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

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

4 Tens of thousands of rock slopes are adjacent to the nation’s highway systems as part of the transportation network. Together with many additional features, this highway network permits commerce, facilitates mobility, and contributes to national goals, as stated in FHWA’s mission statement, to “enable and empower the strengthening of a world-class highway system that promotes safety, mobility, and economic growth, while enhancing the quality of life of all Americans.” However, antiquated construction goals and methods often prioritized minimal excavation quantity and speed of construction over long-term slope stability, leaving states nationwide with legacies of marginally performing rock slopes. These aging slopes are more prone to rockfall, posing a hazard to highway users, often requiring unexpected closures, and necessitating increased efforts from maintenance personnel. Removing loose rocks by means of scaling increases safety, improves highway resilience and functionality, and maintains highway assets that were not designed or constructed with the tools available today. Role of Rock Slopes Within Transportation Corridors Well-performing rock slopes are important contributors to a functioning and reliable trans- portation corridor. Poorly performing slopes often produce rockfalls that threaten traffic flow and pose safety hazards to both the traveling public and the department of transportation (DOT) personnel (Federal Highway Administration, 1989; Turner and Jayaprakash, 2012). Recent research has indicated that economic benefits can be realized by DOTs that perform preventive maintenance on rock slopes, rather than wait for significant failures to close the road: the “worst-first” approach (Thompson et al., 2016; Beckstrand et al., 2017; Mines et al., 2018; Anderson et al., 2017; Vessely et al., 2019). Like other transportation infrastructure, rock slope failures and associated rockfall disrupt highway corridor functionality and increase life-cycle costs when not properly addressed. While detailed national data do not exist for rockfall occurrences inhibiting traffic flow, a review of one DOT maintenance database suggests much higher disruption and maintenance response frequency, with one 12-mile highway corridor in Alaska (Seward Highway, mile post 104 to mile post 116) recording 248 maintenance call-outs for rockfall between December 2005 and March 2019, a rate of nearly 19 rockfall events annually over the 12-mile segment (Alaska Department of Transportation and Public Facilities, n.d.). This suggests that the contribution of rock slope performance to corridor functionality and maintenance is significant. By com- parison, bridge failures have been estimated to occur approximately 128 times annually on a national basis, disrupting traffic flow and requiring maintenance response (Cook et al., 2015). Maintaining rock slopes is an important aspect of maintaining corridor functionality, and as with bridges and pavements, their performance is critical. C H A P T E R 1 Introduction

Introduction 5 Synthesis Objectives Frequently, scaling of loose rock from the rock cuts is one of the first activities in both pre- ventive maintenance programs and rockfall hazard mitigation projects (Andrew and Pierson, 2012; Pierson and Vierling, 2012). Descriptions and applications of rockfall mitigation measures beyond rock slope scaling can be found in recent comprehensive publications (Wyllie and Mah, 2004; Turner and Schuster, 2012; Wyllie, 2017). Currently, there are no published design guides, surveys of standard practices, or best management practices (BMPs) specific to rock slope scaling. This synthesis project is intended to establish the current state of the practice for slope scaling in use by U.S. departments of transportation. This synthesis’s scope was to collect scaling information regarding • Methods for estimating scaling type and quantity for a project; • Methods for estimating scaling production rates (e.g., project duration and cost estimation); • Typical scaling contract plans for contractor bidding; • Scaling specifications (e.g., contractor qualification requirements, methods used for measure- ment and payment, and incidentals); • Typical DOT scaling administration methods (e.g., on-call, design-bid-build, and emergency); • Typical methods for measuring work projects (e.g., man-hours, volume, and slope face area); • Approaches to temporary roadway protection and traffic control during scaling activity; • Methods for determining project completion; and • Documented methods for assessing scaled slope performance. Synthesis Methodology This synthesis used various methodologies to identify how DOTs administer and contract rock slope scaling projects. These included (1) a literature review of available rock slope design books, manuals, journal articles, and conference proceedings; (2) an online questionnaire for geotechnical leads of each state, containing 31 questions specific to scaling practice; and (3) six case examples from DOTs that feature unique perspectives or programs. Terminology As with any specialized field, rock slope scaling employs specific terminology. This section provides definitions for the common terms that will be used throughout the report. Photographs of various items were also added to illustrate certain tools or methods. Scaling. Removing loose rock from a slope using hand tools and/or mechanical equipment. This is frequently subdivided into hand scaling or heavy scaling, as described below. Hand or General Scaling. Scaling work performed by scalers on ropes using scaling bars (Figure 1). In some cases, hand scaling may also be performed from a crane or lift basket, depending on site conditions. Scaling bars are steel, or aluminum with steel tips for weight considerations. Heavy or Intensive Scaling. Concentrated scaling work and/or effort to remove or nearly remove a specific rock mass or a concentration of loose rocks. Typically, it combines hand scaling with additional tools, additional time, mechanical scaling, and/or blasting. Some of the most common pieces of heavy scaling equipment are shown in Figure 2.

