Rehabilitation of Culverts and Buried Storm Drain Pipes (2022)

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41   This chapter presents a summary of the overall findings and identifies possible research topics to advance the state of the practice. The key findings, research gaps, and possible future research provided in this chapter are based on the literature review (Chapter 2), results of the question- naire (Chapter 3), and case example interviews (Chapter 4). The survey was completed by 42 state DOTs. Four of these state DOTs (Georgia, Iowa, Oklahoma, and South Carolina) did not rehabilitate culverts (i.e., their response to Question 4 of the survey was “No”), as shown in Figure 3-1. Therefore, the analyses of the responses to Questions 5 through 9 in Chapter 3 are based on the responses submitted by 38 state DOTs. Case examples presented in Chapter 4 were developed on the basis of the interviews of the Delaware, Maine, Oregon, and Wisconsin DOTs. 5.1 Key Findings CMP culverts are rehabilitated regularly and more frequently as compared with other cul- verts. Half of the state DOTs responding to the survey frequently rehabilitate CMPs, while only five do not rehabilitate them (Figure B-1). This finding is consistent with the expected life of culverts, which is estimated as approximately 20–50 years for CMP and ≥50 years for concrete culverts (Figure 3-3). On the other hand, thermoplastic pipes are the least-rehabilitated culverts: 71% of the state DOTs do not rehabilitate them (Figure B-4). Thermoplastic pipes are relatively new (e.g., in the states of Delaware, Maine, and Oregon). Of the 42 state DOTs responding to the survey, 25 (60%) indicated that they have an estab- lished rehabilitation program (Figure 3-2). Each state DOT has its own priorities, depending upon unique factors. For example, the Delaware DOT has a policy to address deterioration as early as possible when it is observed in larger-span metal pipes. The Delaware DOT has a large number of shallow or submerged culverts (Figure 4-3). For the Maine DOT, rehabilitation of deeper pipes is better than replacement, if possible, because culvert depth (hence, cost) is a driving factor for culverts deeper than 10 ft. For the Oregon DOT, owing to cost and impact on the traveling public, trenchless methods may be the only option for a deteriorating shallow culvert under a busy roadway. The Wisconsin DOT uses a performance-based approach that emphasizes maintenance to minimize future rehabilitation work as part of its CAMP. Survey results show that hydraulics and impact on the users of the roadway are the most critical factors affecting the selection of the rehabilitation method. According to 87% of the responding state DOTs, hydraulics is the most critical factor affecting the decision in the selection of reha- bilitation method (Figure B-7). Adoption of the Maine DOT’s culvert diffuser system to decrease hydraulic losses and increase culvert capacity (Figure 4-8) may make many culverts viable subjects for rehabilitation. Additionally, the reduced energy at the diffuser outlet limits scour and erosion. C H A P T E R 5 Summary of Findings

42 Rehabilitation of Culverts and Buried Storm Drain Pipes Impact on users is another critical factor, according to 74% of the state DOTs responding to the survey (Figure B-11). To reduce the impact on the traveling public, open-cut replacement methods are avoided. As a result, trenchless methods, including sliplining and CIPP, are the most commonly used by the state DOTs (Figures 3-6 and 3-7). Similarly, methods involving concrete paving or lining of the invert are also relatively common because of their minimal impact on the traveling public (Figure 3-5). Concrete paving or lining of the invert is one of the most common methods of rehabilitating CMP culverts. Research data, best practices, and guidance documents for these commonly used rehabilitation applications are limited. During the interviews, it was revealed that state DOTs would consider using certain rehabilitation methods if more research data and design guidelines were available (e.g., for spray-applied structural lining methods). As shown in Figure B-27, 71% of the 38 responding state DOTs have used spray-applied structural liners, but 39% of the 38 state DOTs rarely use it. Some state DOTs prefer to use selected rehabilitation methods more than others. For example, the Wisconsin DOT commonly uses sliplining and has developed specifications and require- ments for sliplining and grouting. It extended the application of sliplining methods to noncir- cular shapes (Figure 4-14). Currently, the Wisconsin DOT discourages spray-applied structural lining. The Delaware DOT, however, uses spray-applied structural lining more frequently and has developed specific requirements for installation of spray-applied liners and materials. Most state DOTs consider the ability to perform the rehabilitation work in-house to be an insignificant factor in the selection of a rehabilitation