Following catastrophic failures encountered by the original owner of the Oakland Macomb Interceptor System — the Detroit Water and Sewerage Department (DWSD) — the need for condition-based asset management was identified. At DWSD’s request, NTH Consultants, Ltd. (NTH) performed a condition assessment of the entire Oakland Macomb Interceptor System primarily through the use of closed circuit television.
The Oakland Macomb Interceptor System, which includes approximately 42 miles of sanitary sewer, provides service to 830,000 people in Oakland and Macomb Counties. The interceptor was constructed in the early to mid-1970s by DWSD. Ownership of the system was recently transferred from DWSD to the Macomb County Public Works Commissioner’s Office and an inter-county drain authority jointly owned by the Oakland County Water Resources Commissioner’s Office and the Macomb County Public Works Commissioner’s Office.
The individual interceptor components consist of inside pipe diameters ranging from 36-inches to 153-inches; inverts ranged from 20 to 110- feet below ground surface; and construction materials ranging from cast-in-place non-reinforced concrete, reinforced concrete pipe and concrete cylinder pipe. Installation methods included both tunneling and open-cut construction. The soils around the interceptor are a mixture of granular and cohesive soil, with ground water levels as much as 30 to 80-feet above the invert of the interceptor.
Over the service life of the system there have been three catastrophic failures, each of which resulted in multi-million dollar repair costs. The current investigation, evaluation and assessment program was undertaken after the most recent of these catastrophic failures.
The principal challenge to the investigation and evaluation of the interceptor system was that the system had to remain in service during the investigation. As a result, flow levels within the system during the investigation ranged from as low as six-inches to as much as 60 inches. Access to the manholes was a challenge as the alignment of the various portions of the interceptor system are located in road rights-of-way, railroad rights-of-way, through an existing high tension power corridor and easements through both public parks and private property. Work hours were adjusted to take advantage of low flow periods and, when possible, were coordinated with the flow controls used during repairs to what is now the Macomb County-only portion of the system. Work in the areas adjacent to wetlands was scheduled after periods of dry weather to minimize damage to the easements.
The investigative techniques utilized included closed circuit television (both tractor mounted and raft mounted), sonar, laser measurement, surface geophysical investigation, geotechnical investigation and man entry inspections. In addition, both chemical and atmospheric testing was performed as a part of the inspections.
Closed circuit television (CCTV) is considered the standard for the evaluation of collection systems. In the Detroit area, the use of CCTV to evaluate sewers first appeared on a regular basis in the early 1980s. The initial cameras were rigidly fixed to a skid and sometimes the hardest part of the job was getting the pull line through the sewer to pull the camera. Defects were recorded by hand on paper forms and each operator recorded what they saw in their own words. As a result, if a sewer was televised at different times by different firms, the descriptions often varied greatly. Descriptions also varied widely between members of the same firm. Because of the variances in descriptions, it was difficult to monitor change from inspection to inspection.
The Water Resources Centre (WRc) in London recognized the need to standardize descriptions. The WRc first published their Manual of Sewer Condition Classification in 1980 but it was a number of years before it reached the Detroit area. By the mid-1990s with the advent of computerized recording systems, the WRc classifications were included in the dropdown menus. In 2002, the National Association of Sewer Service Companies, in conjunction with WRc, released their Pipeline Assessment Certification Program (PACP).
PACP provides an objective system for the classification and documentation of observations during a CCTV inspection. Based on the research by the WRc, the PACP system provides a 1 to 5 rating for the defects, with 1 being the best and 5 indicating immediate action required. The PACP provides a computational methodology for establishing the average rating of a portion of a collection system and a four-digit Quick Code that lists the number of the two most severe ratings encountered using an alpha numeric code. PACP is subdivided into four areas; Structural Defect Coding, Operational and Maintenance, Construction Features Coding and Miscellaneous Features Coding. Each of these four areas is in turn divided into multiple subsections. For example, the Structural Coding is divided into cracks, fractures, broken, hole, deformed, collapse, joint, surface damage, buckling, lining failure, weld failure, point repair and brickwork. Certified users are provided with a complete reference manual and a key which lists codes by family and provides a page reference guide.
A series of Condition Inspection and Evaluations were performed on the Oakland Macomb Interceptor System in 2007 and 2008. Within the lower portions of the Oakland Macomb System, the CCTV was supplemented with surface seismic testing using the multi-channel analysis of surface waves (MASW) technique. In addition the CCTV in the lowest portion of the system was also supplemented with sonar testing to identify debris along the invert of the interceptor.
The reports for these inspections and recommendations for remedial measures were issued in January 2008 and August 2008. Based on these reports, a program was developed that addressed the defects with PACP grades of 4 or 5. The initial phase of this defect restoration program was to construct flow control gates to provide in-line storage such that restoration could be performed while portions of the interceptor were in a dewatered condition.
As the initial phase of the restoration was concluding, the Edison Corridor was re-inspected using a combination of CCTV, laser measurement and sonar. The need for the re-inspection and proposed schedule for this work was identified in the 2008 reports. In addition, suspect areas identified by the MASW testing were investigated with a series of test borings. The CCTV data was again recorded using PACP and compared to the reports from the initial inspections. The data from the current inspection indicated that the deterioration had advanced from the 2007/2008 condition investigations. Areas where the coarse aggregate was exposed in the previous studies had lost additional cement paste due to microbial induced corrosion (MIC) such that the coarse aggregate was now projecting. In areas where the coarse aggregate was projecting in the previous condition survey now exhibited aggregate lost. When comparing the reports for the 2007/2008 reports to the present investigation, it was also clear that the distressed areas had expanded in size. Due to the increase in distressed areas, an adjustment to future repair program component was warranted
Without the use of the standardization of PACP, a direct comparison of the CCTV data between inspections would have been difficult. With the ability to directly compare the data, it allows for a data based adjustment to the proposed restoration program. Further, the comparison allows the project owner to understand the necessity for the program adjustments. The laser, sonar and other supplemental testing in conjunction with the CCTV data, facilitates the development of restoration drawings.
In summary, the NASSCO’s Pipeline Assessment and Certification Program is a useful tool in condition based asset management. The use of PACP’s standardized descriptions allows the rate of change in distress to be monitored. Further, NASSCO has also undertaken the approval of the software used to record the condition survey data; thereby assuring that regardless of who provides the program used to record the data, if it is NASSCO approved it can be read and imported into another manufacturer’s approved software program. This ability to transfer data between programs is critical when the CCTV records are prepared by different providers, each of which may be using different data management software to record their data.
ABOUT THE AUTHORS:
Harry R. Price, P.E., F. SAME, Principal Engineer, NTH Consultants, email@example.com, (313) 237-3935
John R. Kosnak, P.E.,M. SAME, Senior Project Engineer, NTH Consultants, Jkosnak@nthconsultants.com
FOR MORE INFO:
NTH Consultants Ltd., Detroit, MI, (800) 736- 6842, http://nthconsultants.com