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6State of the Practice Summary To develop a national recommended practice for alterna- tive bidding of culverts and storm sewers, it was necessary to understand the state of gravity drainage system practice for roadway projects across U.S. DOTs. As such, part of NCHRP Project 10-86 focused on reviewing the state of the practice for design, specification, and bidding of drainage pipe systems for highway projects through a DOT survey and a literature review. The state of the practice reviews consisted of technical and policy reviews of federal standards, specifications, and guide- lines; relevant research reports; academic papers; case studies; industry literature; and the existing state of the practice across U.S. DOTs. The state of the practice reviews provided informa- tion across all aspects of gravity drainage pipe system design, selection, installation, quality control, performance, rehabili- tation, and modes of failure. There is a vast amount and range of information available on drainage systems from text books, published literature, research reports, and product information generated by pipe manu- facturers and their trade associations. The following national guidelines and specifications are intended to guide and control design on federally funded roadway projects: â¢ AASHTO Highway Drainage Guidelines (AASHTO, 2007) â¢ AASHTO LRFD bridge design specifications (AASHTO, current year) â¢ AASHTO LRFD bridge construction specifications (AASHTO, current year) â¢ AASHTO Standard Practice documents â¢ ASTM and AASHTO test and material standards â¢ Federal Highway Administration (FHWA) Hydraulic Design Series and Hydraulic Engineering Circulars â¢ FHWA Culvert Design Software, HY-8 â¢ FHWA Storm Drain Design Software, HY-12 â¢ United States Army Corps of Engineers (USACE), Hydro- logic Engineering Center River Analysis System (HEC-RAS) However, these national resources and guidelines are then integrated and modified into state guidelines, specifications, and practices. Based on the current review, state practices for drainage pipe system design, bidding, installation, inspection, maintenance, and other factors vary tremendously, with limited consistency across states on many issues. The state of U.S. DOT practice greatly influenced the direction and choices made in developing the Recommended Practice, and key elements are presented in this report to provide context. The results of the DOT survey under- taken as part of this project provided detailed information on the current state of the practice for highway drainage system design. The results and summary findings of the sur- vey, along with the results of a literature review that sup- plements the questionnaire responses, are presented in this chapter of the report. This report notably does not pre- sent a full summary of the range of state practices, and the reader is referred to the following resources for additional information: â¢ White and Hurd (2011) â¢ Taylor, C. and Jeff, M. (2012) â¢ Mitchell et al. (2005) â¢ Zhao, J. Q., et al. (1998) â¢ Gabriel and Moran (1998) (The update is NCHRP Synthe- sis 474, which will be published in spring 2015) â¢ Caltrans Design Information Bulletin (no. 83-02 (2011)) 3.1 Survey of State DOTs A survey of all state DOTs was undertaken by communi- cating with each DOTâs AASHTO Subcommittee on Materials representative or alternate designee. The intent of the DOT survey was to determine the current state of practice regard- ing the use of bidding alternative materials for drainage systems. The research team received responses from 37 state C H A P T E R 3
7 DOTs, a 74 percent return rate, as shown in Figure 1. The DOT survey questions and responses are presented in Figures Q1 through Q14 with brief commentary. It is acknowledged that in some instances the responses to the survey represent sub- jective personal knowledge of the current state of practice for a given agency, as formal tracking of various survey items is not completed by all agencies. Question 1: Does your agency have a current policy for allowing the selection of alternative pipe systems? Of the 37 responses to Question 1, 32 DOTs (86%) indicated that they do have a current policy addressing alternative pipe system selection. Only one DOT indicated that no policy was in effect. Four others indicated that some restrictions apply in accordance with their policies. 37 â Survey Completed 8 â Communicated with DOT â Survey Response Not Received 5 â No Response from DOT Figure 1. NCHRP Project 10-86 state DOT survey response graphic (Alaska and Hawaii not to scale). 32 1 4 0 5 10 15 20 25 30 35 Yes No Yes,with restrictions N um be ro fR es po ns es Figure Q1. Number of DOTs that use an alternative pipe system selection policy.
826 11 0 5 10 15 20 25 30 oNseY N um be r o f R es po ns es Figure Q2. Number of DOTs that have or are evaluating new pipe types. 1 1 2 2 4 4 3 4 6 8 8 9 21 0 5 10 15 20 25 Co m bi ne d Se w er S ys te m Se rv ic e Li fe Cr ite ria Fl oo d Zo ne /C ro ss in g Lo ad in g (T ra ï¬ c) O th er N on e Pe rm af ro st Ti da la nd Sa ltw at er Ph ys ic oc he m ic al (W at er ) W ild lif e Pa ss ag e De pt h Hy dr au lic Ph ys ic oc he m ic al (S oi l) N um be ro fR es po ns es Type of Unique Condition Figure Q3. Number of DOTs whose states have unique design conditions. Question 2: Has your agency approved (or is currently evaluating) any new or non-traditional pipe materials? Approximately 70% of DOTs have approved or were evaluating new or non-traditional pipe types. Of those DOTs, the percentages considering the following pipe types were as follows: â¢ Polypropyleneâ34% of DOTs â¢ Steel reinforced polyethyleneâ32% of DOTs â¢ Fiberglassâ12% of DOTs â¢ PVCâ12% of DOTs â¢ HDPE (7% solid wall and 3% single wall)â10% of DOTs This response confirms the active nature of the pipe supply industry and the extent to which research and new product development are ongoing. The openness of state agencies to evaluate new products is also encouraging and highlights the need to coordinate these evaluation efforts and to capture best practices across the country. Question 3: For culvert design, does your state have any unique conditions that require a special design focus (such as hydraulic or structural) above and beyond standard prac- tices (such as AASHTO recommendations)? Unique conditions could include very low pH, saltwater environments, heavy log- ging trucks, lack of stone backfill, aquatic organism passage, etc.
9 21 6 10 0 5 10 15 20 25 YES NO NO ANSWER / UNKNOWN N um be ro fR es po ns es Figure Q4a. Number of DOTs whose fill height tables are agency-specific. 8 7 7 3 9 1 0 1 2 3 4 5 6 7 8 9 10 U nk no w n LR FD R ati on al En gi ne er in g In du st ry Su pp lie d N o Re sp on se O th er N um be ro fR es po ns es Response Figure Q4b. How were agency-specific fill height tables prepared? Many of the DOTs (57%) have soil conditions that may affect the durability of pipe systems. Therefore, it was recognized that durability analyses, particularly for corrosion and abrasion, should be a key component of an alternative pipe selection pro- cess. Some of the other conditions and criteria, such as wildlife passage requirements, are left to be independently assessed by those agencies during a pipe system selection process. It was recognized that alternative pipe system bidding needed to be streamlined and to be able to handle routine designs efficiently. This response allowed the research team to evalu- ate what special design conditions could be handled within the framework and which ones would need to be addressed outside it. Question 4: If your agency utilizes fill height tables for structural design, are those tables state/agency specific? How were the tables developed? Most state DOTs utilize fill height tables that were devel- oped specifically for that agency. However, the development of those fill height tables came from a variety of sources or was unknown. The responses suggested that the use of fill height tables was universally accepted as the most practical means for structural design of typical highway drainage pipe systems. Question 5: Does your agency consider the potential impact of changes in Manningâs n values over the service life of the pipe? The majority of DOTs do not consider the potential impact on pipe capacity of Manningâs n values over the service life of
10 26 11 0 5 10 15 20 25 30 Ye s N o N um be ro fR es po ns es Figure Q6. Number of DOTs that use the concept of Design Service Life. 8 26 3 0 5 10 15 20 25 30 Ye s N o N o An sw er / U nk no w n N um be ro fR es po ns es Figure Q5. Number of DOTs that consider changes in Manningâs n values over the service life of the pipe. the pipe. Of those DOTs that do, typically culverts with natural bottoms are those that require a more detailed evaluation of appropriate roughness values in design. Additionally, some DOTs increase the Manningâs n value from the manufacturer as a factor of safety. Question 6: Does your agency currently use the concept of pipe Design Service Life? Most of the DOTs indicated they use the concept of Design Service Life (DSL) in the design of pipe systems. The DSL ranged from 25 to 100 years, primarily based on roadway classification or Average Daily Traffic (ADT) criteria. Question 7: During the design stage, does your agency consider or plan for future remediation of the pipe system? Of those DOTs that do plan for future remediation of the pipe system (including due to special conditions), the most cited example was oversizing culverts in very deep fills to allow for sliplining in the future for repair if needed. Question 8: Based on past experience, how would you rate your reliance on in-situ treatments for extending the life of drainage pipes? If in-situ treatments are routinely used, which techniques are used? Almost all the DOTs surveyed have routinely or occasion- ally used in-situ treatments for pipe system rehabilitation. Most agencies tend to have a program in place to consider options for trenchless pipe system rehabilitation. Several DOTs who occasionally used in-situ treatments were trying to avoid disruption to the public for a culvert replacement. The most common method of in-situ rehabilitation is slip- lining followed by Cured-In-Place Pipe (CIPP) and invert paving.
