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41 Introduction to the Recommended Practice The Recommended Practice in AASHTO format developed during the execution of NCHRP Project 10-86 is presented in this report with commentary as Appendix A. Worked examples of the Recommended Practice are presented in Appendix B to aid designers in the use of the Recommended Practice. Appen- dix B is not published herein but is available on the NCHRP Project 10-86 webpage at www.trb.org. 5.1 Overview The evaluation and selection of suitable, and cost-effective drainage pipe systems for highway projects involves consider- ation of a range of engineering suitability criteria, installa- tion requirements, and construction and post-construction maintenance costs. The availability of a streamlined, rational, and reliable design approach that identifies a wide range of appropriate pipe system alternatives on a consistent and unbiased basis would allow owners and agencies to take advan- tage of increased product competition with lower overall costs for procuring highway drainage systems. In addition, if such an alternative drainage pipe design and selection system also took serviceable life and durability into account, it would allow the appropriate pipe systems to be matched to the func- tional requirements of the highway, resulting in improved drainage system performance and lower long term mainte- nance costs. This Recommended Practice aims to achieve these objectives. By delivering a consistent and technically sound design and selection process for drainage pipe systems this Recommended Practice also provides agencies the ability to systematically track bid selections and drainage pipe system inventories and performance records for input into asset management systems. Additionally, as agencies systematically track design evaluations and compare them over time to actual in-service performance, it will provide the opportunity to continually improve the state of knowledge regarding service life predic- tion and evaluation methods. This Recommended Practice is intended to guide agencies and industry in implementing a performance-based process for contractor selection and delivery of drainage pipe systems on highway construction projects. The Recommended Practice provides guidelines and procedures for (1) agency definition of drainage requirements and (2) contractor bidding of drain- age pipe systems to meet those requirements. 5.2 Scope of the Recommended Practice This Recommended Practice presents a methodology to guide transportation agencies in implementing a performance- based process for selecting alternative drainage pipe systems on highway construction projects and is intended for use by transportation agencies, design consultants, and contractors. The Recommended Practice is intended to provide a system- atic, rational, comprehensive, and technically sound process for the evaluation of alternative highway drainage pipe systems, which includes the pipe dimensions, material and joints, bed- ding, embedment, and backfill. The Recommended Practice uses recognized methods for pipe system selection, design, and post-construction acceptance based on performance-based criteria including hydraulics, structural capacity, durability, and environmental compatibility. The Recommended Practice also provides guidance for post- installation inspection and agency acceptance of drainage pipe systems. The Recommended Practice is not intended to provide specific guidance for every potential design decision that may arise during a drainage project. Instead, the intent is to pro- vide guidance and recommendations for evaluating suitable alternatives for the majority of routine highway drainage applications. The Recommended Practice is intended to be as inclusive and flexible as possible so as to address specific agency needs and requirements. Agency-specific regulatory policies and C H A P T E R 5
42 practices can be considered within the framework of this Recommended Practice. The Recommended Practice indicates which related design issues are not inherently addressed, so that these issues may be addressed outside of this methodology. The Recommended Practice is applicable to circular, elliptical and arch-shaped culverts and storm sewers where a number of alternative pipe systems are readily available for selection as suitable alter- natives. Box culverts, large span structures, and pressurized pipes are not specifically addressed or intended to be evaluated through the Recommended Practice. 5.3 Summary of the Recommended Practice The Recommended Practice is intended to be transparent with all inputs, methodologies, and evaluation results clearly defined and presented. The process recommends undertaking evaluations using each agencyâs full inventory of pipe systems, including incorporation of available variations in installation type and backfill material and compaction. Pipe systems are technically evaluated as to their suitability in each of three main design functions: hydraulic, structural, and durability. The Recommended Practice recommends evaluating the widest practical range of drainage pipe system options against the system performance requirements for each highway drain- age application. This decreases the potential for bias in the selection of pipe system alternatives included in the bid docu- ments. An inventory of available pipe systems within a juris- diction may not currently be available and may have to be developed by the agency. The Recommended Practice should be applied to each drainage application individually, so that site specific condi- tions affecting the performance and projected service life can be adequately considered. The Recommended Practice is intended to promote techni- cal evaluation of entire pipe systems as opposed to separately evaluating pipe system components. This allows