National Academies Press: OpenBook

Guidelines for Certification and Management of Flexible Rockfall Protection Systems (2016)

Chapter: Chapter 5 - Discussion of Management of Rockfall Fence Systems After Inventory and Condition Assessment

« Previous: Chapter 4 - Proposed Inventory and Condition Assessment of Rockfall Fence Systems for Agencies
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Suggested Citation:"Chapter 5 - Discussion of Management of Rockfall Fence Systems After Inventory and Condition Assessment." National Academies of Sciences, Engineering, and Medicine. 2016. Guidelines for Certification and Management of Flexible Rockfall Protection Systems. Washington, DC: The National Academies Press. doi: 10.17226/23519.
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Suggested Citation:"Chapter 5 - Discussion of Management of Rockfall Fence Systems After Inventory and Condition Assessment." National Academies of Sciences, Engineering, and Medicine. 2016. Guidelines for Certification and Management of Flexible Rockfall Protection Systems. Washington, DC: The National Academies Press. doi: 10.17226/23519.
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Suggested Citation:"Chapter 5 - Discussion of Management of Rockfall Fence Systems After Inventory and Condition Assessment." National Academies of Sciences, Engineering, and Medicine. 2016. Guidelines for Certification and Management of Flexible Rockfall Protection Systems. Washington, DC: The National Academies Press. doi: 10.17226/23519.
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Page 18
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Suggested Citation:"Chapter 5 - Discussion of Management of Rockfall Fence Systems After Inventory and Condition Assessment." National Academies of Sciences, Engineering, and Medicine. 2016. Guidelines for Certification and Management of Flexible Rockfall Protection Systems. Washington, DC: The National Academies Press. doi: 10.17226/23519.
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Page 19

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16 C H A P T E R 5 After completion of the inventory and condition assessment phase of an asset management plan, the data is analyzed using life-cycle cost models to forecast future investments. The life- cycle cost analysis principles for assets such as pavement are well developed with readily defined variables. However, the data required to perform a life-cycle analysis for a rockfall fence sys- tem is not as readily quantified as it is for pavement. Section 5.1 and Section 5.2 provide a comparison of the two systems. 5.1 Pavement Asset Management Concept For pavement asset management most variables can be quantified or tested. The specific items associated with pave- ment design and construction include the following: • pavement thickness • mix types • aggregate types • binders • compaction • density testing The preceding quantities are measurable items associated with pavement asset and used in modeling and design, however the one item that specifically affects the asset is traffic volume. Traffic volume is also a measurable element that can be quan- tified in real time relatively inexpensively compared with the cost of an overall pavement project. With the above information and a well-defined deterioration model, a life-cycle analysis can be performed to evaluate treatment options for the short term and long term. This analysis guides the decision making pro- cess to maintain the pavement in the best condition possible given budgetary and other constraints. Figure 5-1 depicts a generalized life-cycle activity profile that ties pavement performance to traffic volume, showing pavement deterioration over time. The example life-cycle activity profile for pavement illus- trates pavement deterioration to the replacement threshold versus performing several surface treatments to maintain the pavement in a better condition over the life cycle. The deterio- ration models and treatment alternatives that are incorporated into the life-cycle analysis are relatively well known based on the current knowledge of pavement performance. This concept fits well into asset management and program- ming of funding given that the parameters are measurable and well defined and can be anticipated and predicted to a reasonable confidence level. 5.2 Rockfall Fence Asset Management Concept For rockfall fence asset management most variables can- not be readily quantified. The main variables associated with rockfall fence design include the following: • the estimated design rock diameter • the estimated design impact energy level associated with a given design rock diameter • the estimated bounce height for the design rock diameter These quantities are used in modeling and design of the rockfall fence asset; however, these events would need to be measured to provide a basis for managing the rockfall system. The rockfall energy, frequency, and bounce height are not readily measurable elements that can be quantified for use in asset management of rockfall fences. These events are more analogous to a catastrophic event for pavements such as a water main break that undermines the pavement section. Anticipating and attempting to predict the actual rockfall size, frequency, and bounce height is not readily performed. Typically rockfall systems are located in historic rockfall areas, but the time, intensity, frequency, and amount of rockfall from Discussion of Management of Rockfall Fence Systems After Inventory and Condition Assessment

