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36 Summary of Gaps in Knowledge and Practice This chapter summarizes noted gaps in the current state of practice related to the design and subsequent bidding of gravity drainage pipes for roadway applications. NCHRP Project 10-86 was tasked with identifying gaps that had the potential to substantially affect the development and implementation of the Recommended Practice. The techniques used to identify critical gaps included the following: â¢ Consideration of the current knowledge gaps identified in the draft report of NCHRP Project 20-07 (White and Hurd 2011). These relate to the definition and selection of DSL, lack of a comprehensive quantitative model for predicting pipe service life, and lack of an approach to defining levels of joint performance. â¢ Review of literature including state practice guidelines and specifications with regard to DSL assessment for pipe selec- tion and the data currently available for assessing long term pipe failure mechanisms, and the rates of deterioration. â¢ Use of a survey of state DOTs (state drainage and material engineers) as outlined in Chapter 3 to solicit, amongst other things, broader feedback on the main concerns with pre- dicting pipe service life and any recurring problems identi- fied with premature pipe failures. â¢ Discussions with state agencies, the Province of Ontario, and amongst the research team of technical resources (e.g., with Dr. Ian Moore) to determine limitations and shortcomings of existing drainage pipe design and selection procedures. Gaps in practice can typically be grouped into one of the following categories: â¢ Knowledge Gapsâareas where there is a deficiency in design theory/methodologies or performance data to support rigorous and robust design and performance decisions. â¢ Implementation Gapsâareas where there is clear and technically valid information (design method, performance data, etc.) to support use of a design method, evaluation criteria, pipe product, and so forth, but implementation has not yet been instituted. 4.1 Knowledge Gaps Knowledge gaps are areas where the state of knowledge has not reached maturity and/or consensus has not been reached on the appropriate approach to a given design problem or in the evaluation of a particular aspect of performance. To date, the following critical knowledge gaps have been identified that impact the design and bidding of drainage pipe systems: â¢ Standardization of DSLâStandard (universal) and objec- tive guidelines for defining service life requirements for various drainage pipe system applications are not defined in AASHTO. â¢ Service Life Prediction and Evaluation (Durability)â the prediction and evaluation of drainage pipe system (pipe material, backfill, etc.) service life is a complex pro- cess involving the evaluation of chemical (corrosion) and mechanical (abrasion) resistance (material properties) and loading (service conditions). â¢ Time-Dependent Performance Dataâin general there is a lack of statistical data of long term field performance for the full range of drainage system and service conditions. â¢ Pipe Joint Evaluationâthe evaluation of structural and hydraulic performance impacts from various pipe joint systems results in both knowledge and implementation gaps. â¢ Installation Qualityâa clear and universally accepted methodology to quantify the impacts of installation quality on drainage system performance is not known to exist, and sufficient performance data to generate such an evaluation system may not exist for all pipe systems and installation conditions. It is noted that the hydraulic and structural design of new (virgin) drainage pipe systems is generally well understood C H A P T E R 4
37 and the methodologies presented in reference documents (e.g., AASHTO LRFD Bridge Specifications; Chapter 12) are accepted as appropriate. However, while knowledge gaps related to these functions do not exist, implementation gaps related to these basic design functions do exist as detailed below. 4.1.1 Standardization of DSL Establishing the minimum required life at an adequate level of service for a pipe system is a necessary guideline to con- sider for selecting alternative pipe systems. This issue recog- nizes that different pipe types will deliver different service lives under defined conditions, but that not all highway applications require the same service life or level of service. While some DOTs have guidelines on defining DSL, there currently is not a standard approach for this process. There are a number of different aspects to this. On a simple level, most agencies relate DSL to the highway classification or the strategic importance of the route. Thus design service lives of 25, 50, 75 or 100 years can be assigned. Other factors that need to be considered are the ease of replacement of a particular pipe system. For example, if a cross culvert is at the base of a high rockfill embankment, and replacement would require the construction of a tempo- rary detour, the DSL may need to be increased irrespective of the road classification. The research team is not aware of any comprehensive life cycle costing studies done on the differential between a 25-year pipe design and a 75-year pipe design. 188.8.131.52 Future Considerations The development of a standardized set of objective guide- lines for use in setting DSL requirements would likely serve to benefit the industry and may warrant consideration by AASHTO. 