Design Practices for Rock Slopes and Rockfall Management (2022)

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4 Introduction Over the last century, construction of the nation’s highway system required the construction of tens of thousands of rock slopes. The constructed highway system has required the mainte- nance, reconstruction, and hazard mitigation of thousands of rock slopes. This has been accom- plished by either installing rockfall mitigation devices, reconstructing a slope to improve safety and reduce risks of service disruptions, or performing simple ditch maintenance when required. Design guidance for rock slope and rockfall mitigation was developed by research starting in the 1960s (Ritchie 1963). In the intervening decades, research has been conducted on the design and construction of rock slopes and rockfall mitigation—both by public agencies and specialized manufacturers offering products for rock slope construction and rockfall mitigation (Turner and Schuster 2012). In recent years, computer-based modeling has become invaluable in the design process as well as in the analysis of existing slopes. Rapid advances in computer and survey technology have made complex data analyses faster and easier. Required or recommended national minimum standards that utilize these advances for rock slope or rockfall mitigation have not been advanced at the federal level. However, many state departments of transporta- tion (DOTs) have developed internal goals and objectives to help guide design. Role of Effective Rock Slope and Rockfall Mitigation Design High-performance rock slopes, although easily overlooked, facilitate mobility and commerce by not generating rockfall that reaches the roadway. Maintenance needs, such as cleaning roadside ditches of rockfall debris, are minimal and accomplished at regularly scheduled times. Emergency response work is not required, leaving budget available for other projects vital to maintaining a functional and reliable transportation corridor. Like other transportation infra- structure, rock slopes age and degrade over time and have an associated life-cycle cost. Failures and associated rockfall disrupt highway corridor functionality and create elevated safety threats when not accounted for in initial rock slope design or addressed by mitigation work. To facilitate adequate design of rock slopes and rockfall mitigation, research guiding design has been in use by some DOTs since the 1960s (Pierson et al. 2001; Ritchie 1963). Multiple DOTs, federal agencies, and private companies have also conducted research on rock slope and rockfall mitigation design (Arndt et al. 2016, 2009; George et al. 2018; Hofmann et al. 2019). In the absence of national minimum design standards, some DOTs have formally or informally adopted their own design standards, goals, or objectives. Examples of design goals that DOTs have adopted, either on a project-specific or statewide basis, include the following: • Acceptable containment percentages of rockfall debris that will be captured prior to entering the roadway • General reductions in maintenance efforts and/or safety concerns C H A P T E R 1

Introduction 5   • Reductions in hazard “scoring” using evaluation systems such as the Rockfall Hazard Rating System (RHRS) All these design goals aim to provide metrics that allow the DOT to guide design efforts with a practical, uniform approach. Synthesis Objectives The objective of this synthesis is to document the state of the practice for DOTs regarding the design of new rock slopes, the rehabilitation of existing ones, and effective rockfall mitiga- tion measures. Recent advances in computing have made complex analyses more accessible to designers and have become an invaluable tool in rock slope design and evaluation. Descriptions and applications of various tools and methods for design and evaluation or rock slope projects can be found in technical books and recent comprehensive publications (Andrew et al. 2011; Brawner 1994; FHWA 1989; Turner and Schuster 2012; Wyllie 2017). Currently, there is a wealth of research and regional expertise, but there are no published national standards for design goals and objectives regarding rock slope construction and rock- fall mitigation design. In the absence of federal performance requirements, some DOTs have formally or informally adopted design standards, goals, and objectives for their own jurisdic- tions. This synthesis is intended to document the current state of the practice for design of rock slopes and rockfall mitigation in use by state DOTs. The synthesis scope was to document current state DOT design practices for rock slopes and rockfall mitigation, including the following: • Relative percentages of new and rehabilitated rock slopes managed by DOTs • Relative percentages of design work completed by DOTs, consultants, or contractors • Documented agency design goals and objectives • Methods for including additional components in the design process, such as project delivery, constructability, maintenance, asset management, aesthetics, life-cycle costs, resilience initia- tives, highway system designation, or risk management plans • Incorporation of risk concepts into rock slope design and management • Typical technical factors that are used to inform design, such as new versus rehabilitated slopes, slope stability models, rock mass characteristics, rockfall frequency, rockfall containment percentages, and other performance standards • Employment or development of performance measures for rock slopes and rockfall mitigation Synthesis Methodology This synthesis used three methods to collect data on current DOT design practices for rock slope and rockfall mitigation projects. These methods included (a) a literature review of available rock slope design publications; (b) a 33-question online questionnaire sent to lead geotechnical engineers or engineering geologists for each state, FHWA divisions, Washington, D.C., and Puerto Rico; and (c) in-depth interviews with representatives of four DOTs to provide case examples of unique perspectives or programs. Responses were provided by 49 DOTs and the 3 FLHD offices—a 94% and 100% response rate, respectively. Terminology Rock slope and rockfall mitigation design employs some specific terminology not used in other areas of highway design. This section provides definitions for the specialized terminology that will be commonly used throughout the report. Select photographs have been added to

