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Estimating and Contracting Rock Slope Scaling Adjacent to Highways (2020)

Chapter: Chapter 2 - Literature Review

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Suggested Citation:"Chapter 2 - Literature Review." National Academies of Sciences, Engineering, and Medicine. 2020. Estimating and Contracting Rock Slope Scaling Adjacent to Highways. Washington, DC: The National Academies Press. doi: 10.17226/25824.
Page 9
Page 10
Suggested Citation:"Chapter 2 - Literature Review." National Academies of Sciences, Engineering, and Medicine. 2020. Estimating and Contracting Rock Slope Scaling Adjacent to Highways. Washington, DC: The National Academies Press. doi: 10.17226/25824.
Page 10

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9 This synthesis project included a review of the limited published literature on rock slope scaling. Identified sources included textbooks, workshop manuals, and conference presenta- tions. Scaling was not a major topic in most of these sources. However, the review nevertheless identified relevant information on the following topics: • Typical scaling methods and associated tools. • Typical life span of a scaling project. • Interaction between rock slope scaling and other methods of rock slope stabilization/rockfall control. • Recommendations for developing scaling specifications. The broadly accepted definition of rock slope scaling is the removal of loose or potentially unstable material and rock from a slope (Pierson and Vierling, 2012). Implicit in this statement is that scaling work only removes those rocks that are already ready to fall. Vegetation removal (i.e., tree removal) is typically a component of any scaling operation, particularly in highly vegetated slopes where root-jacking is a concern (Wyllie and Mah, 2004). Multiple methods are available for completing scaling work, ranging from hand scaling performed by crew members using pry bars, to mechanical scaling, to trim blasting of selected rock features (Andrew et al., 2011; Brawner, 1994; Pierson and Vierling, 2012; Wyllie and Mah, 2004). Frequently, multiple methods are employed at a given site. Hand scaling may be done alone or in combination with blasting. Mechanical scaling may augment hand scaling by covering large areas in a shorter amount of time through the use of long-reach excavators or walking “spider” excavators (Andrew et al., 2011; Brawner, 1994; George et al., 2016). Within the mechanical scaling category, there are certain methods, such as hydraulic scaling or cat-track dragging, that have become less common and would still require hand scaling (Andrew and Pierson, 2012). Small-scale, or trim, blasting is frequently used to remove a particular rock feature within a larger project area, with post-blast rock removal via hand scaling (Andrew and Pierson, 2012). In the reviewed literature, there was broad agreement on the recommendations for worker safety and safety of the traveling public. For worker safety, scaling is conducted starting from the crest of the slope to its base. If no additional barriers or forms of protection are in place, then areas below active scaling areas should be temporarily closed to the public (Andrew and Pierson, 2012; Brawner, 1994; Wyllie and Mah, 2004). In emphasizing the importance of selecting the appropriate rock removal method, one source noted that once scaling of any large feature has begun, it should be considered unstable, and the area below should not be reopened until the feature is removed or otherwise deemed stable (Andrew et al., 2011). For safety of the traveling public, traffic control is a crucial project component. Complete road closure below the slope is optimal but is not always achievable. Temporary barriers are also C H A P T E R 2 Literature Review

10 Estimating and Contracting Rock Slope Scaling Adjacent to Highways necessary to protect adjacent structures or sensitive areas from rocks that fall during the scaling operation (Andrew et al., 2011). Design and selection of appropriate rockfall mitigation measures, such as rock bolts, draped mesh, and rockfall barriers, were discussed in a variety of sources (Brawner, 1994; Turner and Schuster, 2012; Wyllie, 2017; Wyllie and Mah, 2004). Throughout the reviewed documents, scaling was identified as a temporary rockfall reduction measure that must be performed again after some period of time. The range of scaling effective- ness, in the absence of additional mitigation methods, varied from as low as 3 to 5 years (Wyllie and Mah, 2004) to 2 to 10 years (Andrew et al., 2011; Andrew and Pierson, 2012; Pierson and Vierling, 2012), and up to 8 to 15 years (Brawner, 1994). In all of these examples, the durations were apparently based on author experience as opposed to analysis of consistent data sets, and the range of effective project life spans may reflect the variation in geology and climate (e.g., freeze-thaw cycles, frequency and intensity of storms) between the authors’ regions of expertise. Although scaling alone is not a permanent solution to a rockfall problem, it is recognized as a valuable component of routine maintenance or may be used to remove a particular unstable feature (Pierson and Vierling, 2012). It is also routinely used as the first component of other rockfall mitigation efforts because it improves site safety (Andrew et al., 2011). Specific recommendations for developing scaling project specifications were not typically included in the published literature. One exception, a publication by the National Highway Institute (Brawner, 1994), the training and education arm of the FHWA, made the following recommendations for scaling specifications in the Rockfall Hazard Mitigation Methods course: • Require that the scaling crew has performed similar work satisfactorily. • Require 2 years’ experience for scalers, and 5 years for the crew foreman. • Specify construction sequence, crew size, equipment, waste removal plan, and traffic control plan in construction documents. • Measure work on a crew-hour basis. • Require inspection of the face by an engineer after each scaling pass to determine if work has been completed satisfactorily. These FHWA recommendations, made in 1994, did not touch on many other scaling-related contracting issues, such as use of prequalified contractors or methods for measuring scaling volumes and performance (Brawner, 1994). In addition, results from a questionnaire completed as part of a workshop convened in 2018 at the 97th TRB Annual Meeting, Managing Highway Rock Slope Scaling: Design and Construction State of the Practice, indicated that significant variation in scaling project development and delivery methods persisted (Arndt, 2020). Work- shop discussions indicated that departments typically continued to approach scaling projects on a case-by-case basis and were dissatisfied with the continued absence of a broad overview of the current state of the practice beyond their individual departments. In the review of books, articles, and manuals conducted for this synthesis, the following senti- ment appeared to apply: scaling is a well-recognized mitigation technique, but it was treated as an initial or interim mitigation method and typically received only modest attention before the writer’s focus moved on to other, more complex rockfall mitigation methods. However, as the 2018 TRB workshop highlighted, quality completion of scaling work lays the foundation for a successful project and, therefore, warrants the same care given to scoping other rockfall mitiga- tion techniques.

Next: Chapter 3 - State of the Practice »
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Scaling loose rock from highway rock slopes is an important aspect of improving rock slope safety in mountainous areas, according to input from 42 state departments of transportation and two regional divisions of the Office of Federal Lands Highway.

The TRB National Cooperative Highway Research Program's NCHRP Synthesis 555: Estimating and Contracting Rock Slope Scaling Adjacent to Highways documents current rock slope scaling practices adjacent to highways.

An appendices document is also included as part of the publication.


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