The paper also presents suggested standards for defect code infiltration and inflow (I/I) rate assignment. Defect I/I rates are necessary for conducting a full cost-effectiveness analysis. Standard defect code I/I rates are only partially described in the current data collection industry standards. Specifically this paper will address the following:

• Standard defect coding industry practices.
• Risk-based approach to condition data analysis.
• Suggested standards or guidelines for assignment of defect I/I rates.

Introduction

Infrastructure owners are expected to know the condition of their systems and have processes that support their program activities. For instance, the regulatory community generally interprets the Clean Water Act’s National Pollution Discharge Elimination System’s provisional requirement of “properly operate and maintain” to include owner’s knowledge of condition and performance capabilities of the system. Without condition and performance knowledge appropriate preventative measures cannot be properly planned and implemented. In addition, regulators are expanding their push for better condition assessment practices through programs such as Capacity, Management, Operations, and Maintenance and in enforcement actions at both the state and federal levels.

Wastewater collection systems are a major capital investment and utilities must ensure they are properly maintained if they are to perform as desired. Pipelines can account for the largest percentage of capital investment and a significant portion of the operating cost investment of an annual utility budget.

Effective condition assessment and data analysis are important to ensure that a pipeline will provide for effective and continuous service. In order to fully leverage condition assessment data, it is important that analysis of the condition assessment data consider the risk of failure of each asset and prioritise asset needs and any reinvestment on the most critical needs first.

Significant advancements have occurred in technology used to collect asset condition data. These advancements include improved CCTV cameras, digital imaging, laser profiling, sonar profiling and collection of other environmental factors such as hydrogen sulfide concentrations, as well as improved data collection software applications. Improvements have also been made regarding industry standards for data collection and defect coding for both manholes and pipelines. National Association of Sewer Service Companies’ (NASSCO) Pipeline Assessment and Certification Program (PACP) coding for pipelines, Manhole Assessment and Certification Program (MACP) for manholes, and American Society of Civil Engineers’ (ASCE) Manhole Inspection and Rehabilitation Manual No. 92 provide standard methods for collecting the condition of pipes and manholes and building laterals.

The analysis and decision-making process using the condition data (including hydraulic data) is a critical aspect of the condition assessment process and one that requires a structured, yet flexible approach to meet the unique needs of each utility owner. Translating the condition data into actionable items requires analysis of condition data into failure probabilities and consequences so that risk can be estimated and overall priorities be established. Risk should consider structural, maintenance and I/I condition of each asset.

City of Rochester – studying the sewer

The City of Rochester, located in Minnesota, sanitary sewer system is comprised of 505.8 miles of sewerage networks with over 488,320 ft of trunk system, 12 inches and larger, and serves 38,755 acres. The focus of this study includes about 140,000 ft of public sewer system that is mostly small diameter pipe of 8 and 10 inches in diameter contained in two neighbourhoods, namely Kutzky Park and Slatterly Park (study area). These areas were being studied due to sewer backups experienced during severe storm events.

The purpose of this pilot I/I investigation study was to develop alternatives to mitigate the city’s wet weather sewer backups in the study area; maintain the constructed pipe service capacity of the collection system by reducing wet weather flow; provide consultation for private sector I/I reduction; and, develop processes and procedures to evaluate the condition of the collection system. The pilot I/I study is to result in a collection system improvement plan for the study area. In addition, the study will provide the city with a process, procedures and tools that can be used to assess condition of the rest of its sewer collection system.

The following activities were necessary to conduct the evaluations:

• Field inspections – comprehensive manhole inspection and internal CCTV inspections were completed for every manhole and line segment in the study area. The manhole inspections were performed using ASCE Manual No. 92 protocols and CCTV inspections were performed using NASCCO PACP protocols. Each field form was reviewed to ensure that the field data was complete and free from obvious errors or inaccuracies. Data was also reviewed to ensure correlation with the city’s GIS and related data bases, and all field data will be checked to ensure correlation with the hydraulic network. Data from the field inspection was entered into an electronic database for storage and processing.
• Analyse field data – field inspection data was analysed and summarised. The defects identified were flow quantified using observed or estimated I/I rates as a basis. The defects were prioritised based on the ratio of repair cost to estimated I/I removed (dollar per gallon per day) and based on a risk evaluation. It should be recognised that the sewer main inspections and manhole inspections were part of a larger inspection program that also included inspection of building sewer connections, building service laterals, and smoke and dyed water testing of the public sewer mains. The discussion of these tests and results is beyond the scope of this paper.

