Fracture Failure of Large Diameter Cast Iron Water Mains [Project #4035]

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1 Fracture Failure of Large Diameter Cast Iron Water Mains [Project #4035] ORDER NUMBER: 4035 DATE AVAILABLE: July 2013 PRINCIPAL INVESTIGATORS: Balvant Rajani and Yehuda Kleiner BACKGROUND Large diameter transmission (trunk) mains are an essential and critical component of the water supply system. Their risk of failure can be defined as the expected magnitude of the consequences of failure, which is a function of both the probability of occurrence and the probability of the consequences of failure. The failure consequences of large diameter mains typically include service interruption to a large community, direct costs (breakage repair, property damage, lost water, liabilities), indirect costs (loss of production, deterioration of roads, damage to adjacent utilities, interruption of traffic), and social costs (discomfort, illness, service disruption, traffic, and business disruptions). This large scale of failure consequences dictates an asset management strategy that is based on failure prevention in contrast with failure frequency management that is prevalent in small diameter distribution mains, where failure consequences are much smaller. The definition of a large diameter (trunk or transmission) water main is invariably relative to the size of the distribution network at hand. Some large water utilities consider as large pipes that are at least 24 (600 mm) and even 30 (750 mm) in diameter. Other utilities tend to emphasize pipe functionality (no service connections) in the definition and thus might include 16 (400 mm) and even smaller diameter pipes in the definition. Some utilities distinguish between primary and secondary trunk mains, depending on whether there are service connections. Distribution (small-diameter) mains typically have diameters less than ( mm) and have service connections. The structural response of small-diameter pipes to typical external loading conditions leads to predominantly longitudinal stresses, which leads to circular (circumferential) breaks while the response of large (>12, 300 mm) diameter pipes subject to the similar loading conditions leads to predominantly in-plane stresses, which leads to mainly longitudinal fractures or split failures. This report addresses the structural performance of the latter pipe sizes.

2 OBJECTIVES Over the past few years, several efforts have been undertaken to understand the causes of failure of cast iron mains. However, few of these studies have focused on large diameter cast iron (CI) mains. This report addresses specific issues deemed important to improve our understanding of the failures of large diameter CI mains since their failure can be (and often is) catastrophic. The broad objective of this research was to determine and assess mechanisms of fracture failure of large diameter CI water mains. The principal sub-objectives of this study were: To review the history of design, installation, and manufacturing standards for large diameter cast iron mains in the United States and Canada; To develop a protocol for forensic investigations in the field at the time of failure and failure analysis of large diameter cast iron mains; To demonstrate how the collection and analysis of data on inventory, vintage, size, past failures, etc. can improve the understanding of the condition of pipe inventory; To develop a model that characterizes lead caulked bell-spigot joints to gain insight on their behavior under different loading scenarios; To apply mechanics of fracture failure to garner insight into crack initiation and propagation in old, large diameter CI pipes; and To comment on features in large diameter CI mains that non-destructive inspection techniques should be able to detect and/or identify. APPROACH Historical review of design, installation, and manufacturing standards for cast iron pipes Historical information on the production and installation of cast iron pipes has been reviewed with special emphasis on how the understanding of historical conditions and practices can help address problems and issues faced by many water utilities today. Lessons from many documented anecdotal experiences and from experimental and field studies can be summarized as follows: Cast iron pipes have non-uniform wall thickness across the pipe cross section or along pipe segment length whatever the manufacturing process (although wall thickness was more uniform in later vintages than in earlier ones). Recognition that only certain pipe sizes were produced by certain casting processes at certain segment lengths can help to correctly identify pipe inventory data (vintage, material type) that are missing or otherwise erroneous. It is possible that some of the bells cracked during handling or caulking and that these cracks went undetected at the time of installation. These incipient cracks

3 could manifest in sudden failure (no visible deterioration) a long time after installation. Some water utilities have no choice but to repair some existing bell-spigot lead joints. In these cases, trained crews should carry out this repair since excessive and inappropriate hammering and caulking process can damage the bell that was not previously damaged or can enlarge/lengthen a previously existing incipient crack. If cast iron mains are found to be laid on wooden blocks or resting on isolated rocks or stones during routine repair activities, then steps should be taken to remediate this condition. Pipes laid on wooden blocks or resting on rocks have a higher risk of failure. Pipes thus laid become especially vulnerable when subjected to high external vertical loads, such as heavy traffic. Review of engineering properties of cast iron mains Engineering properties obtained from different sources such as standards, experimental tests, and earlier studies were thoroughly reviewed and collated. Some of these properties that are inexpensive and easy to obtain were correlated with other properties that are important but difficult and expensive to obtain. These correlations provide an alternative means to obtain preliminary estimates of engineering properties of cast iron. Engineering mechanical properties such as tensile strength and rupture modulus (determined from specimens taken from cast iron pipes) can vary significantly from those quoted in standards and guidelines. In general, the properties can vary significantly even within a single pipe segment (both circumferentially and longitudinally). This fact together with variations in pipe wall thicknesses will lead to a range of estimates for the pipe structural capacity of an undamaged pipe. Therefore, it is vital to consider uncertainties when an assessment is sought to determine the margin of safety of cast iron mains. Protocol to examine failure surfaces and identify underlying failure mechanism(s) A practical protocol was developed to collect and collate pertinent data from failures of large diameter cast iron mains. It is suggested that the creation of one North American database where all information on the failure of large diameter cast iron is collated may prove useful for the water industry, especially because failures of large pipes are not as frequent as for small diameter pipes. The protocol for investigation was broken down into four principal stages of information gathering: (1) basic information on failed pipe (pipe size, pipe age, etc.), site features, (road type, traffic loading, etc.), excavation details (bedding details, soil type, soil properties, etc.), and weather and operational details (minimum and maximum temperatures records prior to the event, valve closure, etc.), (2) identification of breakage patterns, (3) fractography, and (4) metallography and chemical analysis. Of course, not all large diameter cast iron mains merit investigation at all four stages. The first two stages are deemed important for a good understanding of the pipe failure and are relatively inexpensive to conduct. The last two stages are costly and therefore optional