11 2 29 6 0 5 10 15 20 25 30 35 Ye s N o Sp ec ia l Co nd iti on s O nl y N um be ro fR es po ns es Figure Q7. Number of DOTs that plan for future remediation of the pipe system. 19 16 1 1 0 2 4 6 8 10 12 14 16 18 20 R ou ti ne ly U se d O cc as io na lly U se d N ot U se d N o A ns w er N um be r of R es po ns es Figure Q8a. Number of DOTs that routinely use in-situ treatments for pipe systems. 20% 50% 80% 40% 15% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% Sh ot cr et e/ G ro uti ng CI PP Sl ip lin in g In ve rt Pa vi ng Pa tc hi ng % of DO Ts ro uti ne ly us e Type of In-Situ Treatment Figure Q8b. Types of in-situ treatments used by DOTs.
12 Question 9: In the last 5 years, which of the following pipe types has your agency installed for highway drainage systems: concrete, steel, HDPE, PVC, other. Figure Q9 shows an average across all DOTs for the pipe types used in drainage systems. However, the responses were also examined to see how pipe type use varied by DOT. For approximately 50% of DOTs, at least half of the pipe used in drainage projects was concrete. For approximately 14% of DOTs, both HDPE and steel were used in at least one-half of the projects. Questions 10 and 11 (aâf): For rigid and flexible drainage pipe systems, what post-construction inspection methodologies does your agency use? For both rigid and flexible pipe, visual and video inspection are the most common post-installation inspection methods for pipe drainage systems. Mandrel testing is still fairly common for flexible pipe. Questions 10 and 11 (g): For rigid and flexible pipe systems, at which stage of the construction contract are the inspec- tions undertaken? 56.1% 23.1% 18.8% 2.4% 0.4% 0% 10% 20% 30% 40% 50% 60% Concrete Steel HDPE PVC Other Av er ag e Pi pe T yp e U se d Type of Pipe Figure Q9. Average percentage of pipe type used by DOTs. 4 9 17 25 5 2 6 24 10 16 20 5 5 4 0 5 10 15 20 25 30 M an dr el te sti ng La se r p ro ï¬ lin g V id eo in sp ec ti on (C CT V) V is ua l i ns pe cti on Le ak te sti ng O th er N on e N um be r of R es po ns es Type of Inspection Rigid Pipe Flexible Pipe Figure Q10â11 (aâf). Post-installation inspection methods used by DOTs.
13 11 18 4 5 11 21 3 3 0 5 10 15 20 25 Du rin g Co ns tr uc ti on Po st C on st ru cti on N ot In di ca te d N o In sp ec ti on s Co nd uc te d N um be r o f R es po ns es Timing of Inspection Rigid Pipe Flexible Pipe Figure Q10â11g. When does pipe system inspection occur? Installation of pipe systems most commonly occurs after construction; however, many DOTs aim to have inspection complete by the time construction is done. While 8 responses indicated no inspections were conducted for either rigid or flexible pipes, only one responding agency indicated they do not inspect any pipes. Question 12: Does your agency require a guarantee or warranty period on drainage pipe systems? Only a few DOTs had a requirement for a 1-year warranty after the contract. It is noted that a greater percentage of instal- lations may have warranties even if not required by agency policy. Question 13: Has your agency undertaken any docu- mented case studies on the durability, structural integrity, hydraulic performance, or corrosion resistance of drainage pipe systems? Approximately 43% of DOTs have well-documented studies on the performance and durability of pipe. The corrosion and durability of pipe and the use of flexible pipe are the most common topics. Some of these studies were used in the devel- opment of design guidelines. Many of the more recent studies are available online. Question 14: Does your agency have a practice of inspecting pipe systems on a periodic basis? The DOTs that regularly inspect pipe systems are generally focused on larger structures or those associated with bridges. Some DOTs have initiated an asset-management database and may develop a pipe system inspection protocol in the future. 8 26 3 0 5 10 15 20 25 30 Ye s N o N o An sw er / U nk no w n N um be ro fR es po ns es Figure Q12. Number of DOTs that require a guarantee on drainage pipe systems.
14 16 17 4 0 2 4 6 8 10 12 14 16 18 Ye s N o N o An sw er / U nk no w n N um be ro fR es po ns es Figure Q13. Documented studies on pipe system performance/durability. 17 12 4 4 0 2 4 6 8 10 12 14 16 18 Ye s N o N o An sw er / U nk no w n Pa rti al / In te rm itt en t Pr og ra m N um be ro fR es po ns es Figure Q14. Number of DOTs that inspect pipe systems on a regular basis. 3.2 Commentary on Select Portions of the State of Practice A literature review was performed during the execution of NCHRP Project 10-86 to research and document current practices and was used to develop and guide the framework for the Recommended Practice, as well as to identify gaps in knowledge and application that had the potential to impact the Recommended Practice. While the review of the state of highway drainage practices completed during the execu- tion of NCHRP Project 10-86 covered a significant breadth and depth of topics related to the materials, design, bidding, installation, inspection, maintenance, and other aspects of highway drainage systems, only select summary commentary on specific topics most relevant to the understanding and use of the developed Recommended Practice for Alternative Bidding of Highway Drainage Systems are summarized in this report. The literature review focused on the following topics: â¢ State agency bid practices â¢ Design and construction considerations â¢ Long term performance and service life of pipe (durability) â¢ Post-installation inspection and acceptance criteria Summaries of each of these reviews are included in the following subsections with the resources in the bibliography providing sources of additional information on other related topics. It is acknowledged that certain aspects of the state of the practice have been updated since completion of the main literature review for this project at the end of 2011, and that aspects of the review summary presented do not capture recent updates.
15 3.3 Existing Alternative Pipe Bidding Systems A review of state agency bidding practices and guidelines indicated that while the majority of states surveyed indicated that they have an alternative pipe bidding system in place, the extent to which such a system is implemented and the scope of pipe alternatives permitted are extremely variable. As reported by White and Hurd (2011), none of the current state policies provides a complete protocol for alternate pipe material selection. The bidding of alternative pipe materials is facilitated in different ways by different states as highlighted in the selected summaries below that outline some portions of the state of practice. â¢ In Alaska, the use of a particular bid item code indicates that a choice is available to the bidder. The choice is limited to four options and no further refinement of the options available is possible. Thus, if a choice exists, it is between all four of the allowable pipe materials. In practice, however, only HDPE and metal pipes are used. â¢ A system using groups of allowable pipe materials is also used by MoDOT. MoDOT utilizes a standard (across all projects) list of three groups and specifies the allowable group directly in the bid documents. Both hydraulically smooth and rough pipes are included in the same groups, but no adjustments are made for the difference in hydraulic performance. MoDOT also includes different inspection requirements for the different pipe groups, coupling pipe selection to quality control. â¢ Kentucky and California specify the allowable alternative pipes in the construction drawings, rather than in the bid documents (tabulation sheets). â Kentucky uses the fill height tables to indicate what alternatives are available, and every pipe bid item in the bid documents appears to allow the contractor to choose from all those available for that diameter. This approach closely couples the structural performance (fill height tables) to the pipe material selection. No adjustments are made for differences in pipe system hydraulic capacity or durability. â California specifies in the drainage quantity sheets (part of the construction drawings) which pipes may be bid using alternative materials. For each item a number of alternatives are presented, and where necessary the thick- ness of the metal pipe is specified. â¢ Pennsylvania specifies the allowable alternatives directly in the bid documents using an either/or system. The as-designed pipe is shown, along with either 2 or 3 alternatives based on the roadway classification requirements. Each item (both as-designed and alternatives) is uniquely identified by a bid item number. A master list is maintained for all items that could possibly be included in contracts. â¢ Michigan previously required alternative bidding but removed the requirement based on the provisions of MAP-21. â¢ Nebraska policy specifies that designers select the allowable pipe material options for each installation. The contractor has the option to choose the final pipe material from the list of options provided. In most cases the contractor has the option of selecting the class of pipe and type of installation in accordance with the fill height tables shown on the plans. Nebraska uses a pipe type tender code with numerical des- ignations of 8 digits used by designers to streamline the specification of suitable pipe options. â¢ Florida policy is based on an optional culvert material system and states that optional culvert materials must be considered for all culverts. After the initial hydraulic design, available culvert materials shown in the FDOT Drainage Manual must be evaluated as potential options. The evalu- ation must consider functionally equivalent performance in durability and structural capacity. Florida offers the publicly available Culvert Service Life Estimator (CSLE) software to facilitate evaluations of expected service life (ESL) for con- sidered culvert materials in comparison to the required DSL. â¢ The Ministry of Transportation Ontario (MTO) system allows for a range of pipe sizes and makes allowance for differences in hydraulic performance between rough and smooth pipes. The MTO system is based on the premise that designers specify a list of allowable pipe material types (with applicable minimum material specifications and installation and bedding requirements) and allows the con- tractors to use whichever of the products they prefer. The MTO bidding process uses a succinct bidding code format to identify qualifying pipe types from the master list of pipe culvert tender items maintained by the agency for use in contracts. The differences between current agency practices can materially affect the proposed alternative pipe systems. This is a reflection of the differences in local experience, the risk tolerance of given agencies toward various design criteria, and the structure of alternative pipe selection processes. The review of agency practices and the DOT survey responses found that the absence of both a policy and a comprehensive rational mechanism to facilitate the selection of alternative pipe materials tends to restrict the specification, selection, and installation of available alternative pipe materials and in the teamâs view, can lead to the exclusion of a wide range of viable pipe options from consideration. Because the range of bidding practices and procedures was seen to vary widely across the information reviewed, the Rec- ommended Practice was designed to be flexible, customizable (to accommodate specific DOT requirements), and able to be easily integrated into existing state DOT bidding structures to allow for rapid and easy incorporation into current practice.