6 Design Practices for Rock Slopes and Rockfall Management illustrate certain terms as well as provide typical examples of rock cut slopes and rockfall mitigation structures. • Rock Slope: A constructed slope excavated through rock materials for construction of roads or related facilities. • Rockfall: The occurrence of detached rock fragment(s) sliding, toppling, or falling uncontrolled down natural or constructed slopes onto the right-of-way (ROW). The falling rock(s) may proceed down slope by bouncing and flying along ballistic trajectories or by rolling on talus or debris slopes. Rockfall is typically both a maintenance concern and a risk to safety and mobility. • New Rock Cut: New major rock excavation activities resulting in a new, steep cut slope face comprised largely of intact rock materials (Figure 1). A new rock cut may be an enlargement of an existing cut for minor roadway realignments (Figure 2) or an entirely new cut where no roadway previously existed for major construction projects. • Rockfall Mitigation: Engineered or otherwise purpose-built measures to reduce the effects of rockfall hazards. Mitigation approaches may include hazard avoidance, removal, protection from falling rock, and/or stabilization of potentially unstable slopes or slope features. • Avoidance: Mitigating rockfall hazards by moving the roadway away from the slope producing the rockfall. An example includes a centerline shift into a former shoulder to increase the effective ditch width. • Protection: Mitigating rockfall hazards by capturing falling rocks with constructed measures or improved roadside containment. Examples include draped mesh, flexible rockfall barriers, rockfall attenuators, and roadside barriers (Figure 3 and Figure 4). • Stabilization: Mitigating rockfall hazards through installation of active or passive stabilization measures. Examples include rock drains, rock bolts and dowels, and pinned high-strength mesh (Figure 3). • Catchment Ditch: Sections of flat or negatively sloped ground between the highway and rock slope that are used to dissipate rockfall energy and to collect rocks and other debris that have detached from the slope. Catchment areas can be ditches dug along the foot of a slope or hybrid ditches, which combine a ditch with a barrier (typically a wall, barrier, or berm). • Percent Containment: Percentage of falling or rolling rock detached from the adjacent slope that the catchment/roadside ditch can collect and hold. Uncontained rock reaches the roadway. Figure 1. Construction of new roadway alignment with new rock cuts. Eagle’s Nest, Minnesota (photo courtesy of J. Hudak).

Introduction 7   Figure 2. New rock cut with enlarged catchment ditch constructed as part of minor realignment for a highway safety improvement project. Glacier Highway, Alaska (photo courtesy of C. McCormick). Figure 3. Rockfall mitigation projects with mitigation components installed on the rock slope. Draped mesh protection (left) and rock bolt stabilization over a tunnel portal (right) (photos courtesy of R. Group).

8 Design Practices for Rock Slopes and Rockfall Management Report Organization This synthesis of the current state of the design practice for rock slopes and rockfall manage- ment is organized as follows. Literature Review. This review summarizes currently available recommendations for design of rock slopes and rockfall mitigation, including design goals adopted at the DOT level. Design and analysis methods are briefly discussed; information on design and analysis techniques is beyond the scope of this synthesis. State-of-the-Practice Questionnaire. The results of the 33-question survey sent to lead geo- technical personnel in various DOTs and federal agencies are presented in this section. Responses to each question will be discussed. The text of the questions is contained in Appendix A, compiled responses are in Appendix B, and individual responses are in Appendix C. Case Examples. Based on their responses to the State-of-the-Practice Questionnaire, four DOTs were interviewed to provide detailed responses. Geotechnical specialists from the DOTs of Oregon, Tennessee, Utah, and Virginia gave additional insight on their DOT rock slope and rockfall design practices, design considerations, and project planning. Summary of Findings. This section summarizes the information obtained via the literature review, questionnaire, and case histories, providing an overview of current design practices for rock slope and rockfall management. It identifies knowledge gaps and suggests research for DOTs seeking to adopt more formal design goals and objectives in their own jurisdictions or incorporate additional design goals capturing risk considerations or other nontechnical aspects. Figure 4. Rockfall mitigation project with protective mitigation components installed at toe of slope. I-40, North Carolina (photo courtesy of J. Kuhne).