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75 Future and Parallel Research Through the course of NCHRP Project 10-86, it became clear that an increased level of standardization of specific components of highway drainage systems would simplify the implementation of national design and bidding standards including the Recommended Practice. In particular, addressing the following issues would improve the current practice as it relates to bidding alternative drainage systems: â¢ Standardized backfill and installation requirements â¢ Standardized DSL requirements â¢ National pipe system inventory â¢ Standardized structural design elements and criteria Each of these topics is described in the following subsections. They are also the basis for future consideration by the highway drainage community. 15.1 Standardized Backfill and Installation Requirements Highly variable backfill standards exist across North Ameri- can transportation agencies. In addition, AASHTO standards include different pipe system backfill classifications and design bases for different pipe material types (concrete, metal, and plastic). Note that the ASTM standards are more streamlined (but vastly different) in this area. Specification of alternative backfills is thus both agency and pipe-type specific, which complicates comparison and evaluation of multiple backfill options during alternative or optional bidding. One possible approach to standardizing backfill requirements is to base the specification partly on a stiffness modulus value that can be measured in-situ and that has been shown through research and experience to produce a satisfactory installation. Due to the different ways that flexible and rigid pipes interact with embedment materials, different modulus values would be expected for the different pipe types. Modulus values should be based on the expected pipe system performance over the design life of the installation. 15.2 Standardized DSL Requirements Lack of nationally accepted DSL requirements for each roadway classification and variable definitions and bases for determining DSL mean that pipe suppliers and contractors have to accommodate a diverse range of requirements. This hampers the optimization and standardization of products. The development of standard national definitions for DSL would likely benefit the industry and increase competition by creating a more consistent national market. This would also encourage pipe manufacturers and suppliers to market their products on the basis of service life delivery and to improve products to meet agency defined design lives. 15.3 National Pipe System Inventory A review of new product evaluation procedures currently used by state DOTs was recently completed (White and Hurd 2011). It also presented a recommendation for a three-phase approach for determining if a new product submitted by a vendor is acceptable. This New Product Evaluation (NPE) Protocol comprises an initial evaluation, evaluation of previous performance, and field and laboratory testing. This document could be used as a basis for developing a standard new pipe product evaluation process and guidance on compiling and maintaining lists of all acceptable pipe products. A key com- ponent of a pipe system evaluation process is previous field performance, and there is currently no mechanism to allow this information to be compiled and shared between agencies. The absence of a recognized national pipe inventory and standard acceptance criteria limits the formation of a ânational marketâ and requires agency-specific acceptance of new prod- ucts. Greater use of AASHTOâs NTPEP or other similar pro- grams could greatly benefit the development of more national product acceptance. Use of an approval process defined in terms of component materialâs characterization, controlled field trials, and monitored performance on trial highway projects C H A P T E R 1 5
76 would encourage greater product innovation because there would be more certainty as to what was needed to gain accep- tance in the marketplace. 15.4 Standardized Structural Design Elements and Criteria Increased standardization across structural design elements (most specifically across backfill criteria) could greatly simplify and standardize structural evaluations of pipe systems. Several key elements for consideration in greater standardization are highlighted below. â¢ Performing calculations in accordance with AASHTO LRFD Bridge Design Specifications is a technically involved task and the lack of universal fill height tables is a barrier to implementing a nationally standardized framework for alternative pipe bidding. Alternative bidding can move forward on an agency-by-agency basis but standardization of the AASHTO code (potentially going as far as providing baseline standard fill height tables within the AASHTO code if input criteria become more standardized) would simplify development and implementation of the Recommended Practice. â¢ With no widespread adoption of standardized input param- eters, fill height tables are often not compatible with equi- table pipe system selection. â¢ The thermoplastic pipe industry has not standardized the structural classification of their products with respect to LRFD structural design. Structural capacity is currently manufacturer-specific and the lack of target specification classes creates confusion in the design and specification of these products. This lack of meaningful pipe classes is believed by the research team to be a component that is potentially reducing the acceptance and wider use of thermoplastic pipes. It is noted that discussions in the thermoplastic pipe industry have occurred on this topic but currently have not resulted in the development of meaningful structural classifications within the LRFD design framework. 