Condition assessment data

Condition assessment data provides information on the physical state of pipes and manholes in a system. In most cases, condition data will be available for only part of the system and findings and conclusions from the data will need to be drawn for all pipes. For this study, all pipes and manholes were investigated. There are many approaches for categorising condition assessment data. Most approaches include an evaluation of the number and magnitude of defects or breaks per length of pipe and recording of manhole defects. Some approaches also include scores to indicate defect severity that help in prioritising pipes and manholes from worst to best. Regular collection of condition assessment data, including asset failure data, is an essential part of system maintenance planning, prioritisation and funding.

All defects identified during field inspections were compiled into a master Microsoft Access Database called the Master Defect Database. This database lists each defect by asset and the condition rating or defect observation.

This database was used to summarise all defects, flow quantify these defects, and assign unit costs for repair. The database presents a detailed source-by-source defect printout showing the defect, defect location, estimated flow rate, rehabilitation construction cost and the cost/flow ratio. The cost/flow ratio is the rehabilitation cost divided by the estimated flow removed from the system once the defect is fixed. The defect source list can be sorted by this ratio resulting in prioritisation of the most cost-effective sources to remove at the top of the list and the least cost-effective sources at the bottom of the list. The Master Defect Database and the estimated unit I/I flow rates and unit repair costs used are presented in subsequent sections.

One of the challenges of prioritising defects is the estimate of I/I flow rates and the corresponding rehabilitation costs by defect. For manholes, ASCE MOP No. 92 was used as a guide for flow rates. For main sewer pipes, I/I flow from each defect type was estimated based on the severity of the defect using PACP observation coding and the monitored flow. The initial flow rate estimates for manholes by defect type is shown in Table 1. The initial flow rate estimate for pipes by defect type is shown in Table 2. It should be noted that these rates were adjusted up or down by subsystem depending on the actual flow monitoring results obtained from in-pipe flow meters.

Manhole prioritisation

The prioritisation process for the manholes inspected in the pilot study area utilised the I/I quantification results, condition findings and the location of the manhole. The condition rating for the manhole was converted to a numeric score. Table 4 shows the score assignment for each condition rating. Then the total score per manhole was calculated by summing the scores for each component of the manhole. Second, the total I/I flow per manhole was calculated based on condition and standard ASCE MOP No. 92 rates. A total score was determined by the product of flow rate and condition score and ranked accordingly.

Following development of the overall score by manhole, criteria were established to develop a preliminary rehabilitation schedule. The criteria were coded and the defect database queried resulting in the preliminary rehabilitation items. The majority of the rehabilitation items are in the upper part of the manhole.

Sewer main pipe evaluation and prioritisation process

The prioritisation process for the public sewer mains inspected utilised the structural and maintenance condition findings, hydraulic analysis results and other system characteristics in a risk-based approach. First, asset scores were determined for each sewer main inspected – one score for maintenance, one for structural and one for I/I. Next, the scores are used to identify improvement action items for each defective public sewer main. These action items were then used to develop a preliminary pipe rehabilitation schedule and pipe maintenance program to identify initial improvement activities for the sewer mains in the study area. To prioritise and rank the assets, a risk-based analysis utilising factors for likelihood of failure and consequence of failure was performed.

The PACP observation codes used to record the condition of the public sewer mains were associated with grades on a 1–5 scale. A detailed list of the score assignment for each PACP observation code was developed. Each code was also grouped into three categories – structural, maintenance, and I/I – to represent the type of observation depicted. Certain observation codes were flagged as a major repair item which requires repair regardless of the condition of the rest of the public sewer main segment inspected. Codes where a repair flag was assigned include the following defects: broken, hole, collapsed and defective service lateral tap.

To score the public sewer mains – each observation code was assigned a score on the 1–5 scale and the sum of the assigned scores was calculated. This total raw score was then normalised per a 100 ft of televised length basis in order to provide perspective on the relative scores of all the public sewer main assets inspected. A total score and a normalised score was developed the three categories of observation type – structural, maintenance, and I/I.

To assess the structural condition category of each public sewer pipe segment, the scores were ranked to determine the range of scores for each condition category. The categories selected included, good, moderate, fair, poor and deteriorated. Based on the structural scores, the public sewer mains were assigned to one of the condition categories. This information was used in the asset prioritisation process.