acceptable combinations of backfill material, joint type, installation criteria, pipe linings, and so forth to be considered as sepa- rate alternatives. This may require agencies to develop a wider range of specifications to cover the construction aspects of these variations. The Recommended Practice is intended to be flexible to account for individual state policies and procedures as well as potential future changes in policy, regulation, or availability of new pipe products and evaluation methods. The Recommended Practice follows a systematic five-phase approach to evaluating, bidding, inspecting, and tracking alternative drainage pipe systems. The five phases (identified numerically) each consist of multiple steps (identified alpha- betically), as illustrated in Figure 22: Phase 1âData Gathering and Project Definition Phase 2âTechnical Evaluation Phase 3âFinal Design and Policy Checks Phase 4âReporting Results and Incorporating Alternatives into Bid Documents Phase 5âConstruction Quality Control, Inventory Manage- ment, and Performance Feedback, with these latter compo- nents forming part of a highway drainage asset management system. The Recommended Practice promotes the implementation of a thorough and inclusive performance-based evaluation process that considers all technically suitable alternatives for a given highway drainage application, leaving economic judg- ment of the most cost-effective suitable alternative to be deter- mined through competitive bid. Phase 1 Project Definition Phase 2 Technical Evaluation Phase 3 Final Checks Roadway & Geometrical Hydrology & Baseline Hydraulics Geotechnical & Environmental Additional Considerations Hydraulic Structural Durability Agency Policies Phase 4 Results & Bidding Results Matrix Bid Documents Inventory Results Phase 5 Installation, Maintenance & Tracking Material & Construction QA Post-Installation Inspection Maintenance & Tracking Final Design A B C D Performance Feedback System Feedback Figure 22. Primary steps in the Recommended Practice.
43 Successful evaluation of a large number of pipe system options, as completed during application of the Recommended Practice requires a systematic process for completing and track- ing the results of each evaluation phase. To achieve the goals of systematically and clearly presenting the large number of technical and policy evaluations involved in the Recommended Practice, a matrix approach was devel- oped to track and report the pipe system selection process. The matrix approach consists of all pipe system types com- piled into rows, with circular equivalent pipe sizes listed in columns. Three individual matrices for each of the technical evaluation steps: hydraulic (âHâ), structural (âSâ), and dura- bility (âDâ) are constructed and serve as the pallet for com- pleting the individual steps of the Recommended Practice. A composite matrix with four sub-cells for each pipe system type and size combining the three technical evaluation steps with the final design and policy check (âFâ) stage is then used to create the overall Recommended Practice results matrix. Schematics of the individual and overall composite results matrix are shown in Figures 23 and 24. This matrix format illustrates the results from each step in the systematic process with a âgoâ or âno-goâ decision for each Notes: 1. H denotes hydraulic; S denotes structural; and D denotes durability. 2. Cells highlighted in green pass the relevant design check. 3. Cells with an âXâ and highlighted in red fail the relevant design check. Hydraulic Structural Durability Figure 23. Individual results matrices for technical evaluations.
44 pipe option. Matrix cells for which that type and size of pipe are not available are left blank to indicate lack of availability as the reason for elimination of that option. In addition to the systematic advantages of the matrix approach, the visual presentation of the evaluation results from adjacent sizes in adjacent columns and for similar pipe system types in adjacent rows allows for rapid visual assessment of trends in the presented results. The ability to perform a visual review of trends in the results provides significant advantage for identifying errors in calculation or transcription in the Rec- ommended Practice process and also identifying gaps or areas of improvement in technical methods and/or agency policies. While not required as part of implementation, the Recom- mended Practice is intended to facilitate database tracking of an in-service drainage pipe system inventory to allow for more systematic and efficient maintenance, renewal, and replace- ments in line with the goals of the Second Strategic Highway Research Program (SHRP2) and other ongoing AASHTO and agency initiatives. The Recommended Practice could also aid in tracking drainage pipe system failures, failure causation mechanisms, and achieved in-situ service life values across each adopting agencyâsâ drainage pipe system inventory. For example, if a specific culvert fails within 15 years of installation, it could be back-analyzed using the Recommended Practice to confirm that that specific pipe should not have been selected for that site and application. The tracking of failures and the loss of service life mechanisms, in combination with the tracking Notes: 1. H denotes hydraulic; S denotes structural; D denotes durability; and F denotes final design check. 2. Cells highlighted in green pass the relevant design check. 3. Cells with an âXâ and highlighted in red fail the relevant design check. 4. Pipe system options that pass all four technical evaluations (with the H, S, D, and F cells all highlighted in green) represent pipe systems that are acceptable design options. A yellow border is used to further highlight and identify acceptable design options. Figure 24. Overall composite results matrix for the Recommended Practice.