17 the event cannot be predicted with a reasonable confidence level such as can be performed with traffic volumes. There are systems that can be used to anticipate and predict rock slope failures, but these are generally expensive real-time monitoring systems that collect data on an ongoing basis. For example, the data collection for evaluating stability of a rock slope in an open pit mine may occur for years or even decades in an effort to predict an impending rock slope failure. The prediction of the rock slope failure becomes more accurate as the time nears to the actual failure; however, vast amounts of data are required over long time periods to determine what the threshold may be for an impending rock slope failure. These methods to anticipate and predict rockfall by gathering data are generally many times the cost of the actual rockfall system. For example, in many cases the rockfall fence system may be less than $60,000, but to gather actual data on the rock slope or rockfall activity using radar or LiDAR methods may be in excess of $10,000 to 60,000 a month depending on the system. To further clarify the comparison between pavement asset management and rockfall fence asset management, the fol- lowing three scenarios are presented for a rockfall fence sys- tem. Figure 5-2 depicts a life-cycle activity profile in which a rockfall event impacts a rockfall fence system with energy in excess of the system capacity. After one large rockfall event the entire system requires repair or replacement. Figure 5-3 depicts a life-cycle active profile for multiple smaller rockfall events that are within the system capac- ity but because of the frequency of events, the condition of the system deteriorates to the point of requiring repair or replacement. Figure 5-4 depicts a life-cycle activity profile Optimum Pavement Condition Threshold for Pavement Replacement Pa ve m en t C on di tio n Traffic Initial Pavement Condition Threshold for Preventative Maintenance Managed Lower Cost Treatments Pavement Deterioration over Time Figure 5-1. Generalized pavement life-cycle activity profile. System Replacement/Repair Threshold R oc kf al l F en ce C on di tio n Rockfall Events Initial Rockfall System Condition Event 1 - Large Rockfall (Exceeds Fence Capacity) Figure 5-2. Life-cycle activity profile for a rockfall event that exceeds rockfall system capacity.

18 for a single rockfall event at the service energy level resulting in a slight reduction in fence condition followed by a large rockfall event exceeding the system capacity, requiring system repair or replacement. 5.3 Rockfall Fence Asset Management Practice The preceding example illustrates the difficulty of apply- ing a well-known concept that works well for pavements to rockfall fences. For an agency to apply the principles and con- cepts of asset management from pavement design to rockfall fences, the following elements would need to be addressed: • Frequency of rockfall impacts of various energies • Location of rockfall impacts within the system • Bounce height of the rockfall • Climatic conditions including high precipitation events and freeze-thaw cycles that affect rockfall frequency In lieu of gathering this information through relatively expensive data gathering systems, an agency can estimate the measurements by the following: • Perform an inventory and condition assessment of the rock- fall systems on a periodic basis such as yearly or every 2 years to obtain basic information on the status of the systems. System Replacement/Repair Threshold R oc kf al l F en ce C on di tio n Rockfall Events Initial Rockfall System Condition Events 1, 2, 3, and 4 - Small Rockfalls (Service Energy Level) Figure 5-3. Life-cycle activity profile for multiple smaller rockfall events that require repair or replacement of the system. System Replacement/Repair Threshold Ro ck fa ll F en ce C on di tio n Rockfall Events Initial Rockfall System Condition Event 2 - Large Rockfall (Exceeds Fence Capacity) Event 1 - Small Rockfall (Service Energy Level) Figure 5-4. Life-cycle activity profile for one smaller rockfall event and one larger rockfall event that requires repair or replacement of the system.

19 • Review the inventories and compare the change in system conditions in the time between assessments. • Program funding based on the rate at which system condi- tions fall below a threshold defined by the agency. During project evaluation and prioritization based on condition assessment data and analysis, agencies should con- sider the following: • Cost of system repair or replacement versus overall con- struction contract costs because rockfall fence systems are relatively inexpensive compared with project costs that include traffic control, mobilization, inspection, and so forth • Cost of repairing a rockfall fence system from an unaccept- able condition to an acceptable condition versus complete system replacement • Potential cost savings of repairing or replacing multiple systems under one project As agencies collect additional condition assessment data, they can develop deterioration models for life-cycle analy- sis that will help guide the decision making process. More elaborate models can be developed over time as described in Appendix C of the Final Report. The initial process is depen- dent on creating a condition inventory and database of the rockfall fence systems and evaluating the performance over time. Rockfall fences in less active areas will require little to no maintenance whereas rockfall fences in highly active areas may require yearly maintenance or repair and funding can be programmed accordingly. 5.4 Discussion of Maintenance and Repair of Rockfall Fence Systems Based on the survey results from the manufacturers, there does not appear to be a standard maintenance and repair pro- tocol for rockfall fence systems. There are many issues and challenges surrounding a set guideline or protocol for a fence system including but not limited to the following: • Rockfall fence systems are proprietary and have specific manufacturer designed elements such as braking systems, posts, wire rope anchors, and so forth, that if damaged or broken, would require the manufacturer to provide feed- back or specialty parts for a particular system. • Specific elements of propriety rockfall fence systems may be extremely critical to the manufacturers’ tested system whereas similar elements on a competitor’s system may not be as critical for a given system. • Agencies may have spare rockfall fence parts at their dis- posal and may choose to replace a damaged panel but then would be modifying the manufacturer’s system. This may or may not be an issue with an agency maintaining the rockfall fence systems. Overall, the maintenance of rockfall fence systems needs to be performed on a case-by-case basis, because it will be neces- sary for the reviewers to evaluate the need to repair and main- tain a system rather than attempt to generalize all rockfall damage into one set of pre-determined protocols. An agency will need to assess the amount of repair or replacement nec- essary and how it aligns with the asset management plan.

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TRB's National Cooperative Highway Research Program (NCHRP) Report 823: Guidelines for Certification and Management of Flexible Rockfall Protection Systems provides advice on rockfall fence systems for transportation agencies. It also outlines data that are needed to evaluate the results of rockfall fence systems tested using the procedure recommended for acceptance. Finally, the report presents guidelines for asset management for rockfall fence systems to assist transportation agencies in incorporating these systems into existing transportation asset management plans.

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