Improvement action items

Based on the score of the public sewer main segment, an improvement action item was defined including – no action, targeted maintenance, structural repair/rehabilitation or hydraulic relief. The three improvement action items are discrete actions, meaning that a public sewer main segment could potentially be designated for all three actions – cleaning, repair and relief. The hydraulic relief item is identified based on the results of the hydraulic analysis of the existing sanitary sewer system in the pilot study area for the ten year design event. The peak flow (Qpeak) simulated by the hydraulic model and the design flow (Qdesign) calculated based on pipe characteristics are used to develop the utilisation ratio for the pipe. The ratio of 125 per cent was selected to represent the maximum flow limit. Ratios over 125 per cent represent sewer system bottlenecks requiring critical relief action.

Table 5 summarises the criteria for assigning improvement action items.

Each public sewer main was evaluated based on these criteria, and a preliminary list of improvement projects was developed. This represents the preliminary improvements recommended for the public sewer mains, lists the projects with the pipe segment ranking, and will be incorporated into the overall system improvement plan for the study area.

Risk-based analysis

After the pipes were scored based on condition, and assigned an improvement action item, the pipe segments were prioritised using a risk-based approach. The prioritisation factors reflect likelihood of failure (LOF) and consequence of failure (COF) of the asset. Using these factors, the risk of failure was calculated and utilised to rank the pipe segments as shown in below:

Risk = LOF * COF

Likelihood of failure analysis

The likelihood of failure factors reflect ways in which the asset might fail. Structural condition, maintenance condition and hydraulic capacity were identified as the performance parameters for pipes. The structural and maintenance condition are determined based on the pipe segment scoring described previously. The hydraulic capacity was determined based on the ten year design rain event.

These parameters are weighted based on city input. Next, the likelihood of failure factors was applied using a 1–5 scoring scale.

Consequence of failure analysis

The consequence of failure factors express the implications associated with pipe failure. For this project, the consequences were associated with the following:

• Major users – this represents the impact on service interruption for major users such as hospitals, school and industries. The criterion for this factor is defined as a public sewer main segment which is within five segments, or about 1,000 ft, downstream of a major user.
• Community and environmental impact – this represents the impact associated with community health, safety and perception, as well as environmental protection requirements. The criterion for this factor is defined as a public sewer main segment which is within the confirmed area of sewer backups into basements, at or near a creek crossing, or both
.
• Service area – this represents the impact associated with service interruption throughout the city as the area served increases. The criterion for this factor is defined based on the diameter of the public sewer main segment.
• Constructability – this represents the difficulty associated with constructing the replacement or relief public sewer main segment should a failure occur. The criterion for this factor is based on land use, traffic and geology associated with the particular public sewer main segment.
• Critical crossings – this represents the construction issues associated with replacing or installing a relief line at critical crossings such as utilities, creek or river, railroad and major thoroughfares. There are issues related to design considerations, permitting, jurisdiction, coordination and other matters for each type of crossing. The criterion for this factor is based on whether a public sewer main is at a critical crossing and which type.

Risk of failure

After the LOF and COF were assigned for each pipe, the weighted sum of both factors was calculated, and the product of the two was determined. This calculated value represents the risk of failure for the pipe. This risk value is used to sort all the inspected pipes and develop a prioritised list. This list will be used with the preliminary improvement lists to develop the overall system improvement plan. The list also can be used to develop a prioritised list for a targeted maintenance program.

Conclusions

An effective condition assessment of manholes and pipelines includes not only a quality program to safely collect condition data but also the tools and processes to effectively process the information into a meaningful format. For I/I studies this includes estimates of I/I rates by defect, by asset category and by sub-system. For pipe and manhole rehabilitation programs, the risk-based approach is best to properly prioritise improvements. Maintenance programs can greatly benefit by using a risk-`based analysis for prioritising where maintenance efforts and funds will be best spent.

Author details

1 Director of Public Works, Rochester, MN 2 Project Manager, Rochester, MN 3 Director Conveyance Technology, CH2M HILL, Kansas City, M 4 Principal Technologist, CH2M HILL, Montgomery, AL 5 Project Technologist, CH2M HILL, Kansas City, MO 6 Project Engineer, CH2M HILL, Kansas City, MO

Acknowledgements

The authors wish to acknowledge the work of the City of Rochester staff, CH2M HILL and WHKS & Co. team members whose contributions were essential to this study. In addition, the City of Rochester is thanked for allowing use of the information contained herein.