16 3.4 Literature ReviewâDesign and Construction Considerations Design and construction considerations were reviewed to determine current practice and guidelines for implementation into the Recommended Practice. In particular, hydraulic, struc- tural, and durability design considerations were reviewed exten- sively. For culvert hydraulic considerations, flow control and Manningâs n values were reviewed. For structural considerations, the basic structural design of buried culverts was reviewed, as well as state DOT fill height tables. The state of the practice to account for durability considerations was also reviewed, with practice found to be more variable and less theoretically based than for hydraulic and structural considerations. While design criteria necessarily vary from agency to agency, the state of the practice review aimed to capture the range of underlying design principles to ensure that the process adopted for the Recom- mended Practice is sufficiently rigorous to gain acceptance. 3.4.1 Hydraulic Design Considerations Hydraulic design is an integral and fundamental component of specifying any highway drainage system. This hydraulic design literature review included a brief summary of state of the practice design methodologies and analysis into variations in design methods and hydraulic parameters used in typical DOT practice. Hydraulic Design Series 5 (HDS 5) (FHWA 2012) provides a list of potential hydraulic considerations for highway drainage system design as follows: â¢ Flow Control and Measurement â¢ Low Head Installations â¢ Section Variations (e.g., Bends, Junctions, Wyes, Transi- tions, etc.) â¢ Siphons â¢ Aquatic Organism Passage â¢ Scour at Inlets/Outlets â¢ Sedimentation and Debris Control â¢ Skewed Barrels/Inlets â¢ Multiple Barrels â¢ Perforated Pipes One or many of these considerations may apply to a given culvert and storm sewer design, and may control pipe selection from a hydraulic perspective. While many of these aspects are accounted for in the Recommended Practice through evalu- ation of the baseline hydraulic design, others are not directly incorporated into the decision framework and require outside consideration by the design engineer. 184.108.40.206 Hydraulic Pipe System Evaluation The state of practice for hydraulic design of roadway drainage systems (culverts and storm sewers) often involves evaluation of the two types of flow control (inlet and outlet) to determine the controlling mechanism for each drainage element configuration. The hydraulic capacity of a culvert depends on a range of factors for each type of control, as summarized in Table 1. FACTOR INLET CONTROL OUTLET CONTROL Headwater Elevation X X Inlet Area X X Inlet Edge Configuration X X Inlet Shape X X Barrel Roughness X Barrel Area X Barrel Shape X Barrel Length X Barrel Slope * X Tailwater Elevation X *Barrel slope affects inlet control performance to a small degree, but may be neglected. Table 1. Factors influencing culvert performance (FHWA 2012).
17 The hydraulic factors in Table 1 can be broken out into the following: â¢ Inlet Characteristics (inlet area, inlet edge configuration, and inlet shape) â¢ Site/Geometric Characteristics (headwater elevation, barrel length, barrel slope, and tailwater elevation) â¢ Pipe Selection Characteristics (barrel roughness, barrel area/size, and barrel shape) â¢ Hydraulic Design Constraints (headwater elevation, tail- water elevation, outlet velocity) â¢ Design Determination (barrel roughness, barrel area/size, barrel shape, inlet area, inlet edge configuration, and inlet shape) The inlet characteristics are the only characteristics that affect both inlet and outlet control design, and thus they can be modified to help improve the performance of a culvert system for both types of control. Site and geometric characteristics are determined primarily by the location of the pipe, and typically remain constant in the design. Typically, pipe selection characteristics are iteratively modified and the resulting headwater elevation is compared with the headwater elevation hydraulic design constraint and outlet velocity design constraints, if any, to help deter- mine the design of a pipe. The barrel roughness is a function of the material used to fabricate the barrel and represents the effect of friction loss within the pipe. It is usually set at a recommended value based on pipe material type to deter- mine the minimum required barrel area/size and in special circumstances, the barrel shape (the barrel shape is typically determined based on the site/geometric conditions, e.g., low available pipe cover, wider bottom opening of pipe). The barrel roughness is represented by a hydraulic resistance coefficient, the Manningâs n value. 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. 220.127.116.11 Review of Recommended Manningâs n Values The following references were reviewed to develop a data- base of typical Manningâs n values used in practice that could be analyzed to evaluate the trends within the range of practice. It is noted that there are numerous other references that provide tabulated n values, but the research team believes this sampling of references satisfies the intent to evaluate the range of n values utilized in common practice. â¢ Software References â CulvertMasterâBentley (previously Haestad Methods) culvert design and analysis software â HEC-RASâUnited States Army Corps of Engineers (USACE) Hydrologic Engineering Center River Analysis System software â HY-8âFederal Highway Administration (FHWA) culvert design and analysis software â Hydraflow ExpressâAutoCAD Civil 3D Hydraflow Express Extension hydrology and hydraulic software â¢ Design Manual References â FHWA HDS 5âMay 2005 edition â Caltrans Highway Design Manual, October 4, 2010 â PennDOT Drainage Manual, Publication 584, 2010 edition Each reference provided either a recommended n value, a range of n values, or both. As noted, some non-standard pipes from the references were not considered within this review. Figures 2 through 8 provide summary plots showing the ranges of Manningâs n values across several pipe types for these references. The literature review into the variability and state of practice regarding Manningâs n values suggests the following: â¢ Recommended Manningâs n values are not consistent across reference guidelines. â¢ It appears that consideration for variations in pipe rough- ness over time via abrasion, corrosion, or other mechanisms may explain some of the variations observed in the recom- mended Manningâs n values; however, it is often unclear if and to what magnitude such considerations are included in setting the recommended Manningâs n values for design, for example, â HDS 5 states that n values for concrete pipe were increased from 0.009 to between 0.011 and 0.013 based on field installation and aging, and â The American Concrete Pipe Association indicates that a general âdesign factorâ of 20 to 30 percent has histori- cally been used to account for the difference between laboratory n values and actual installed conditions. â¢ Although theoretically and experimentally proven to have significantly different flow characteristics (and Manningâs n values), many reference guidelines group different cor- rugation types (annular vs. helical) and profile sizes (e.g., 3 Ã 1 vs. 2-2â3 Ã 1â2) together.