method (Figure B-12). This is an indication that most state DOTs work with contractors for culvert rehabilitation. This is consistent with the lack of response to Question 8 (about the ability of the state to perform in-house rehabilitation) by 11 state DOTs (26%), as shown in Figure 3-11. Through training (or expertise), the availabil- ity of equipment, and experience, some of the easier-to-apply and commonly used rehabilitation methods may be applied in-house. For example, sliplining applications are generally done in-house by the Maine DOT. As discussed in Section 3.3 (see also Figure B-14), the Utah and Vermont DOTs mentioned aquatic organism passage and regulatory requirements for fish passage as significant factors that affect their selection of rehabilitation method. Similarly, all of the four interviewed state DOTs (Chapter 4) indicated that fish passage has been a critical factor, especially for culverts in regions close to the coastline in Delaware, Maine, and Oregon. Similarly, protection of desig- nated salmon habitat in Maine; trout, steelhead, and sucker fish in the Klamath basin of Oregon; and the trout passage in Wisconsin is a priority. Examples of unique rehabilitation designs to address fish passage are shown in Figures 4-7, 4-10, and 4-11. 5.2 Information Gaps • The synthesis findings indicate that state DOTs could benefit from new research to develop best practices, guidance documents, policies, and standards for different rehabilitation appli- cations that would provide minimum requirements and additional guidance. Such general or nationwide guidance is especially needed for the following methods on the basis of their potential common usage by the state DOTs: CIPP (Figure 3-6), spray-applied structural lining methods (Figure B-27), sliplining methods (Figure 3-7), methods to stabilize the surrounding backfill soil (Figure B-29), end treatment methods (Figure 3-8), and trenchless replacement methods (Figure 3-9). State DOTs that participated in the synthesis were interested in learning about best prac- tices, the latest research data, and examples of successful field applications. They tend to avoid or discourage the application of new or certain established methods if their state has limited or no experience or poor performance has been observed on a previous project. For example, the

Summary of Findings 43   Wisconsin DOT reported discouraging spray-applied structural lining but intending to use it when more guidance and research results become available (Najafi et al., 2021). On the other hand, the Wisconsin DOT uses a unique sliplining method to rehabilitate elliptical or arch- shaped culverts by using smooth polymer-coated CMP (Figures 4-13 and 4-15). • CMP culverts and surrounding backfill material work as a system to resist applied loads. CMPs develop their strength through the interaction of the soil backfill with the metal struc- ture (Yeau et al., 2009). One of the identified research gaps is related to the structural capacity of CMP culverts when the backfill soil is lost through joints, holes, or perforations in the walls of the deteriorated culverts. Research is needed because, as reported by the Delaware and Oregon DOTs, there is no equation or method to quantify the effect on the remaining structural capacity of the soil–pipe system. This affects the selection of an appropriate repair or rehabilitation method. For example, foam or nonstructural injection may be used to fill in the gap behind the culvert walls if the strength is not compromised; otherwise spray-applied structural lining, sliplining, or a similar structural rehabilitation may be used (e.g., Figures 4-2 and 4-13). • Half of the state DOTs that responded to Question 7 of the survey indicated that the struc- tural capacity of the host pipe is not considered in their rehabilitation design (Figure 3-10). Follow-up interviews with the state DOTs showed that it was not easy to respond “Yes” or “No” to this question. For example, the Maine DOT does not perform structural strength calculations if no erosion or piping exists in the host pipe. Similarly, the Oregon DOT does not perform structural load rating when the invert pavement is installed (Figure 4-12). The Oregon DOT assumes full deterioration of the culvert for spray-applied lining and CIPP rehabilitation projects. The Delaware DOT requires spray-applied structural lining to have sufficient structural capacity and does not take into account the existing capacity of the dete- riorated culvert. Similarly, the Wisconsin DOT assumes full deterioration of CMP culverts and does not consider the existing structural capacity when applying sliplining (Figure 4-14). Given the different approaches and implementations employed by the state DOTs and the lack of guidance, new research and data could provide the state DOTs with the ability to quantify the existing structural capacity of deteriorated culverts. • The most commonly used structural rehabilitation methods are identified as