45 of estimated and actual material service lives will allow for research and data-mining to improve and calibrate existing culvert design methods through feedback loops within the Recommended Practice. Specifically, the developing research topics of service life prediction and failure modes can be significantly improved through the tracking and sharing of actual drainage pipe system service life data across and within AASHTO agencies. The Recommended Practice could also form the basis to simplify and facilitate a highway drainage asset manage- ment system by tracking and integrating system feedback based on the transparent processes included in the Recom- mended Practice. The independent parallel assessment for each functional/technical category used in the Recommended Practice allows the designer to observe why a pipe system was determined to be unsuitable. Regular agency review of techni- cal evaluation results and trends in bidding and field perfor- mance are encouraged for incorporation into agency policy reviews and updates. 5.4 Recommended Use The selection and design of drainage pipe systems for use in transportation projects depends on both economic and technical considerations. Individual agencies currently develop and maintain independent policies to guide the design, bidding, post-construction inspection, and long term asset manage- ment of highway drainage pipe systems. This Recommended Practice is intended to provide a national AASHTO standard for agency implementation of drainage pipe system evaluation and alternative bidding to foster greater harmonization and standardization across AASHTO agencies. With implementa- tion, it should serve to reduce costs through more efficient design, identification of cost-effective solutions, and increased local competition between contractors and suppliers. It should also encourage the development of better pipe products and the formation of a national marketplace for drainage system pric- ing as policies become more nationally standardized. The current functionality of the Recommended Practice is that it selects pipes that have an EMSL that is longer than the desired DSL. However, as EMSL methods improve, the Recommended Practice could facilitate application of a life cycle costing approach to pipe selection whereby even pipes with EMSLs less than the DSL could be selected provided their service life could be extended by in-situ remediation. This would allow even more options to be considered and could facilitate a âstaged-constructionâ approach to drainage design. Traditionally, transportation agencies have used a âmeans and methodsâ approach for selection and specification of products such as drainage pipe systems. In this approach, the agencies specify a particular drainage pipe system during the design process and the cost of the specified system is included in the contractorsâ bids for the project. This system often restricts or impedes competition by eliminating many technically suit- able alternatives. The inclusion of multiple equivalent options during the bid phase of projects has been shown to reduce costs through increased competition. This Recommended Practice presents a methodology to guide transportation agencies in implementing a performance- based process for evaluating alternative drainage pipe systems with the intent to increase competition and reduce costs while maintaining safety and performance standards. The Recom- mended Practice contains elements to guide development of a holistic program that would allow for systematic inventory management and tracking of results that could improve service life predictions and lead to better management of highway drainage assets. The Recommended Practice applies rational performance- based criteria to the selection of pipe systems. It is not intended to be a stand-alone design document, but rather a design guid- ance and process framework when used in conjunction with other resources including AASHTO LRFD, FHWA Hydraulic Design procedures, and agency policies and design manuals. This methodology promotes the implementation of the lat- est national standards and other state-of-the-practice design evaluation methodologies with the intent of being as compre- hensive as possible while also allowing the flexibility to incor- porate agency-specific standards or requirements. The matrix approach developed for technical evaluations within the Rec- ommended Practice is intended to provide clarity of design decisions and to allow for data tracking and mining for future agency use or for research to improve policies and methods. The Recommended Practice methodology presents a sim- plified systematic process for identifying drainage pipe systems for a specific defined application based on the application of hydrological, hydraulic, structural and durability principles. However, it is expected that the Recommended Practice be applied only by engineers experienced in drainage pipe design principles and that the use of the Recommended Practice will not eliminate the need for the results to be reviewed and checked by a drainage design engineer. The Recommended Practice incorporates a final design check step to allow for more detailed analyses, where necessary, beyond the basic evaluations and to allow for agency- or project-specific provisions to be applied. The Recommended Practice addresses the design of circular and standard elliptical and closed arch (i.e., pipe arch) drainage elements. Large and special design drainage pipe systems such as box culverts, large span open bottom arches, pressure pipes, and so forth are not directly addressed or incorporated. Above all, the Recommended Practice is intended as a streamlined process for the design of routine highway culvert and storm sewer systems.