18 0. 01 5 0. 01 4 0. 01 3 0. 01 3 0. 01 3 0. 01 3 0. 01 3 0. 01 2 0. 01 2 0. 01 2 0. 01 2 0. 01 2 0. 01 2 0. 01 1 0. 01 1 0. 01 6 0. 01 7 0. 01 4 0. 01 4 0. 01 7 0. 01 4 0. 01 3 0. 01 1 0. 01 2 0. 01 2 0. 01 1 0. 01 2 0. 01 1 0. 01 1 0. 01 0 0. 01 0 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 Cu lv er tM as te r W oo d fo rm s H EC -R A S - U nfi ni sh ed , sm oo th w oo d fo rm CA LT RA N S Ca st -in -p la ce Cu lv er tM as te r H EC -R A S - C ul ve rt w ith b en ds , co nn ec Âo ns , a nd s om e de br is H EC -R A S -U nfi ni sh ed , st ee l f or m H yd ra flo w E xp re ss CA LT RA N S Pr e- ca st H EC -R A S Fi ni sh ed H Y- 8 H Y- 8 - E lip Âc al H Y- 8 - P ip e A rc h Pe nn D O T Cu lv er tM as te r St ee l f or m s H EC -R A S -C ul ve rt , st ra ig ht a nd fr ee o f d eb ri s H D S 5 M an ni ng 's n V al ue State or Agency Reported Value Recommended n High n Low n Figure 3. Distribution of Manningâs n values for concrete pipes. 0. 01 2 0. 01 2 0. 01 2 0. 01 2 0. 01 1 0. 01 0 0. 01 0 0. 01 3 0. 01 1 0. 01 0 0. 00 9 0. 00 9 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 CA LT RA N S HD PE & P VC Cu lv er tM as te r HD PE HY -8 -H DP E Pe nn DO T- HD PE HY -8 -P VC Hy dr afl ow Ex pr es s- PV C Pe nn DO T- PV C HD S 5- HD PE HD S 5- PV C M an ni ng 's n V al ue State or Agency Reported Value Recommended n High n Low n Figure 2. Distribution of Manningâs n values for smooth thermoplastic (HDPE and PVC) pipes.
19 0. 01 6 0. 01 6 0. 01 5 0. 01 5 0. 01 4 0. 01 3 0. 01 3 0. 01 3 0. 01 3 0. 01 2 0. 01 2 0. 01 2 0. 01 1 0. 01 0 0. 01 7 0. 01 7 0. 01 5 0. 01 5 0. 01 4 0. 01 6 0. 01 5 0. 01 4 0. 01 3 0. 01 3 0. 01 3 0. 01 3 0. 01 2 0. 01 1 0. 01 0 0. 01 1 0. 01 1 0. 01 0 0. 00 9 0. 01 2 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 H EC -R A S - S te el a nd B ra ss Ri ve te d an d sp ir al H EC -R A S - W ro ug ht Ir on G al va ni ze d CA LT RA N S - C as t I ro n CA LT RA N S - S te el P ip e U ng al va ni ze d H EC -R A S - W ro ug ht Ir on Bl ac k CA LT RA N S Sp ir al R ib M et al P ip e H EC -R A S Ca st Ir on - Co at ed H EC -R A S Ca st Ir on - U nc oa te d Pe nn D O T D uc Âl e Ir on P ip e CA LT RA N S- C om po si te Sp ir al M et al P ip e H EC -R A S - S te el a nd B ra ss Lo ck ba r an d w el de d Pe nn D O T Co rr ug at ed M et al P ip e H yd ra flo w E xp re ss Sm oo th W el de d Pi pe H EC -R A S St ee l a nd B ra ss H D S 5 Sp ir al R ib M et al P ip e M an ni ng 's n V al ue State or Agency Reported Value Recommended n High n Low n Figure 4. Distribution of Manningâs n values for smooth metal pipes (e.g., cast iron, ductile iron, spiral rib, etc.). 0. 01 4 0. 01 3 0. 01 3 0. 01 3 0. 01 2 0. 01 7 0. 01 7 0. 01 1 0. 01 1 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 HE C- RA S Vi tr ifi ed S ew er CA LT RA N S HE C- RA S Co m m on D ra in ag e Ti le Hy dr afl ow E xp re ss Pe nn DO T M an ni ng 's n V al ue State or Agency Reported Value Recommended n High n Low n Figure 5. Distribution of Manningâs n values for clay pipes.
20 0. 03 5 0. 03 5 0. 03 3 0. 02 8 0. 02 7 0. 02 7 0. 02 6 0. 02 5 0. 02 5 0. 02 4 0. 02 4 0. 03 5 0. 03 7 0. 02 8 0. 02 6 0. 02 7 0. 02 7 0. 03 3 0. 03 3 0. 02 7 0. 02 5 0. 02 2 0. 02 2 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 CA LT RA N S - C M P 6 x 2 An nu la r a nd H el ic al CA LT RA N S - C M P 9 x 2- 1/ 2 An nu la r a nd H el ic al Pe nn DO T - S te el a nd A lu m in um 6 x 2 An nu la r CA LT RA N S - C M P 3 x 1 An nu la r a nd H el ic al HE C- RA S - C M P - A nn ul ar u np av ed (a ll di am et er s) Pe nn DO T - S te el a nd A lu m in um 3 x 1 An nu la r CA LT RA N S - C M P 5 x 1 An nu la r a nd H el ic al CA LT RA N S - C M P 2- 2/ 3 x 1/ 2 An nu la r a nd H el ic al Pe nn DO T - S te el a nd A lu m in um 5 x 1 An nu la r HE C- RA S - C M P An nu la r un pa ve d 2. 67 x 2 , H el ic al 1 .5 x 0 .2 5 Pe nn DO T - S te el a nd A lu m in um 2- 2/ 3 x 1/ 2 An nu la r HD S 5 - C M P 2- 2/ 3 x 1/ 2 An nu la r M an ni ng 's n V al ue State or Agency Reported Value Recommended n High n Low n Figure 7. Distribution of Manningâs n values for corrugated steel and metal pipesâannular corrugations. 0. 02 4 0. 02 4 0. 02 2 0. 02 0 0. 01 8 0. 01 7 0. 01 5 0. 02 5 0. 02 5 0. 02 0 0. 01 8 0.000 0.005 0.010 0.015 0.020 0.025 H Y- 8 Co rr ug at ed P E Pe nn D O T Pl as c CA LT RA N S H D PE & P VC Cu lv er tM as te r (1 8- 24 in . d ia ) Cu lv er tM as te r (1 2- 15 in . d ia ) Cu lv er tM as te r (1 0 in . d ia ) Pe nn D O T H D PE H D S 5 Co rr ug at ed P E M an ni ng 's n V al ue State or Agency Reported Value Recommended n High n Low n Figure 6. Distribution of Manningâs n values for corrugated thermoplastic pipes.