Figure 1. Hand scaling via rope access and with scaling bars. Photograph courtesy of B. Black. Figure 2. Select equipment used in heavy scaling. Clockwise from top left: air pillows, hydraulic bottle jack (circled), and “boulder busters.” Photographs courtesy of B. Black.

Introduction 7 Mechanical Scaling. Scaling performed using heavy equipment such as mobile walking exca- vators (commonly referred to as “spider excavators”), traditional excavators, or high-reach excavators (Figure 3). This can also include clearing of benches that have been filled by loose rock because of deferred maintenance. Trim Blasting. Small-scale blasting used to remove overhanging or protruding blocks from a slope. This method is typically followed by scaling efforts to remove any remaining loose rock. Hydraulic Scaling. A historic scaling method involving the use of pressurized water to remove loose rock from a slope. This method is typically less effective than scaling performed by a quali- fied scaling crew, and its use has potential environmental concerns. Cat-Track Dragging. A historic scaling method involving knocking loose rock from a slope by dragging cat tracks across a slope face using heavy equipment. This method is typically less effective than scaling performed by a qualified scaling crew. Moveable Rockfall Barrier. A temporary barrier used to contain rockfall generated by scaling activities. In its most common form, such a barrier consists of rockfall mesh stretched between steel beams mounted on steel plates. Figure 3. Select equipment used in mechanical scaling. Clockwise from top left: excavator, “spider” excavator, high-reach excavator. Photographs courtesy of B. Black and C. Hammond.

8 Estimating and Contracting Rock Slope Scaling Adjacent to Highways Safety Scaling. Scaling requested by the scaling contractor that lies outside the contract plans and specifications. Temporary Rockfall Protection. Temporary rockfall control measures intended for worker or public safety, typically intended to prevent uncontrolled runout of scaled debris. Temporary Roadway Protection. Temporary rockfall control measures intended to protect against damage to pavement, bridge abutments, or other constructed or sensitive features. Report Organization This synthesis of the current state of the practice for rock slope scaling is organized as follows: Literature Review. The literature review summarizes the currently available recommendations for scaling project life span, methods, and project components, including any considerations for project scoping and construction practices. State of the Practice Questionnaire. The results of the broad 31-question survey sent to leading geotechnical personnel in various DOTs and federal agencies is presented in this section. Responses to each question are discussed. The section concludes with a review of the lessons learned by respondents regarding scaling work in their jurisdictions. Case Examples. On the basis of their responses to the state of the practice questionnaire, six departments of transportation were interviewed to provide detailed responses. Geotech- nical specialists from the DOTs for California, Colorado, Idaho, New Hampshire, Ohio, and Tennessee gave additional insight on their departments’ scaling practices, project manage- ment, post-project slope assessment, and tracking of project performance over time. Conclusions and Research Opportunities. Following a review of the information obtained though the literature review, questionnaire, and case histories, this section provides an overview of the current state of the practice for rock slope scaling. It describes research opportunities to address knowledge gaps identified by the synthesis and highlights possible next steps for research to improve on the current state of the practice. Appendices. This synthesis contains four appendices. Appendix A contains the survey as presented to each respondent, while Appendix B contains their anonymized responses. Each respondent had the opportunity to upload example scaling plan sheets and specifications, and these are contained in Appendix C and Appendix D, respectively. Example contractor submit- tals, redacted to remove identifying information, are provided in Appendix D for select states.