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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2022. Design Practices for Rock Slopes and Rockfall Management. Washington, DC: The National Academies Press. doi: 10.17226/26636.
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Page 1
Page 2
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2022. Design Practices for Rock Slopes and Rockfall Management. Washington, DC: The National Academies Press. doi: 10.17226/26636.
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Page 2
Page 3
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2022. Design Practices for Rock Slopes and Rockfall Management. Washington, DC: The National Academies Press. doi: 10.17226/26636.
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1   State departments of transportation (DOTs) are responsible for thousands of rock slopes—both natural slopes adjacent to roadways and constructed slopes cut to enable roadway construction. Since the 1960s, DOTs, federal agencies, universities, and private practitioners all have invested in research on the design of rock slopes and rockfall mitiga- tion, with many DOTs using design guides. In recent years, software improvements have made it increasingly faster and more cost-effective to conduct complex modeling and analysis. These guides and software tools are invaluable to practitioners, who use them to evaluate and design rock slopes and rockfall mitigation structures. Unlike for many other aspects of transportation design, prior research has not yet been uniformly translated into standard- ized goals and objectives for design of rock slopes and rockfall mitigation. Instead, several DOTs have formally or informally adopted their own design goals and objectives to meet the needs of their states and the expectations of the traveling public. The objective of this synthesis is to document current DOT practices for the design of rock slopes and rockfall mitigation systems. The information was gathered via a literature review and a detailed questionnaire sent to DOT geotechnical leads of each state DOT, Puerto Rico, Washington, D.C., and the three regional offices of the Federal Lands Highway Division (FLHD) of the FHWA. Responses were provided by 49 DOTs and the 3 FLHD offices—a 94% and 100% response rate, respectively. Follow-up interviews with four state DOTs provided additional insight regarding design practice within their departments. Not all DOTs regularly design rock slopes or rockfall mitigation measures, and therefore filter questions were formulated to permit responding without requiring a full question- naire response. Of the 52 respondents, 34 dealt with these design matters with sufficient frequency to respond to the more detailed questions and serve as the primary basis of the survey results. These 34 responding DOTs and FLHD offices are termed “rock slope” states in the body of this synthesis. The survey questionnaire was subdivided into categories focusing on (a) rock slope and rockfall occurrence, (b) design efforts and design goals, (c) design considerations, (d) design tools, (e) performance measures, and (f) lessons learned. Each category had several questions asked, for a total of 33 focused questions. The major findings of the survey are categorized as follows. Rock Slope Occurrence. The rock slope inventory under the responsibility of the 34 rock slope DOTs range from 50 to 10,000 slopes. State DOTs do not have mandates to perform comprehensive rock slope inventories, so the quality of these counts can range from judg- ment of a DOT’s problematic rock slopes to comprehensive, statewide inventories of all rock slopes. A review of survey responses indicated that the occurrence of rock slopes S U M M A R Y Design Practices for Rock Slopes and Rockfall Management

2 Design Practices for Rock Slopes and Rockfall Management ranged from 6 to 50 per 100 miles of highway for those DOTs with more mature inventory systems. The higher occurrences generally related to states with greater topographic relief. Design Efforts and Goals. The survey identified that nearly half of the rock slope DOTs have formal design goals. The goals may be documented as policy, internal memos, or technical drawings guiding roadway designers. When in place, the design goals are frequently applied across highway system designations [e.g., interstate, National Highway System (NHS)] or functional classifications (e.g., primary arterials, secondary collectors). Of those DOTs that do separate between systems, the Interstate system or the broader NHS received the higher-goal targets. The survey found rockfall ditch catchment percentages (e.g., 90% of all rockfall is contained in the roadside ditch and prevented from entering the travel lanes) were the most common metric used. The containment percentages ranged from 90% to 100% for the interstate, NHS, or primary systems. Containment percentage goals fell to 80% to 95% when goals were adjusted for typically lower-volume highway classification systems. Other design goals included percentage of rockfall impact (i.e., location of first ground strike of a falling rock, not including rollout), target scoring in rating systems, maintenance reductions, and frequency of rock reaching the roadway. Design Considerations. Design of rock slopes and rockfall mitigation measures require consideration of several design factors. DOTs were queried regarding the relative impor- tance of technical design considerations, with constructability, long-term maintenance, and funding considerations cited as “most important.” “Least important” technical consider- ations were often project delivery methods, aesthetics, and resilience initiatives. DOTs were also asked for relative rankings for risk considerations, with consequences of a major failure, liability, and minor failure ranked as most important. Design Tools. There are a large variety of design tools available to DOTs for investigation, analysis, and design. The survey provided 15 design tools that DOTs could assign a propor- tion (100% to 0%) of rock slope projects on which they use the tool. Surface reconnaissance, ditch design guides, rock mass characterization, stereonet analysis, and rockfall modeling were commonly used for all design purposes. Use of specific design tools, such as drilling or point cloud analysis, varies based on the degree to which the rock is exposed in cuts or outcrop. For instance, geotechnical drilling is used 85% of the time for new cuts without exposed rock but only 27% of the time for rockfall sites where rock is frequently exposed in detail. Likewise, technologies that generate point clouds of rock outcrops are used at about three times the rate for rockfall projects versus those for new cuts where rock is not exposed. Performance Measures. The survey indicated that nearly half of the rock slope DOTs have performance measures in place to gauge slope performance. About two-thirds of those DOTs track slope performance informally by their geotechnical group. Most DOTs, however, do not regularly engage in data collection efforts to determine if design goals or performance objectives are met. Reported Lessons Learned. The literature review revealed that DOTs have been addressing rock slopes and rockfall since the 1960s. Since that time, DOTs have been exposed to a variety of technical, contractual, and geologic factors that help improve practice over time. The survey respondents focused on the following general themes: (a) difficulty in identifying and obtaining funding for rock slope improvements and rockfall mitigation; (b) importance of open and clear communication between the DOTs and their stakeholders, contractors, designers, and internal DOT management; (c) benefits of comprehensive data collection,

Summary 3   ranging from statewide inventory and condition assessments to site-specific data collection; and (d) benefits of building internal experience and fostering relationships with experienced contractors and designers. The synthesis identified knowledge gaps in current practices that could be addressed with research that does the following: • Identifies relationships between design goals and improved performance. • Develops methods to track deterioration of rock slopes and rockfall mitigation measures. • Provides a roadmap for identifying, prioritizing, and justifying rockfall mitigation and/or major maintenance projects. • Develops risk-based design criteria that can assist DOTs in developing design criteria that consider rockfall activity, route significance, and performance expectations.

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While there are no national standards for rock slope design and rockfall management, many state departments of transportation (DOTs) have developed their own design goals and objectives.

The TRB National Cooperative Highway Research Program's NCHRP Synthesis 588: Design Practices for Rock Slopes and Rockfall Management documents DOT practices for the design of rock slopes and rockfall mitigation systems.

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