21 0. 02 7 0. 02 6 0. 02 5 0. 02 4 0. 02 4 0. 02 3 0. 02 3 0. 02 1 0. 02 0 0. 01 9 0. 01 6 0. 01 4 0. 01 3 0. 01 2 0. 01 1 0. 01 5 0. 01 5 0. 02 3 0 .0 25 0. 02 6 0 .0 28 0. 02 4 0. 02 4 0. 02 4 0. 02 4 0. 02 4 0. 01 3 0. 01 1 0. 01 1 0. 01 1 0. 02 2 0. 02 5 0 .0 27 0. 01 4 0 .0 16 0. 01 9 0. 02 0 0. 02 1 0. 01 2 0.000 0.005 0.010 0.015 0.020 0.025 0.030 HE C- RA S - C M P (7 8- in d ia ) He lic al 3 x 1 HE C- RA S - C M P (7 2- in d ia ) He lic al 3 x 1 HE C- RA S - C M P (6 6- in d ia ) He lic al 3 x 1 HE C- RA S - C M P (6 0- in d ia ) He lic al 3 x 1 Pe nn DO T - S te el a nd A lu m in um He lic al 3 x 1 , 5 x 1 , a nd 6 x 2 HE C- RA S - C M P (4 8- in d ia ) He lic al 3 x 1 HE C- RA S - C M P (5 4- in d ia ) He lic al 3 x 1 HE C- RA S - C M P (6 0- in d ia ) He lic al 2 .6 7 x 2 HE C- RA S - C M P (4 8- in d ia ) He lic al 2 .6 7 x 2 HE C- RA S - C M P (3 6- in d ia ) He lic al 2 .6 7 x 2 HE C- RA S - C M P (2 4- in d ia ) He lic al 2 .6 7 x 2 HE C- RA S - C M P (1 8- in d ia ) He lic al 2 .6 7 x 2 CA LT RA N S - S pi ra l R ib M et al P ip e CA LT RA N S - C om po sit e Sp ira l M et al P ip e HE C- RA S - C M P (1 2- in d ia ) He lic al 2 .6 7 x 2 HD S 5 - C M P - H el ic al 2 -2 /3 x 1 /2 HD S 5 - C M P - H el ic al 6 x 1 HD S 5 - C M P - H el ic al 5 x 1 HD S 5 - C M P - H el ic al 3 x 1 Pe nn DO T - S te el a nd A lu m in um (1 8- in d ia ) - H el ic al 2 -2 /3 x 1 /2 Pe nn DO T - S te el a nd A lu m in um (2 4- in d ia ) - H el ic al 2 -2 /3 x 1 /2 Pe nn DO T - S te el a nd A lu m in um (3 6- in d ia ) - H el ic al 2 -2 /3 x 1 /2 Pe nn DO T - S te el a nd A lu m in um (4 8- in d ia ) - H el ic al 2 -2 /3 x 1 /2 Pe nn DO T - S te el a nd A lu m in um (6 0- in d ia ) - H el ic al 2 -2 /3 x 1 /2 HD S 5 - S pi ra l R ib M et al P ip e M an ni ng 's n V al ue State or Agency Reported Value Recommended n High n Low n Figure 8. Distribution of Manningâs n values for corrugated steel and metal pipesâhelical corrugations. â¢ Manningâs n values can vary significantly with pipe diameter for corrugated and structural plate sections, but are essen- tially independent of pipe size for smooth walled pipes. â¢ Many references lack independent consideration of helical corrugation profiles. Based on the literature review it seems that preliminary hydraulic screening of suitable pipe materials is often com- pleted by assuming one or more generalized Manningâs n values. Implementation of such an approach is useful for pre- liminary screening in cases when an automated database or software system is not available. 3.4.2 Structural Design Considerations Structural design of culverts is necessary to ensure the strength and serviceability of the drainage system. In the structural design of culvert pipe systems, an integral relation- ship exists between the pipe and the surrounding material in which it is installed. The design of the pipe products, the installation procedures, trench or embankment geometry, and the quality and compaction of bedding and backfill materials are all integral parts of the structural design of buried pipes. Designers shall thus specify in the documents, as appropriate, the bedding detail and installation method for each pipe material or class of pipe selected. In general, the structural design of buried culverts depends on a number of factors including the following: â¢ Pipe â Material type â Material class â Diameter â Wall thickness â Wall profile â¢ Installation configuration â Depth â Trench width â Slope angle of trench â¢ Material properties â Native soil/rock properties â Bedding properties and compaction â Backfill properties and compaction â Foundation material 18.104.22.168 Structural Pipe System Evaluation The primary reference available to practitioners today for buried pipe design is the AASHTO LRFD Bridge Design Specifications, Section 12 (AASHTO current version). This reference provides design guidelines for different materials, including those most commonly used for highway drainage
22 pipes (metal, reinforced concrete, and thermoplastic). How- ever, different state DOTs are often using different versions of the AASHTO LRFD code at any given time, with several states having not yet fully integrated LRFD-based design method- ologies into practice. Within the AASHTO LRFD Bridge Design Specifications, buried structures are designed such that they resist factored loads given by specified load combinations. Tables of load com- binations and load factors are provided. The factored loads are then compared with the factored resistance to determine the suitability of the design. Tables of resistance factors for buried structures are provided, along with procedures for calculating the nominal resistance. Performance criteria are usually established by the design engineer based on required performance and capacity of the specified products. When a product capacity is reached or exceeded, it is said that a performance limit has been reached. Performance limits are established for each product to pre- vent conditions that may interfere with the design function, including the ability to meet the specified service life. AASHTO requires that both service and strength limit states be checked. For metal and thermoplastic pipes the governing service limit state is deflection. For reinforced concrete pipe the governing service limit state is crack width. The strength limit states are as follows (AASHTO 2010): â¢ Metal Pipes â Wall area â Buckling â Seam failure â Flexibility limit for construction â Flexure of box and deep corrugated structures only â¢ Concrete Pipes â Flexure â Shear â Thrust â Radial tension â¢ Thermoplastic Pipes â Wall area (including local buckling) â Buckling â Flexibility limit The specific corrugation profile geometry for corrugated pipes often controls the structural evaluation for flexible pipe systems. It is important to note that corrugation pro- files for metal pipe are nationally standardized and defined in AASHTO material and industry trade association (e.g., American Iron and Steel Institute [AISI]) specifications, whereas thermoplastic pipes consist of manufacturer specific (and often patented) corrugation profile geometries. The thermoplastic pipe industry has discussed development of pipe classes based on LRFD design considerations but none are known to exist at this time, thus requiring individual evaluation and consideration of thermoplastic pipes on a manufacturer by manufacturer basis. Loads on unpressurized gravity pipes include the following: â¢ Soil pressure â Rigid pipes â Flexible pipes â¢ Wheel loading (live loads) â¢ Soil subsidence â¢ Seismic loads â¢ Frost loading â¢ Loads due to expansive soils â¢ External water pressure (internal water pressure includ- ing effects such as water hammer need to be considered in pressurized applications) â¢ Flotation When fill height tables are used for routine design, soil loads and wheel loads are considered. If other non-typical structural loadings are anticipated for a given application, fill height table evaluations usually need to be supplemented with more detailed designs. In addition to the structural evaluation of the typical pipe wall section, joints, transitions, and other components of the pipe system are also evaluated for structural adequacy. Manufacturing practice for pipe systems typically strives to achieve structural capacities for joints, transition pieces, and other non-standard sections equal to or greater than the main pipe profile for a given pipe class so that structural evaluations only need to be completed on the standard pipe section. Formal methods for the structural design of joints are provided in the final report from NCHRP Project 15-38 found in NCHRP Web-Only Document 190: Structural Design of Culvert Joints. While joints, transitions, and other special pipe sections may have performance issues, these issues are often related to instal- lation deficiencies (rather than to inherent structural failure) and as such do not impact the basic structural assessment and adequacy of given piping systems. In other words, no consid- eration for the impacts of improper installation is considered in the screening of pipes for structural adequacy to meet a given loading condition in this work. Consideration for proper instal- lation, including development of detailed specifications and post-installation inspection protocols are critical elements to successful pipe installations, and are assumed adopted in the standard practices of each transportation authority that uses an alternative pipe bidding system. Current practice for confirming the structural adequacy of a particular pipe is to refer to âfill height tablesâ that indicate the maximum acceptable loading (expressed as a height of fill
23 0 40 80 120 160 200 240 AI SI M O N D M N M T O H U T W V CO PA (2 5) N Y AK F L ID IN O R W A GA T X M D KY O K W Y N V AR M S KS LA M E M A AZ N C TN N E N M W I IL IA VA PA (5 0) M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 9. Distribution of maximum fill heights for 24â 12 gage galvanized steel pipe (2-2/3 î³ 1/2 corrugations). (Note: Values in parentheses indicate design life.) material) for each pipe product. Minimum fill heights are also specified to protect the pipe from in-service (traffic) loading and construction equipment (although protection from con- struction loadings are often contract requirements left to the installer and enforced via post-installation inspections and quality control protocols). Fill height tables generally consider different pipe material types, bedding classes, pipe profiles and configurations, and diameters. Some tables also consider other variables such as trench/embankment installations, service life, and foundation conditions. The designer can use these tables to quickly evaluate the structural adequacy of pipe materials (e.g., pipe wall thickness and/or corrugation profile and/or class of pipe) for most applications. The AASHTO LRFD Bridge Design Code does not specify or provide recommendations for standardized bedding and backfill requirements for all material types (nor does any other nationally standardized guidance). While concrete pipe design is grouped into standard installation classes that are generally recognized and used nationally, flexible pipe instal- lations are generally not standardized, and have resulted in each state agency developing agency-specific protocols and guidance for installation and subsequent structural evaluations, which has created large variations in the structural capacity evaluated for pipe systems in different jurisdictions. 22.214.171.124 Review of Fill Height Tables The NCHRP Project 10-86 research team completed a review of state agency fill height tables circa 2011 and compiled the graphs in Figures 9 through 21, which demonstrate the wide range of structural design values in current practice. Information on the structural design of culverts using fill height tables was reviewed from 46 state DOTs as well as from relevant trade associations, such as the American Concrete Pipe Association (ACPA). Additional information on this topic was obtained through the DOT survey questionnaire, which was returned by 37 DOTs. Based on the review of state DOT fill height tables, it can be seen that significant variation exists across the practice for all pipe types and classes. Additionally, many pipe products do not have specific fill height tables. Because the use of fill height tables is the standard of practice and greatly simplifies structural evaluation, products that do not have fill height tables or have tables that are not accepted by a state agency are often excluded from consideration. 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.
