient expenditures. A hypothetical example of this could be comparing a rock slope project on a route with a 40% effective ditch, low rockfall activity, and low traffic volumes versus a project with an 80% effective ditch, high rockfall activity, and high traffic volumes. In a scenario blind to rockfall frequency and traffic volume risk, the site with the 40% effective ditch may be selected prior to the site exhibiting 80% effectiveness despite more rocks entering the road where more vehicles pass. Providing DOTs with a family of site selection objectives and site-specific design goals would result in more efficient risk-reduction and safety expenditures. Pragmatism-Driven Decision Making Survey responses indicate that DOTs have been adept at using available guides and recom- mendations as they become available. Starting with rolling rocks off cliffs in the 1960s and developing prescriptive design charts to obtaining highly detailed 3-D surfaces with UAVs and laser scanners to perform detailed rockfall modeling today, geotechnical professionals appear quick to utilize technology advancements. Although many aspects of highway design are nation- ally standardized, DOT geotechnical professionals have demonstrated through their responses their need for design parameters by developing their own. DOTs have repeatedly applied new knowledge to their real-world applications by widely utilizing new publications such as the RCAD. This suggests that well-considered research to develop design guidance informed by additional parameters would be readily implemented, even without national requirements or mandates. Future Research Opportunities Research Relationship Between Goals and Performance Design goals typically focus on the percentage of rockfall debris that can be contained in the roadside ditch, with most criteria falling between 90% and 98% containment. However, this criterion does not directly correlate to the frequency with which rock may enter the roadway. For instance, an active slope that produces 100 rockfalls per year with a 90% containment still results in rocks on the road about once a month. Conversely, a stable, low-activity slope with 10 rockfalls per decade but 50% containment will see rocks on the road about once every 2Â years. In a containment-only design goal scenario, the active slope with rock on the road every month meets the design goal, whereas the other, objectively less hazardous slope does not. Research regarding rockfall frequency, ditch effectiveness, and design goals that focus on slope perfor- mance can reduce associated rock risks and result in a more cost-effective and reliable highway network. Methods to Track Deterioration Survey responses and case histories indicate that some DOTs have had rockfall mitigation measures installed for decades. A few states, such as Oregon, possess high percentages of mitiga- tion measures that no longer function as originally intended. Other states, such as Tennessee, have many rockfall mitigation systems that are comparatively new and have yet to experience significant deterioration. Deterioration of various rock types (i.e., interbedded sedimentary rock versus crystalline igneous rock), rockfall mitigation measures (i.e., rock bolts, draped mesh), and slope performance (i.e., rockfall activity impeding traffic, increased required maintenance)
32 Design Practices for Rock Slopes and Rockfall Management would assist DOTs with life-cycle cost forecasts and inform site selection decision making. Information on the deterioration rate, life-cycle costs, and their contributing factors is lacking based on survey responses. Research to develop practices to track deterioration and costs could be prepared for rockfall mitigation and rock slopes to inform DOTs regarding potential replacement schedules and budget planning. Identification of Funding Mechanisms for Mitigation or Major Maintenance Projects Survey responses indicated that a primary challenge for DOTs is recognizing and prioritiz- ing funding for rockfall mitigation projects. Sharing methods used by various DOTs, such as fostering management buy-in, demonstrating the contribution of high-performance rock slopes to an efficient highway system, or careful documentation of the risks posed by a select group of rock slopes, could benefit DOTs seeking to prioritize rock slopes for repair. Likewise, DOTs may have unique maintenance programs where specialty contractors are available on a task order or on-call basis to maintain or repair rockfall mitigation measures. Research and Development of Risk-Based Design Criteria Design guides have evolved significantly since the prescriptive Ritchie ditch criteria in the 1960s, to analytical methods described by Hoek and Bray in 1981, to statewide rating systems in the 1990s, to modern ditch criteria of the 2000s. During this time, some DOTs have developed a mix of design goals that help drive site-specific design decisions. At the same time, DOTs have faced increasing budgetary challenges as the slopes adjacent to the highway system continue to weather and produce rockfall. DOTs would benefit from research providing a development framework to generate consistent, risk-informed design goals that factor in items such as route classification, traffic volumes, and performance history.
