Design Practices for Rock Slopes and Rockfall Management (2022)

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9   Literature Review This literature review examines reference material commonly used by DOTs in rockfall and rock slope design analysis. The sources used for this investigation included technical books, FHWA publications, and technical studies. Currently, the literature describes techniques to investigate, analyze, design, and construct rock slopes and rockfall mitigation systems. Along with these summaries of analysis methods, the literature includes overviews of successful and unsuccessful case studies to help inform a DOT’s design process. Although there is consensus between the reviewed sources regarding analysis techniques for new rock slopes and rockfall mitigation, there is little information on what the design goals should be. Answers to questions DOTs may ask themselves, such as “How many rocks on the road and at what frequency is acceptable?” or “How much blast damage is too much?” generally go unanswered or are not the focus of research activities. Some DOTs have adopted specific design guidelines, either through policy, informal general- ized goals, or project-specific requirements, although DOTs do not have federal requirements that have to be met. Based on the existing amount of knowledge and research on rock slope and rockfall analysis, there may be benefits in developing more uniform goals to guide design between DOTs. This would further normalize safety measures implemented during the design process, resulting in safer, higher-performing highways. The following discussion topics pertaining to rockfall and rock slope site selection and design methodology were identified: • Slope hazard rating systems and site selection • Slope stability • Mitigation measures and selection considerations Slope Hazard Rating Systems and Site Selection Sources indicate that many DOTs refer to the RHRS to determine risk and hazard levels of rock slopes (Turner and Schuster 2012; Wyatt and Schenck 2020; Wyllie 2017). Implementa- tion of these programs by state DOTs began in earnest with the Pooled Fund RHRS Study led by the Oregon DOT (ODOT) and the subsequent National Highway Institute course (Pierson 1991; Pierson and Van Vickle 1993). In the past decade, some of these RHRS programs have matured into Geotechnical Asset Management (GAM) programs (Beckstrand et al. 2017b, 2017a; Thompson et al. 2016; Vessely et al. 2015). These state programs have generated large data sets on locations of rock slopes producing rockfall, their susceptibility to failure, and potential failure impacts. Data collected during RHRS field evaluations have provided valuable information for initial site selection for rockfall sites, with simplified cost–benefit analyses built into the systems C H A P T E R   2

10 Design Practices for Rock Slopes and Rockfall Management (Pierson and Van Vickle 1993). More recently, these systems have helped provide rationales guiding design decisions by setting RHRS score targets (Anderson et al. 2017; Anderson and Rivers 2013). The states that have been applying transportation asset management (TAM) prin- ciples, including deterioration, financial, and risk analysis, to their geotechnical assets will have advanced analytical tools to guide site selection and compare funding to other transportation assets. NCHRP Research Report 903: Geotechnical Asset Management for Transportation Agencies details GAM program approaches for DOTs (Vessely et al. 2019). Slope Stability When delving into methods of design for specific rock slope stability issues, both multiple sources and common practice recommend assessing the global stability of a potential hazard slope to determine the risk of failure via kinematic, limit equilibrium, or numerical analysis (Brawner 1994; FHWA 1989; Hoek and Bray 1981; Turner and Schuster 2012; Wyllie 2017; Wyllie and Mah 2004). Kinematic analysis of the geometric interaction of discontinuity orien- tations and the rock face, referred to as “Markland’s tests,” is a common and crucial first step (Hoek and Bray 1981). Broadly speaking, the primary methods identified by the sources to estimate failure likelihood are qualitative kinematic analyses, followed by quantitative factors of safety (FS) and/or probability analyses. Some research has occurred to apply load and resistance factor design (LRFD) to select rock slope failure mechanisms, but this method has not been widely applied to rock slopes (Basha and Moghal 2013). Of these analysis types, the most commonly applied method to quantify rock slope stability is the FS design stability method. Recommended design FS are provided by several sources, but these recommendations are not uniform. Wyllie and Mah (2004) recommend using FS published in 1967 by Terzaghi and Peck. These FS include 1.3 to 1.5 for earthwork, 1.5 to 2.0 for earth retain- ing structures, and 2 to 3 for foundations (Terzaghi and Peck 1967). FHWA (1989) recommends using Pierre Londe’s (technical director of Coyne et Bellier, Paris, France) multiple FS that can define a slope’s stability. These FS are based on the degree of confidence the designer has in the particular parameter being considered, with higher FS applied to parameters that are more difficult to accurately describe. These FS include 1.5 for cohesive strengths, 1.2 for frictional strengths, 2.0 for water pressure, and 1.0 for weights and forces (FHWA 1989, Appendix 3, p. 2). Technical guidance prepared by FHWA for the Federal Lands Highway Division (FLHD) recommends FS on the order of 1.3 to 1.5, depending on the levels of importance and risk (FHWA 2007). Unstable slopes that can be reliably modeled and back-analyzed for representa- tive shear strengths may be mitigated with lower FS of 1.2 to 1.3, although rock slopes are not explicitly included in the “unstable slope” classification. More recently, AASHTO’s LRFD Bridge Design Specifications recommend FS of approxi- mately 1.5 and 1.3 (expressed as resistance factors of 0.65 and 0.75, respectively) for overall stability (AASHTO 2020). AASHTO’s factors vary based on understanding of geotechnical parameters and if the slope is supporting or containing a structural element, with lower allow- able FS for well-defined, non-supporting slopes. Mitigation Measures and Selection Considerations Rockfall mitigation typically relies on a combination of the mitigation concepts: removal, avoidance, protection, and reinforcement. Each of these broad categories contains multiple mitigation methods. Brief descriptions of each type of reinforcement, removal, or protection measure are widely available in the literature and typically include a definition of the method