15.5 Links to Other Research and Identified Research Needs Widespread adoption of a national alternative bidding approach for pipe drainage systems would coordinate well with other identified research needs across the industry. In partic- ular, two TRB committees (AFS40, Subsurface Soil-Structure Interaction; and AFF70, Culverts and Hydraulic Structures) have identified a number of research needs directly related to issues identified during the course of NCHRP Project 10-86. 15.5.1 Design of Bedding Thickness and Backfill Envelopes for Underground Structures This research need highlights the fact that a number of pipe materials and backfills have recently proliferated without a corresponding update of backfill requirements. Further eval- uation of the influence of installation procedure and bedding conditions on the backfill properties that affect structural performance is needed. Adopting the Recommended Practice would allow for a range of backfill options to be specified in conjunction with different pipe materials. Tracking and synthe- sizing the performance of these pipe-backfill systems would be facilitated by having a standardized framework, such as the NCHRP Project 10-86 Recommended Practice. 15.5.2 Modulus-Based Quality Control in Culvert Backfill Installation There is a need for an improvement in how backfill quality is measured and how specifications can be revised to reflect new technologies available for measuring soil modulus. The primary objectives of this identified need are as follows: â¢ Correlate field gage soil modulus measurements to tabulated modulus values based on soil type coupled with Proctor density. â¢ Investigate usefulness of soil modulus gages in the market- place for assessing culvert backfill properties. â¢ Collect soil modulus data for a range of soil, culvert, and con- struction methods to assess modulus values and variability. â¢ Develop recommendations for establishing compaction target values. â¢ Develop a standard test method for soil modulus for incor- poration into AASHTO specifications. A standardized framework such as the Recommended Practice would allow for easier and faster implementation of ongoing or new research, such as modulus-based quality control of backfill. With improved methods of backfill quality control some of the agency concerns about dealing with large numbers of pipe system options would be alleviated. 15.5.3 Long Term Performance of Buried Pipe Systems There is a need to develop a basis for evaluation of the projected longevity of the large number of buried pipes cur- rently installed. There is an associated need to incorporate long term performance considerations in the design of future pipe installations. These factors directly relate to improving the way that culvert pipes are currently designed and specified as well as installed and maintained.
77 This project has identified that there is considerable un- certainty in existing methods used to estimate material service life, and that methods do not exist for all pipe types. Having a standardized framework, such as the Recommended Practice, would allow new methods to achieve widespread adoption in practice. A great deal of guidance has been developed on culvert durability, inspection procedures, and rating systems. This information has been used by DOTs as well as local agencies to estimate service life and level of deterioration of their drain- age infrastructure. In addition, a number of highway agen- cies have implemented culvert inspection programs, which incorporate formalized inspection scheduling and documen- tation. Most of these programs, however, are not tailored to a national audience, are not comprehensive in addressing all structural, hydraulic, geotechnical, and environmental issues, and do not produce condition and performance data compat- ible with culvert management strategies and systems. In conjunction with this effort, there is a critical need for a comprehensive, state-of-the-art computerized tool for docu- menting and managing culvert and storm drain facilities once they are identified, evaluated, and rated. A drainage asset man- agement system would serve as a database for culverts and storm drain pipe inventories and assist with recording locations, tracking evaluations of condition and performance, scheduling inspec- tion and maintenance activities, and selecting and budgeting rehabilitation and replacement activities. The Recommended Practice, in particular if implemented through a software tool, would be a complementary tool to any future development of a system for managing culvert and storm drain inventories. Such systems could be linked to provide information on field performance of pipe systems back into the pipe selection process. Over time, this would be significantly beneficial when evaluating culvert durability. The Recommended Practice to alternative bidding com- bined with the potential implementation of a meaningful and standardized pipe system code classification could facili- tate the tracking of performance data to aid future research projects, such as those mentioned above. The Recommended Practice retains an agencyâs flexibility to manage risk for each design component as per agency preference, but makes a clear distinction between pipe eliminations made for technically justifiable reasons and those made for agency or designer preference. In summary, certain aspects of current practice result in barriers to developing nationally applicable design tools.