invert paving with reinforced or unreinforced concrete materials, sliplining, spray-applied structural lining, and CIPP. The requirements and application of each of these methods vary widely from state to state. Many state DOTs prefer one or two methods over others on the basis of their past experience. Research to develop best practices, standards, and guidance documents for design and application of these methods is suggested to achieve better long-term performance. – Concrete invert paving is used mainly to rehabilitate CMP culverts when the metal near the bottom corrodes and deteriorates. Concrete may be unreinforced or reinforced with welded steel mesh, shear studs, or steel rebar (e.g., Figures 4-10 through 4-12). In addition to portland cement, different materials (e.g., fiber reinforcement) may be included in con- crete. The thickness, pavement width, reinforcement, and concrete application (e.g., spray) requirements are different for each state DOT. New research data (similar to Masada, 2017) and design guidance will benefit the state DOTs when they use the invert paving methods. – There are different spray-applied structural lining methods, materials, and equipment. Some state DOTs (e.g., Delaware) have their own specifications. Other state DOTs (e.g., Wisconsin) currently avoid using spray-applied structural lining because of a lack of research, best practices documents, or clear design guidelines. The survey results suggest that, like the Wisconsin DOT, more state DOTs are likely to use spray-applied lining methods more frequently if guidance documents (e.g., Najafi et al., 2021) can be produced through research. – Four of the 42 state DOTs responding to survey indicated that they do not rehabilitate culverts (i.e., the response of four state DOTs to Question 4 was “No”). All of the 38 state DOTs that responded to Questions 5 through 9 indicated that they use sliplining applications

44 Rehabilitation of Culverts and Buried Storm Drain Pipes (Figure 3-7). Of the 42 state DOTs that responded to Question 3, only 10 provided pub- lished documents or specifications for culvert rehabilitation or replacement. Accordingly, most state DOTs did not report or do not have specifications, best practices, or guidance docu- ments for culvert sliplining. ASTM F2984 (2019) includes specifications for a unique application involving a segmental panel system that uses a grout-in-place liner reha- bilitation method (Section 2.2). Similar limited guidance and requirements are available for sliplining and grouting (e.g., ASTM A979, 2020). However, further research and best practices documents could benefit state DOTs that are interested in rehabilitating circular and noncircular culverts (Figure 4-14) but are experiencing poor performance (e.g., due to poor grouting practices). Figures 2-1, 4-2, 4-5, 4-13, and 4-14 show a few examples of the potential placement of new pipes with respect to host pipe. In addition to geometry and interstitial spacing (space between the new and host pipes), specifications are required for the grout materials to prevent poor performance, as shown in Figure 4-5. – Research on CIPP applications and guidance documents is also needed, especially for the rehabilitation of noncircular culverts. – As shown in Figure 3-8, all state DOTs except for five (Arizona, Louisiana, Maine, Maryland, and New Jersey) use replacement methods that employ trenchless installation. State DOTs could benefit from a guidance document describing the limitations and requirements for different trenchless replacement applications. • Of the 42 responding state DOTs, 12 (29%) reported having either a statewide program or published documents for culverts after defects are identified and indicated that this guidance existed specifically for the purpose of expected service life and life-cycle costs. However, all but one of the state DOTs that reported rehabilitating culverts indicated that service life and life-cycle costs were either critical or important factors when they select the appropriate reha- bilitation method (Figure B-13). Very limited guidance exists in the state DOT specifications or documents that were provided in response to the survey. The Oregon DOT (2014) provides the most detailed information on the service life of different culverts and materials (Sec- tion 4.3). The Wisconsin DOT assumes full service of 50 years in life-cycle cost analysis. New research and guidance would help in calculating life-cycle costs for culverts and in estimating service life on the basis of the culvert type, materials, wall thickness, exposure to different cor- rosive environments, and other critical factors. • Aquatic organism or fish passage was identified as a critical factor affecting the rehabilita- tion design. Different state DOTs develop their own designs to meet the state and federal requirements (Figures 4-7, 4-10, and 4-11). New research and a guide document could pro- vide design options to meet the different needs of state DOTs.