4 and the decision to undertake them should be based on engineering judgment applied on a case-by-case basis. Fractography, which involves the inspection and examination of twin or mating fracture surfaces, is not typically conducted with any rigor. In many instances, focus or emphasis on a particular pipe failure investigation is on one specific aspect and other issues are not addressed with equal rigor, if at all. This inequitable focus is strongly discouraged, as supporting information that could be important and relevant can be easily missed. Statistical analyses of repair event data of large cast iron pipes Inventory and break record databases for large diameter cast iron pipes from three utilities were examined and analyzed. Rudimentary analyses showed that it is possible to gain a preliminary view on the performance of large diameter cast iron mains within a water supply system. The visualization of main breaks on digital maps can offer insight on how traffic patterns, and surficial soil types influence the performance of large diameter cast iron mains. While most utilities collect the same basic data, analyses showed that additional insight can be gained on breakage patterns if the utilities collect additional data such as break location (street address); joint type identification (even if the break was not at the joint); presence of corrosion; historical changes in pressure regime during the life of the pipe; type and date of any rehabilitation, e.g., internal cement lining, etc. Behavior of bell-spigot joints and crack initiation and propagation Two types of failure modes are commonly observed in large diameter cast iron mains: bell split (bell shard) and longitudinal failure. These two types of failures often occur suddenly and without much prior warning in pipes that have had many years of service free of serious problems. Two independent models were developed to explain these two types of failure modes. A moment-rotation relationships model was developed using data from experiments conducted by Prior (1935) and these relationships were then used to determine the response of two or more segments of cast iron pipes subjected to vertical loads and ground movement. This model can identify the threshold rotation that leads to joint leaks and the maximum rotation that bell-spigot joint is able to tolerate before failure. This model was then used to explain the circumstances under which the bell split failure may have occurred in Cleveland in March Sudden occurrences of longitudinal failure and bell splits are simulated through the application of fracture mechanics and fatigue analysis. A cast iron main can only undergo fatigue failure if the pipe has existing cracks or voids. Even if the cast iron mains are free of casting defects, cracks could still have been introduced during loading/unloading activities or during lead caulking of the bell-spigot joint, with a high likelihood of not being noticed on installation and commissioning. Fatigue analysis showed that long fatigue life is assured if an existing crack size is below a certain limit (critical crack length) and the cast iron main is operating under conditions that generate low stresses. However, the nature of the fatigue analysis is based on considerable empirical relationships and, wherever possible, physical evidence should

5 be sought to confirm fatigue failure. It is recognized that this exercise is not easy and not always possible to pursue rigorously. CASE STUDY A 30 diameter pit cast iron main installed in 1880 in Cleveland, Ohio, failed on March 6, A detailed failure analysis of bell split was undertaken and partitioned into three distinct parts, each of which considered a specific failure mechanism. These parts were: (a) operational load analysis or static analysis, (b) joint analysis due to ground movement (settlement), and (c) fatigue analysis. Different plausible scenarios were considered to explain the bell-split failure of the 30 diameter cast iron main installed in Cleveland in Operational load analysis clearly showed that while the factor of safety is significantly reduced in the presence of the concrete vault, the margin of safety is still sufficient for the mains to have performed safely. Ground movement analysis of jointed pipes showed that the construction of the concrete vault above the joint of the 30 cast iron main made the situation worse by subjecting the joint to additional rotation. A crack may have developed in the bell had this additional rotation been large enough. Under this circumstance, two failure scenarios are possible; namely, (a) additional rotation was large enough to cause the bell to split or (b) additional rotation was sufficient to induce a minor crack in the bell but fatigue (repeated) loading caused the crack to grow uncontrollably to cause the bell to eventually split. As noted previously, Lewandowski (2009) asserted that color differences along the fracture surfaces indicated that the fracture occurred in two distinct stages. This observation suggests that the second scenario is more plausible than the first. RESULTS/CONCLUSIONS This research addressed important issues that pertain to the management of aging large diameter CI mains. A protocol to collect significant details on failures was developed. Creation and sharing of data in the form of national or international databases of pipe failures could be beneficial for water utilities to identify and to compare causes of these catastrophic failures. Rudimentary statistical analyses of break and pipe inventory data from three water utilities showed that it is possible to gain a reasonable perspective on the performance of large diameter cast iron mains within a water supply system. While most utilities collect the same basic data, analyses showed that additional insights can be gained on breakage patterns if the utilities collect more data than they currently do. Two models were developed to explain commonly observed failure types of bell split (or bell shard) and longitudinal failure in large diameter CI mains. Based on these models, a few insights could be gained: Removing asphalting coating from the spigot when repairing a joint will result in lower tolerance to rotation and consequently increased failure likelihood. The likelihood that a larger pipe fails due to rotation is much higher than that of a smaller pipe when buried in the same bedding type.

6 Fatigue failure requires the pipe to have an initial small crack and a continual exposure to cyclical loads. Initial cracks can be a result of manufacturing defects (voids, etc.) or mishandling during transportation, and/or installation. Fatigue failure analysis involves considerable empirical relationships and should be undertaken with engineering judgment and verified as much as possible using physical evidence. The application of these models to analyze a failure that occurred in Cleveland, Ohio, is described. Forensic analyses of field failures with the aid of models developed here will lead towards an improvement in our understanding why and how these failures occur in large diameter cast iron mains.