Literature Review 11   and a brief physical explanation of the system, with Turner and Schuster (2012) providing a thorough summary of modern rock slope and rockfall mitigation analysis approaches, methods, and considerations. Beginning in 1963 with what became known as the “Ritchie ditch design criteria” (Figure 5), state DOTs began to understand and acknowledge the risk posed by rockfall and poorly performing rock slopes (Ritchie 1963). This design guide was based on empirical field research, developed by purposefully rolling rocks off rock slopes constructed for an unopened highway project in Washington. These ditches were successful in capturing rockfall but eventually proved to pose roadside hazards due to the steep, unrecoverable 1.25H:1V foreslope. Roadside concrete barriers to prevent automobile entry into Ritchie ditches became necessary as roadside clear zones became a requirement. Research published 40 years after Ritchie’s research brought the roadside rockfall ditch into the current era of highway design (Pierson et al. 2001). The Rockfall Catchment Area Design Guide (RCAD) presented a variety of containment percentages based on cut height and angle and foreslope width and angle. However, with the RCAD came decisions on what containment percentage would be acceptable to the DOT, as 99% containment often required ditch widths much larger than those commonly used in the past (Figure 6). Considerations to include in selecting a rock slope design have been widely discussed and include design complexity, effectiveness, and long-term durability as well as aesthetics and future maintenance requirements (Andrew et al. 2011; Hoek and Bray 1981; Turner and Schuster 2012). Figure 5. Ritchie ditch design criteria (Ritchie 1963).

12 Design Practices for Rock Slopes and Rockfall Management Understandably, these considerations may vary substantially within an agency’s rock slope port- folio, based on roadway location, policy objectives, relative corridor importance, and concerns from other stakeholders. The literature review did not identify published design requirements or parameters for these nontechnical components. A frequent recommendation is to analyze case studies of similar rock slopes and incorporate lessons learned from previous projects into the final design. Discussion of appropriate application of standard excavation practices, including design and construction guidelines, is widely available in the rock slope design literature. The three most commonly identified forms of excavation in the rock slope literature are blasting, ripping, and breaking, with an overall emphasis on the importance of controlled blasting and design of ditches for adequate catchment. Figure 6. Example RCAD quick reference chart for an 80-foot-tall, 0.25H:1V slope (Pierson et al. 2001).