24 0 20 40 60 80 100 120 140 160 M O U T W V AK F L ID O R W A W Y GA A R M S M D O K N C AZ KS LA VA IA N E IL TN M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 11. Distribution of maximum fill heights for 36â 12 gage galvanized steel pipe (3 î³ 1 corrugations). 0 20 40 60 80 100 120 140 160 AI SI M O N D M N M T O H SD U T W V CO A K FL ID IN O R W A CA PA (2 5) N Y GA K Y KS M D TX O K W Y N V N E N M AR M S LA AZ N C W I TN I L M E M A VA IA PA (5 0) M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 10. Distribution of maximum fill heights for 36â 12 gage galvanized steel pipe (2-2/3 î³ 1/2 corrugations). (Note: Values in parentheses indicate design life.)
25 0 20 40 60 80 100 120 140 160 M O N D U T W V CO A K FL ID IN O R W A CA W Y GA A R M S M D O K TX KS LA N E AZ N C IL VA TN IA M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 12. Distribution of maximum fill heights for 48â 12 gage galvanized steel pipe (3 î³ 1 corrugations). 0 20 40 60 80 100 120 140 160 CA ID IN KY M D M O N Y M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Helical Annular Figure 13. Comparison of maximum fill heights for helical and annular profiles (2-2/3 î³ 1/2 corrugations, 36â diameter).
26 0 5 10 15 20 25 30 35 40 N Y N M (T re nc h) CA U T CO K Y N E N M (+ P ro j) FL N V AC PA AR M O IN M T (T re nc h) N C O H O K (T re nc h) TN M E (Z er o Pr oj ) M I N D SC W Y GA I L PA W A W I M T (E m ba nk ) O K (E m ba nk ) O R SD TX (E m ba nk ) VA W V M E (+ P ro j) IA M A TX (T re nc h) KS M N DE ID M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 15. Distribution of maximum fill heights for 24â reinforced concrete pipe (1350 D, highest quality bedding). 0 5 10 15 20 25 30 35 40 U T CO F L IN M T (T re nc h) N C O K (T re nc h) N M (T re nc h) TN M E (Z er o Pr oj ) M I N D SC AZ N M (+ P ro j) GA I L PA W A W I N V M T (E m ba nk ) O K (E m ba nk ) O R SD VA W V N Y N E AR W Y M E (+ P ro j) TX (E m ba nk ) M A KS O H IA TX (T re nc h) DE ID KY M O M N AC PA CA M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 14. Distribution of maximum fill heights for 24â reinforced concrete pipe (1350 D, lowest quality bedding).
27 0 5 10 15 20 25 30 35 40 U T CO F L M T (T re nc h) N C N M (T re nc h) TN M E (Z er o Pr oj ) IN M I M S N D AZ GA I L N M (+ P ro j) O R PA W A W I M T (E m ba nk ) N V O K (T re nc h) O K (E m ba nk ) SD VA W V AR N Y W Y M E (+ P ro j) DE K S M A TX (E m ba nk ) IA ID N E O H TX (T re nc h) KY M N M O AC PA CA M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 16. Distribution of maximum fill heights for 36â reinforced concrete pipe (1350 D, lowest quality bedding). 0 5 10 15 20 25 30 35 40 AZ N Y N M (T re nc h) CA U T CO K Y N M (+ P ro j) FL N V AC PA AR M O M T (T re nc h) N E N C O H TN M E (Z er o Pr oj ) IN M I N D W Y GA I L O R PA W A W I M T (E m ba nk ) O K (T re nc h) O K (E m ba nk ) SD TX (T re nc h) TX (E m ba nk ) VA W V IA M E (+ P ro j) DE K S M A M N ID M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 17. Distribution of maximum fill heights for 36â reinforced concrete pipe (1350 D, highest quality bedding).
28 0 5 10 15 20 25 30 35 40 N E AK CA KY ID W A AW W A O R CO (9 5) GA IA O H M O (9 5) TN VA F L N C AR I L N Y PA W V W I KS CO (9 0) M O (9 0) SC U T IN M I M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 19. Distribution of maximum fill heights for 36â Type S HDPE pipe. (Note: Values in parentheses indicate % compaction.) 0 5 10 15 20 25 30 35 40 N E AK CA KY O R ID W A AW W A IA M O (9 5) CO (9 5) GA N C O H VA TN F L M O (9 0) KS AR CO (9 0) IL N Y PA U T W V W I SC IN M I M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 18. Distribution of maximum fill heights for 24â Type S HDPE pipe. (Note: Values in parentheses indicate % compaction.)
29 0 5 10 15 20 25 30 35 40 CO (9 5) CO (9 0) M O (9 5) AR CA N E M O (9 0) N M IN KY N C TN O R IL U T W A GA O H VA K S FL PA W V M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 20. Distribution of maximum fill heights for 24â profiles wall PVC pipe. (Note: Values in parentheses indicate % compaction.) 0 5 10 15 20 25 30 35 40 CO (9 5) CO (9 0) M O (9 5) AR CA N E M O (9 0) KY N C IN N M TN W A O R U T GA I L O H VA F L KS PA W V M ax im um F ill H ei gh t ( fe et ) State or Agency Reported Value Figure 21. Distribution of maximum fill heights for 36â profiles wall PVC pipe. (Note: Values in parentheses indicate % compaction.)
30 Differences in allowable fill heights are thought to be the result of the following main factors: â¢ In agency structural evaluations, there are differences in how material factors such as pipe stiffness, corrugation geometries, and backfill/bedding details are considered. The following are some examples of the differences: â Variations in applied loading from trench versus embank- ment installations â Variations in installation and bedding/backfill specifi- cations among agencies and among pipe materials â Changes in structural capacity with time due to pipe material degradation (e.g., metal corrosion, etc.) â Ignoring structural capacity of corrugation geometries â¢ The designs of rigid and flexible pipe systems have inherent differences in design methodology and in the factors of safety or risk tolerance typically applied in practice. â¢ There are variations in the basis for design (i.e., many state DOTs had not fully adopted the AASHTO LRFD Design requirements). â¢ The fill height tables presented in the standard guidelines and design manuals of many state DOTs do not specify the design bases or assumptions used in the development of the presented fill height tables. â¢ There are variations in the factors of safety and/or LRFD factors used to account for variations in construction quality oversight and specification across state DOT practice. While the current study is not focused on providing a fully robust solution to the current biases that exist in practice, there is noted benefit to greater standardization and removal of bias. Given the range of different methodologies used in the development of fill height tables, some variation in the fill height tables was expected but the magnitude and range of variations was surprising. The lack of use of a clearly expressed and consistent methodology (despite one being available) is a cause for concern. Consistent implementation of the LRFD methodology in the form of fill height tables for typical instal- lation conditions and configurations for all available pipe types would benefit designers, constructors, and manufacturers of highway drainage pipe products. 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 that prevents national standardization and direct comparison of performance. The following summary points have been developed from this review of fill height tables: â¢ While standardized design methodology is available from AASHTO and is mandated on federally funded projects, state practice generally does not follow the AASHTO guidance for structural adequacy evaluations because standardized fill height tables developed from these guidelines were not readily available at the time of the survey in 2011, and state agencies are continuing to use historic fill height tables. â Several states are currently working to address this issue as noted in their responses to the survey. â Some (but not all) manufacturers have issued fill height tables in compliance with the AASHTO LRFD design specifications that are available for use. â¢ Available fill height tables were developed with a range of design assumptions and specifications that often are not documented including variables such as the following: â Design Methodology (e.g., both direct and indirect methods for rigid pipes could be used, differing versions of AASHTO or state specifications are often implemented across pipe materials, and the design criteria is often not fully reported). â Loading (e.g., traffic loads, unit weight of fill) â Bedding Materials (e.g., material classifications [typically state specific and not AASHTO based]) â Bedding Conditions (e.g., compaction requirements are often unknown or inconsistent) â Installation Configuration (e.g., trench, embankment, and so forth.) â¢ Not all available pipe products are independently considered, for example, â Corrugated metal pipe (CMP) with helical and annular corrugations and/or with varying corrugation profile geometries are often not considered separately. â The corrugation profile geometry of thermoplastic pipes is manufacturer specific and no national standards exist to group or classify the range of products in the market- place with respect to LRFD limit states. â¢ No explicit consideration of joint materials and stress concentrations is made in typical practice. This would be acceptable if joints where always equal to or stronger than the equivalent barrel sections, but joints often present points of weakness in pipe installations, especially those with below standard installations. â¢ Variations in the degree of compaction are not always con- sidered in fill height tables. â¢ Degradation of structural capacity due to corrosion and abrasion is generally not incorporated into fill height tables (PennDOT is noted to include this), despite AASHTO LRFD 12.6.9 requirements. â The AASHTO code does not provide specific guidance regarding the proper method(s) to account for degra- dation of structural capacity, and this likely is a driving factor in the limited application of structural degradation considerations in practice. â Durability is widely considered not to be fully devel- oped for all pipe products and installation environments. It is an area requiring continued research and practical implementation.