4.1.2 Service Life Prediction and Evaluation (Durability) The potential for changes in system material properties (pipes and surrounding materials) over time (durability) serve to impact structural and hydraulic performance of drainage systems. Additionally, the durability of system components is impacted by a range of chemical and mechanical loading processes that typically fall along the periphery of drainage system designer knowledge and expertise. As such, it is under- standable that this area is the least mature and most variable with regard to available design methods, compilations of field and laboratory performance data, and integration in practice. The process is further complicated in practice by the fact that most prediction models assume the pipe system is correctly installed and are invalid if this is not the case. The knowledge gaps related to pipe durability are well known as reported in a wide range of reference documents including MTO (2007) and NCHRP Synthesis 254 (1998) (NCHRP Synthesis 474 is an update to NCHRP Synthesis 254 that will be published in spring 2015). There is also significant ongoing research at universities, within state DOTs, and by pipe manufacturers and trade associations aimed at improving the knowledge base regarding durability. 184.108.40.206 Future Considerations The Recommended Practice developed in this study acknowl- edges the data gaps that exist in evaluating durability and quan- tifying EMSL. Because the state of knowledge in this area is rapidly changing and likely will continue to change and adapt as new and improved materials, construction techniques, and post-installation verification techniques are implemented, the path forward is to ensure that the developed practice includes the following: â¢ Clear definitions and typical ranges for critical properties (pH, resistivity, sulfide concentrations, chloride concentra- tions, bedloading, etc.). â¢ Inclusion of a range of current and accepted methods for evaluating EMSL with a discussion of limitations and applicability. â¢ Flexibility to allow state specific and/or new developments in evaluation methods to be easily implemented into the process. â¢ Suggested methods for measuring relevant properties/ parameters. 4.1.3 Time-Dependent Performance Data In general, there is a need for additional evaluations of time-dependent performance data on all drainage systems. Drainage systems and pipe products that have longer histo- ries have significantly more data available, but often these collections of data are potentially biased as a result of being presented by industry trade organizations and/or they do not cover the full range of installation conditions. For newer pipe products and systems the need for evaluation and unbiased compilation of performance data is significantly greater and leads to the exclusion of newer pipe products in some jurisdictions. 220.127.116.11 Future Considerations The need for continued and additional studies to collect and analyze drainage system performance data is well known. For this project, the developed Recommended Practice relies on available studies and a flexible framework intended to allow incorporation of changes in performance criteria and data as new information is published.
38 4.1.4 Pipe Joints The responsibility to provide information regarding the impact of pipe joint systems typically falls on the manufac- turer. While some pipe systems and products provide a full range of information to allow the adequate incorporation of joint performance into design, this is not universal amongst available pipe products and joint systems. Additionally, the performance of many joint systems is strongly dependent on the quality of installation (i.e., proper vs. improper installation) and the performance of improperly installed joints is in general not well documented or quantifiable. Instances where joint performance data and/or evaluation tools are not available in the literature (even if they are avail- able internally within pipe manufacturerâs literature) are considered knowledge gaps in the current study. The knowledge gaps related to pipe joint systems are evi- dent by the proportionally large percentage of failures (or other service impacts) that are related to pipe joints. Joints have the potential to impact both the hydraulic and the structural performance of the pipe material, and to further impact the performance of the pipe system through leaks that can lead to degradation or erosion of bedding and embedment mate- rials. Infiltration of soil particles into pipes can also cause an increase in abrasion. 18.104.22.168 Future Considerations The following are potential future considerations related to the existing knowledge and data gaps in evaluating joint performance: â¢ Pipe manufacturers should be encouraged (and/or required) to provide technical information regarding the impacts on hydraulic (Manningâs n) and structural (impact on fill height tables) performance (if any) for each joint system, assuming proper installation is followed. â¢ As it is likely impractical to fully and accurately quantify the impacts of poor or improper joint installation on perfor- mance, the recommended path forward is for state agencies and other owners and their representatives to institute and require the following: â Clear specification of joint requirements in contract documents â Contractor pre-qualifications regarding experience installing various joint systems â Development and implementation of adequate inspec- tion protocols to provide greater assurance of high quality and proper joint installations. The structural design of joints to withstand variations in construction, support, and loading conditions is the topic of NCHRP Project 15-38. The results of that project will improve the state of knowledge on this topic. 