Maintaining Performance and Managing Risk of a Large-Diameter, Aging, Combined Sewer
Abstract:The greater Cleveland metropolitan area contains combined sewers dating over one hundred years old. The underground infrastructure is aging, and inspection and maintenance of the system is critical. The Northeast Ohio Regional Sewer District (District) currently owns and maintains the major conveyance sewers in greater Cleveland and recognizes the importance of maintaining older sewer systems to protect the public, prolong service life, and maintain a functional sewer system. This paper discusses a project which involved inspection, cleaning, rehabilitation, and development of a long-term operations and maintenance (O&M) strategy for one of the District's greatest sewer assets – the Easterly Interceptor.
The District owns and maintains the Easterly Interceptor, which is a large-diameter interceptor that begins in downtown Cleveland, flows northeast, and discharges into the Easterly Wastewater Treatment Plant. The Interceptor is a combined sewer, receiving storm and sanitary sewer flows from local sewers throughout greater Cleveland. The Interceptor is over 100 years old, ranges in diameter from 8′-0″ to 13′-6″, and is greater than 8 miles in length. Approximately 58% of the length was constructed with brick, and the remaining length was constructed of reinforced concrete with a shale brick internal wearing course.
The existing and future flow conditions, the history of settled deposits accumulation, and the location of the sewer are just a few factors leading to the evaluation of the Easterly Interceptor. Upstream sections are largely over-sized causing the dry weather flow velocity to be less than self cleansing and resulting in settled deposits. The District is implementing a system-wide facilities plan to drastically reduce combined sewer overflows (CSOs), and therefore, the hydraulic conditions of the Interceptor will change, which increases the need for assessing the current flow condition and optimizing performance to lead into the future. Finally, the Interceptor is a risky asset; it runs through downtown Cleveland, underneath buildings, and under major traffic corridors.
The project included developing short- and long-term solutions for the operation of the Interceptor with a four-phased approach. Phases I and II involved performing field inspections and evaluating the inspection information for settled debris accumulation. These phases included the following tasks:
Inspect sewer using the National Association of Sewer Service Companies (NASSCO) Pipeline Assessment Certification Program (PACP©)
Evaluate hydraulic performance and grit deposition
Identify cleaning requirements
Evaluate cleaning alternatives
Develop bid documents for cleaning work
Create baseline drawings
The District has previously cleaned upstream reaches of the Easterly Interceptor and wanted to identify the amount of settled deposits that had re-accumulated since those cleanings. The District also wanted to identify if the downstream reaches required cleaning in order to maintain optimal operational performance. The inspections confirmed that the upstream sections of the sewer have excess hydraulic capacity, causing low velocities and the accumulation of significant volumes of settled deposits. The inspections identified that the downstream sections have sufficient flow velocity to keep the sewer self-cleansing. Approximately 1.8 miles of sewer located downstream of the previous cleaning work did have significant volumes of settled deposits that required removal in order to maintain hydraulic capacity. Since the time of this project, the cleaning contract has been awarded and the cleaning work is currently ongoing. Approximately 3,300 cubic yards of debris is estimated to be settled within the identified cleaning limits, and the sewer is being cleaned such that 95% of the cross-sectional area is available for flow. The paper discusses the cleaning techniques, construction issues, and the steps taken during design to reduce potential construction claims.
During Phase III, the inspection information was evaluated for the operational and structural condition of the sewer. The tasks included the following:
Assess operational and structural condition based on inspection data
Identify rehabilitation requirements
The structural and operational defects were evaluated using a risk analysis approach. The major sewer defects and the surrounding conditions were evaluated to determine the risk of each sewer defect, and therefore, the rate of failure. One section of the sewer was identified as requiring replacement. All of the other defects were grouped as lower risk and consist of local repairs and internal linings. The repair of these defects is currently under design. The baseline drawings were created to serve as a record for District regarding the condition of the Easterly Interceptor at the time of those inspections. Plan and profile drawings were created that display the sewer inspection information.
The final phase, Phase IV, involved evaluating the existing operations of the Easterly Interceptor and evaluating the current maintenance practices of the District. A long-term operational & maintenance (O&M) strategy was prepared to lay a framework for future maintenance and inspection work. This strategy includes grit management, an inspection schedule, and a maintenance plan for the Easterly Interceptor and its manholes, as well as for the surrounding structures that directly influence the Easterly Interceptor.
This project encompasses a complete asset management strategy of one of the District's largest sewer assets, including inspection, evaluation, and implementation of rehabilitation. This project involved multiple facets of sewer maintenance and evaluation. Professionals in the sewer industry will benefit from learning about this project, regardless of whether their particular interest is sewer cleaning, hydraulic assessment, structural assessment, evaluating risk, or long-term O&M strategy.
Document Type: Research Article
Publication date: January 1, 2008
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