31 3.4.3 Long Term Performance and Durability A literature review of long term performance and durability of pipe systems was undertaken to evaluate the approaches used for estimating material service life of drainage systems for incorporation into the Recommended Practice. The intent of this project is not to provide an exhaustive review of dura- bility, but to identify the key factors and methods needed for the implementation of a reliable Recommended Practice. NCHRP Synthesis 474: Service Life of Culverts will be published in spring 2015 and will provide a more comprehensive review of this topic. This review examined the performance and durability of various pipe types including reinforced concrete, thermo- plastic, and corrugated metal pipes. The following subsections briefly describe the major factors affecting durability for each pipe type, the current state of practice for addressing these factors, and existing methods for assessing the durability of a pipe. Software and online models for estimating material service life were also reviewed. There are a number of well estab- lished procedures for predicting the service lives of concrete and metal pipes related to material parameters and environ- mental and loading conditions, but prediction methods for thermoplastic pipe are less well developed. The two primary mechanisms of degradation for properly specified and installed culvert pipe systems are corrosion and abrasion. The AASHTO LRFD Bridge Design Manual (Section 12.6.9) requires that the degradation of structural capacity due to corrosion and abrasion be considered in design, but does not provide specific methods for doing so. The specification further allows that if the design of a metal or thermoplastic culvert is controlled by flexibility factors (i.e., construction loads versus service loads) during installation, then the requirements for corrosion and abrasion protection may be reduced or eliminated, provided that it is demonstrated that the degraded culvert will provide adequate resistance to loads throughout the service life of the structure. A summary of the most commonly accepted independent (i.e., not developed or published by a pipe trade organization) quantitative service life calculation methods for concrete and metal pipes is included in Appendix C. No known methods are in use to calculate the estimated material service life (EMSL) of thermoplastic pipes related to application conditions and loading. Extensive research has been undertaken, much of it sponsored by FDOT to better understand deterioration mechanisms of thermoplastic pipes and to develop appropri- ate material specifications to ensure long term performance. The EMSL of thermoplastic pipes is based on the material performance specifications and details of the resins used in the pipe manufacturing process. The materials are thus gen- erally assigned a fixed EMSL regardless of the environmental parameters at the site. Thermoplastic culvert pipes for high- way drainage applications are usually assigned EMSL values between 50 and 100 years. Summary background information on several key culvert durability topics is presented below. The reader is referred to NCHRP Synthesis 474: Service Life of Culverts (to be published in spring 2015) for a more detailed summary of culvert dura- bility issues. 126.96.36.199 Corrosion Corrosion is the destruction of pipe material by chemical action. Most commonly, corrosion attacks metal culverts or the reinforcement in concrete pipe. Similar damage can occur to the cement in concrete pipe if it is subjected to highly alkaline soils or sulfates or to other pipe materials if they are subjected to extremely harsh or aggressive environments. For corrosion to occur, an electrolytic corrosion cell must be formed. This requires the presence of water, or some other liquid to act as an electrolyte, and materials acting as an anode, cathode, and conductor. As electrons move from the anode to the cathode, metal ions are released into solution, with characteristic pit- ting at the anode. The culvert will typically serve as both the anode and the cathode. Corrosion can affect either the inside or outside of a pipe or both. The potential for corrosion to occur, and the rate at which it will progress, is variable and dependent on a variety of factors. Depending on the particu- lar corrosive environment encountered, increased pipe wall thickness, additional cover over reinforcing steel, or special coatings may be required to extend service life. 188.8.131.52 Abrasion Abrasion is the gradual wearing away of the culvert wall due to the impingement of bedload. Abrasion will almost always manifest itself first in the invert of the culvert. As with corrosion, abrasive potential is a function of several items, including culvert material, frequency and velocity of flow in the culvert, and composition of the bedload. Bedload is the leading cause of culvert abrasion. Critical factors in evaluating the abrasive potential of bedload material are the size, shape, and hardness of the bedload material and the velocity and frequency of flow in the culvert. Generally, flow velocities less than 5 ft/s are not considered to be abrasive, even if bedload material is present. Velocities in excess of 15 ft/s, which carry a bedload, are considered to be very abrasive and some modifications to protect the culvert should be considered. It is very difficult to look at a given culvert material and provide an absolute determination of how it will be affected by bedload abrasion. Perhaps the most useful method for making a reasonable determination is to look at the various types of culvert materials and make relative comparisons.
32 184.108.40.206 pH The pH value is defined as the log of the reciprocal of the concentration of hydrogen ion in a solution. Values of pH in natural waters generally fall within the range of 4 to 10. A pH of less than 5.5 is usually considered to be strongly acidic, while values of 8.5 or greater are strongly alkaline. Studies performed in various states have been inconclusive in determining the exact role pH plays in corrosion. The presence of oxygen at the metal surface is necessary for the corrosion to occur and is independent of the pH. However, at the very least a pH reading that is either highly acidic or alkaline is indicative of a heightened potential for corrosion. The lowest pH levels (most acidic) are typically seen in areas that have received high rainfall over many centuries. The runoff and percolation will leach the soluble salts, with the resultant soil becoming acidic. Other likely sources of acidic runoff are mine wastes that often contain sulfuric and sulfurous acids. Milder acids can be found in runoff from marshy areas, which contain humeric acid, and mountain runoff that often contains car- bonic acid. Conversely, arid areas are much more likely to be alkaline due to soluble salts contained in groundwater being drawn to the surface through capillary action and then concentrating after evaporation occurs. Generally, soil or water pH levels between 5.5 and 8.5 are not considered to be severely detri- mental to culvert life. 220.127.116.11 Resistivity Resistivity of soil is a measure of the soilâs ability to conduct electrical current. It is affected primarily by the nature and concentration of dissolved salts, and the temperature, moisture content, compactness, and presence of inert materials such as stones and gravel. The greater the resistivity of the soil, the less capable the soil is of conducting electricity and the lower the corrosive potential. The unit of measurement for resistivity is ohm-centimeters or, more precisely, the electri- cal resistance between opposite faces of a 1-centimeter cube. Resistivity values in excess of about 5,000 ohm-cm are con- sidered to present limited corrosion potential. Resistivity values below the range of 1,000 to 3,000 ohm-cm will usually require some level of pipe protection, depending upon the corresponding pH level (e.g., if pH < 5.0, enhanced pipe protec- tion may be needed for resistivity values below 3,000 ohm-cm; if pH > 6.5, enhanced pipe protection may not be needed unless resistivity values are below 1,500). As a comparative measure, resistivity of seawater is in the range of 25 ohm-cm, clay soils range from approximately 750 to 2,000 ohm-cm and loams from 3,000 to 10,000 ohm-cm. Soils that are of a more granular nature exhibit even higher resistivity values. 18.104.22.168 Chlorides Dissolved salts containing chloride ions can be present in the soil or water surrounding a culvert. Chlorides will also be of concern at coastal locations or near brackish water sources. Dissolved salts can enhance culvert durability if their presence decreases oxygen solubility but, in most instances, corrosive potential is increased, as the negative chloride ion decreases the resistivity of the soil and/or water and destroys the protective film on anodic areas. Chlorides, as with most of the more common corrosive elements, primarily attack unprotected metal culverts and the reinforcing steel in con- crete culverts if concrete cover is inadequate, cracked, or highly permeable. 22.214.171.124 Sulfates Sulfates can be naturally occurring or may be a result of humanâs activities, most notably mine wastes. Sulfates, in the form of hydrogen sulfide, can also be created from biologi- cal activity, which is more common in wastewater or sanitary sewers, and can combine with oxygen and water to form sul- furic acid. Although high concentrations can lower pH, and be of concern to metal culverts, sulfates are generally more damaging to concrete. Typically, the sulfate in one or more various forms combines with the lime in cement to form calcium sulfate, which is structurally weak. Concrete pipe is normally sufficient to withstand sulfate concentrations of 1,000 parts per million (ppm) or less. For higher concen- trations of sulfates, higher strength concrete, concrete with lower amounts of calcium aluminate (under 5%), or special coatings may be necessary. 126.96.36.199 Available Software Durability Evaluation Tools The literature review identified three software programs that have been developed by transportation agencies to automate the calculation of estimated service life for pipe systems: â¢ HiDISC 1.0 developed for the MTO (not yet publicly released) â¢ CSLE (Culvert Service Life Estimator) 2013 developed by FDOT â¢ AltPipe v 6.08 developed by Caltrans HiDISC and CSLE are stand-alone software programs while AltPipe is an online tool. Appendix C includes screenshots showing the use of these programs for a non-aggressive case.