4.2 Implementation Gaps or Inconsistencies 4.2.1 Introduction Implementation gaps are areas where typical practice does not consistently follow known best practices and/or regula- tory requirements (i.e., Code of Federal Regulations (CFR), AASHTO design specifications, and so forth). The following implementation gaps have been identified to date that are anticipated to impact the alternative bidding of drainage systems: â¢ Variations in hydraulic design criteria â¢ Out of date/inconsistent fill height tables â¢ Site specific consideration of durability â¢ Consistent and timely evaluation of new pipe products â¢ Unwarranted exclusion of pipe systems (historical or other bias) 4.2.2 Variations in Hydraulic Design Criteria 22.214.171.124 Variations in Manningâs n Value Recommendations The Manningâs equation is an empirical relationship com- monly used to calculate barrel friction losses in pipe system and design. The Manningâs n value is based on either hydraulic test results or resistance values calculated using a theoreti- cal equation such as the Darcy equation and converting to a Manningâs n. The use of the Manningâs equation for culvert design is the predominant means of evaluating the hydraulic adequacy of various pipe materials for a given drainage appli- cation used in practice. Section 126.96.36.199 presented summary plots showing the ranges of Manningâs n values across several pipe types for the references reviewed. The observed range in values indicates that Manningâs n values are not standardized across the practice and that this represents an implementation inconsistency in current practice. 188.8.131.52 Changes in Hydraulic Performance Over Time Abrasion, corrosion, and bio-sliming (i.e., accumulation) of pipe materials are known to potentially influence the hydraulic performance of pipes over their service life. However, as identi- fied in the review of hydraulic design practice, clear methods to evaluate and incorporate this potential change in performance over time do not exist. As expected in an area without a clear standardized evaluation method, typical practice regarding incorporation of these factors is quite variable. A relatively wide range of Manningâs n values is recom- mended for use in hydraulic design and adequacy evaluations
39 (as summarized in Figures 2 through 8) that vary over a wide range of risk tolerances and do not typically consider the expected length of service. Practice ranges widely, from agencies recommending no change in Manningâs n from the measured virgin material values, to others that recommend potentially conservative upper end values that would only occur in service through significant material changes over the service life of the drainage element. The relevance of these service life changes in Manningâs n need to be evaluated further to establish whether indeed they need to be considered in an alternative pipe selec- tion process or whether they would have no material impact on the outcomes. 184.108.40.206 Future Considerations Additional research and review of available service data would be beneficial to the design and adequate evaluation of time-dependent impacts on hydraulic performance. If avail- able, information could be requested from manufacturers; however, it is unlikely that this information would be available for all pipe types or that a consistent methodology would have been used in evaluating performance. 4.2.3 Out of Date/Inconsistent Fill Height Tables As summarized in Section 220.127.116.11, typical practice for the structural evaluation of drainage systems is to use fill height tables to screen combinations of pipes and installa- tion con ditions for adequacy based on the known loading conditions. This approach is technically valid, is quick, and is not subject to significant errors because the use of such tables greatly simplifies what can be complicated analyses. As such, the use of fill height tables is expected to remain the predominant method for structural evaluations of drainage systems. The inconsistency of the preparation, use, and variables for structural design and fill height tables results in an imple- mentation gap that affects the current state of the practice, and prevents national standardization and in-service tracking of structural pipe system performance. These factors combine to present a state of current practice that contains significant variation and bias in the evaluation of structural capacity of highway drainage elements, and that prevents national standardization and direct comparison of performance. 18.104.22.168 Future Considerations The research team worked with the pipe industry rep- resentatives to develop nationally standardized structural fill height tables across all pipe types, but was unsuccessful in accomplishing that task fully within the project time con- straints. The development of nationally standardized baseline fill height tables based on the AASHTO LRFD code and a set of clear and transparent design criteria would likely be of benefit and save resources spent repeating the design processes across all agencies in the United States. Following the develop- ment and distribution of such standard tables, state agencies could then consider variations from the standard values for state specific or design specific conditions (e.g., increased fac- tors of safety for critical structures and variations in available bedding classes) at reduced effort and cost. It is the teamâs opinion that the development of standardized fill height tables is a key step in providing for equitable com- parison and evaluation of highway drainage elements and would be of great value to the technical community. Addi- tionally, it would provide a clear baseline of standard practice and methodology for new pipe products, which would help ease integration of new products into practice. It is noted that TRB Standing Committee AFF70 identified as one of the key research needs to standardize structural design methods for alternate pipe materials based on equivalent risk factors (TRB 2009). The research statement and objective from that identified research need reads as follows: âThe AASHTO LRFD Bridge Design Specification use different load and resistance factors for the design of pipes made from dif- ferent materials. Seemingly, this practice results in different factors of safety between designs of the concrete, metal and plastic pipes involved. In some instances, the apparent safety factors appear to range from roughly 1.2 to 5.0. This is a historic practice that may or may not be justified by the loads, known strength of the pipe materials and installed backfill soils involved. Load factor evaluations need to account for current day inspec- tion controls, deflection testing, etc. Effective means of offsetting likely variations in design assumptions need to be identified, evaluated and, where appropriate, included in future specifications. Resistance factors are to be developed on the basis of inconsistencies that may actually occur within the pipe and backfill materials as specified as well as the accountability of the materials involved.