33 3.4.4 Inspection and Post-Installation Certification Experience has shown that one of the critical issues impact- ing the performance (short and long term) of pipe systems is the quality of the installation. Drainage systems that are appropriately designed and properly installed will generally perform well throughout the design life of the pipe system. Post-installation inspection of a buried pipe system is one phase of a comprehensive quality assurance program. Mill certificates for all pipe materials should be checked in advance and conformance to relevant project specifications and refer- ence standards (e.g., ASTM/AASHTO) confirmed. Source acceptance test results for all imported materials should be checked against project specifications. Inspections should be performed on the pipe, bedding, and backfill materials prior to and during installation. The agencyâs specifications for com- paction and general requirements for workmanship during construction should be enforced. Some agencies have a pro- gram of periodic routine systemwide inspections for in-service pipe systems. While this is not considered essential, it can identify potential future serviceability problems that can be addressed by routine maintenance rather than by emergency repairs. The inspection of system materials prior to installation and inspection during construction are summarized in the following subsections followed by a more detailed discussion of post-installation inspection procedures. Guidelines for routine systemwide inspection programs of in-service pipe systems can be found in the FHWA Report No. FHWA-IP-86-2, Culvert Inspection Manual (FHWA 1986). As new pipe products (materials, coatings, and rehabilitative liners) and remote access inspection technologies have been introduced since the Culvert Inspection Manual was developed, there is a need for updated culvert inspection guidelines. An update and review of inspection procedures and technologies is proposed to be addressed through NCHRP Project 14-26, Culvert and Storm Drain System Inspection Manual, which is scheduled for completion in fall 2015. The AASHTO LRFD Bridge Construction Specifications provide excellent baseline recommendations for inspection requirements for the three main categories of pipe materials (Metal Pipes in Section 26, Reinforced Concrete Pipes in Sec- tion 27, and Thermoplastic Pipes in Section 30) that can also be applied to other flexible and rigid pipe material systems. 188.8.131.52 Inspection of Pipe Materials In general, state agencies have well-developed and docu- mented policies for evaluating and ensuring the quality of pipe materials delivered to project sites. These procedures often include the following: â¢ Qualification of manufacturer and manufacturing facility and review of mill certificates â¢ Inspection of deliveries, which may include inspection of the following: â Identification markings â Date of manufacture â Shipping papers â Diameter â Net length of fabricated pipe â Evidence of poor workmanship â Identification of damage during shipping and handling â Measurement of surface cracks (for example with leaf gages) â¢ Taking samples of pipe for additional testing (chemical, mechanical, coatings) 184.108.40.206 Inspection During Construction Inspection of the pipe system materials and workmanship during construction allows corrections to be made in assembly and backfill practices before construction is complete, and is of particular importance for deeply buried, high traffic, or other critical and/or costly to repair installations. The timing and frequency of such inspections should depend on the signifi- cance of the structure and depth of fill. In general, inspections should be conducted when materials arrive at the job site, dur- ing pipe installation, during backfilling, and prior to construc- tion of final finishes (e.g., paving). Inspections during construction may include examination of the following: â¢ Foundation material â¢ Trench geometry and dimensions â¢ Groundwater conditions â¢ Bedding material â¢ Line and grade â¢ Assembly techniques â¢ Structure backfill and compaction methods â¢ Joint assembly and materials â¢ Pipe deflection (during construction) â¢ Damage to pipe coatings 220.127.116.11 Post-Installation Inspection Post-installation inspection allows for timely identification of potential installation problems and allows for corrective action to be taken, if needed, within the scope of the construction con- tract. The AASHTO LRFD Bridge Construction Specifica- tions recommend that final post-construction inspections for culvert approval be completed no sooner than 30 days after completion of installation and final fill so that defects under
34 initial conditions can have time to present themselves. The AASHTO construction specifications commentary expands on this recommendation by stating that soil consolidation continues with time after installation of the pipe. While 30 days will not encompass the timeframe for complete consolidation of the soil surrounding the pipe, it is intended to give sufficient time to observe some of the effects that this consolidation will have. Post-installation inspection can be carried out in a number of ways, with the most common methods being the following: â¢ Visual inspection performed manually (usually for larger diameter pipes, typically greater than 36 in.) â¢ Visual inspection performed remotely by video inspection (e.g., closed-circuit television [CCTV]) â¢ Mandrel testing â¢ Laser profiling, upcoming ASTM F36 method â¢ Non Destructive Inspection/Testing (NDI/NDT) techniques Current post-installation inspection requirements of pipe systems across state agencies vary more significantly than current practices for the other stages of inspections. This difference is due in part to the continued introduction of new pipe materials, design methods, and remote access inspection techniques within the industry. Improving the consistency of post-installation inspection practices will mitigate risks associated with broadening the use of alter- native pipe types. 18.104.22.168 AASHTO LRFD Bridge Construction Specifications Standard post-installation inspection recommendations are found in the following subsections of the AASHTO LRFD Bridge Construction Specifications: â¢ Metal Pipes (Section 26) â¢ Reinforced Concrete Pipes (Section 27) â¢ Thermoplastic Pipes (Section 30) 22.214.171.124.1 AASHTO Visual Inspection Recommendations for Flexible Pipe Systems. The recommended inspections for flexible pipe system installations include checks for the following: â¢ Alignment â¢ Joint separation â¢ Cracking at bolt holes â¢ Localized distortions â¢ Bulging, flattening, and racking â¢ Minimum cover levels (for shallow installations) â¢ Deflection Testing 126.96.36.199.2 AASHTO Visual Inspection RecommenÂ dations for Reinforced Concrete (and Other Rigid) Pipe Systems. Reinforced concrete pipes do not deflect appre- ciably before cracking or fracturing, so deflection testing is of limited value. Visual inspection of pipe interiors and joints is the primary means of inspection for rigid pipes. During a visual inspection, observations of the following should be made: â¢ Misalignment â¢ Joint defects â¢ Longitudinal cracks â¢ Transverse cracks â¢ Spalls â¢ Slabbing â¢ End-section drop off 188.8.131.52.3 Other Inspection Techniques. A wide range of other less commonly used culvert inspection techniques are available, several of which are listed as follows: â¢ Destructive Core Sampling and Evaluation â¢ Ground Penetrating Radar (GPR) (Applied from ground surface and from within pipes) â¢ Impact Echo (IE) Testing â¢ Infrared (IR) Thermography â¢ Mechanical Impedance Testing â¢ Microdeflection Testing â¢ Natural Frequency Measurement â¢ Pigs (basic mandrels through Instrumented âSmartâ Pigs) â¢ Spectral Analysis of Surface Waves (SASW) â¢ Ultrasonic Testing â¢ Ultra Wide Band (UWB) Radar 184.108.40.206 Commentary on the State of Practice for Inspections The state of knowledge with respect to short and long term inspection for highway culverts has been benefited tremen- dously in recent decades by significant improvements in a range of inspection technologies. Most notably improvements in CCTV, remote control robotics, laser profiling, optical scan- ning, and other remote techniques make in line inspections of culverts easier, less expensive, and more reliable than ever before. Many agencies routinely use a range of remote and man-entry inspection techniques during installation, post- installation, and for long term monitoring and inventory man- agement. It is noted that some forms of inspection require or are benefited from training and certification programs such as National Association of Sewer Service Companies (NASSCO), and agency-specific training such as that provided by FDOT, Ohio DOT, and others.
35 The survey response graphs included in Chapter 3 and the following observations were made based on the results of the NCHRP Project 10-86 DOT survey: â¢ For rigid pipe systems: visual inspection is the most common, followed by video inspection and laser pro- filing. â¢ For flexible pipe systems: mandrel testing is the most com- mon, followed by visual inspection, video inspection, and laser profiling. â¢ Leak testing is performed equally (although infrequently) on flexible and rigid pipe systems. â¢ Video inspection and laser profiling are performed equally on rigid and flexible pipe systems. â¢ Video inspection is approximately 60% more common than laser profiling. â¢ Rigid pipe systems are less likely to be inspected than flexible pipe systems. Two ongoing NCHRP Projects: 14-19 on Culvert Reha- bilitation to Maximize Service Life While Minimizing Direct Costs and Traffic Disruption and 14-26 Culvert and Storm Drain System Inspection Manual will provide updated sum- maries of culvert inspection techniques.