â âEvaluate the basis of the various AASHTO pipe design specifi- cations (methods), their degree of technical substantiation as well as their dependency on (any) differing loading assumptions, the effect of variations in specified backfill materials and the dependency of the pipeâs performance on contractor workmanship. Develop design methods with equivalent risk factors for pipes of alternate materials. Appropriate soil and live load design assump- tions as well as soil support, shape control and pipe material strength variations are significant factors.â 4.2.4 Site Specific Consideration of Durability As introduced and summarized in Section 3.4.3, the AASHTO LRFD Bridge Design Manual requires that the two main mechanisms of durability (corrosion and abrasion) be considered in design highway drainage systems. While multiple methods for calculating site and pipe system specific EMSL val- ues exist (see Section 3.4.3 and Appendix C), a survey of North
40 American DOTs completed during NCHRP Synthesis 474: Service Life of Culverts (an update to NCHRP Synthesis 254 that will be published in spring 2015) indicated the following state of practice trends show an apparent implementation gap in applying site specific durability evaluations. â¢ Assumed agency-wide values are still the predominant method for estimating EMSL during design of all pipe types. Quantitative methods are more commonly used for pipes with a longer history of use (concrete and metal), and are more rare for pipe materials with a shorter history of use. â¢ For agencies that complete quantitative EMSL evaluations, corrosion and abrasion were the most common factors con- sidered, followed by settlement and stress cracking, and other factors were generally not considered. â¢ The tools and aids used to calculate EMSL typically include some combination of assumed values, agency-specific data, and industry supplied data. Software programs are still relatively infrequently used to predict EMSL values. â¢ There is near universal application of assumed values of EMSL for all pipes other than concrete and metal, which is believed to primarily result from the limited methodologies to complete project specific evaluations of MSL for thermo- plastic and other non-concrete/metal pipe types. â¢ One-third of agencies reported maintaining maps indicating regions of environmentally aggressive conditions. 22.214.171.124 Future Considerations The Recommended Practice developed acknowledges that knowledge and implementation gaps exist in evaluating dura- bility and quantifying EMSL. Because the state of knowledge in this area is rapidly changing and likely will continue to change and adapt as new and improved materials, construc- tion techniques, and post-installation verification techniques are implemented, the suggested path forward is to ensure that practice moves toward more widely including the following: â¢ Continuation of the increasing trend to collect site specific environmental and geotechnical data from the native soil, backfill, flow, and groundwater data necessary to implement site specific durability methods. â¢ Consideration by AASHTO to include a range of current and accepted methods for evaluating EMSL with a discussion of limitations and applicability. â¢ Continued information sharing amongst state DOTs with successful and robust site specific durability evaluation practices. â¢ Continuation of the significant on-going research related to the durability assessment of thermoplastic and other newer pipes types. 4.2.5 Consistent and Timely Evaluation of New Pipe Products This gap is typically temporary and exists from the time of new product development until sufficient product information and performance data exist to encourage implementation in practice. However, the timing to reach maturity and the requirements to reach acceptance are often highly variable across state agencies (often related to the local need and com- petitiveness of each new product). While temporary implementation gaps are necessary and important through the final development and implementa- tion of new products, a unified, consistent and clear system of requirements to allow new products to gain approval status (or to be determined to be unsuitable for widespread use) with federal and state agencies would be beneficial to the pipe manu- facturing industry and would take significant burden off indi- vidual states to have to conduct individual (often repetitive) state specific review and approvals of all new products. New products can represent improved performance and cost savings where they can be introduced into use for appropriate applications. Based on qualitative historic evidence in the piping and other industries, the adoption of an open and clear system applicable on a national level for evaluation (approval/rejection) of new or updated pipe products would increase competition and be beneficial to both industry and state agencies. 126.96.36.199 Future Considerations The most practical solution to this challenge would be the greater use and adoption of the National Technical Product Evaluation Protocol (NTPEP) process or another similar nationally coordinated process to provide a common and clear framework for independently evaluating new pipe products. NTPEP evaluations are often completed in conjunction with specific state DOTs to run field trials under agreed protocols. 4.2.6 Unwarranted Exclusion of Pipe Systems (Historical or other Bias) One of the main objectives of this study is to provide a framework that allows for the reduction or elimination of bias in the bidding and design of drainage systems. It is the intent of the research team that this known implementation gap will be improved through development and implementation of the Recommended Practice.