Managing the Useful Life of Your Segmented Block Combined Sewer Dianne M Sumego, P.E. Black & Veatch ABSTRACT Following two road collapses, as a result of deterioration within the City s segmented block combined sewer system, the City needed to understand the condition, useful life and potential risk of their combined sewer system. Black & Veatch was retained by the City to perform a detailed investigation and condition assessment of 40,000 lineal feet of their segmented block combined sewer system. The goal of this study was to understand the condition and associated risk to the city in this identified area. KEYWORDS Planning, Condition Assessment, Risk Analysis INTRODUCTION One of the biggest challenges our industry is facing is finding resources, both financially and staffing, to maintain our underground infrastructure. Across the globe these combined sewer infrastructure systems were installed 80 plus years ago and have already surpassed their useful life. However, what defines useful life and what are the factors impacting the determination of useful life? Each sewer system varies and with that factors impacting useful like include soil conditions, loading, age and make of pipe material, composition of wastewater, and operation and maintenance to name a few. The challenge, as mentioned above, is having the available resources to assess and maintain this critical infrastructure on a regular basis. The City of Lima s (City) segmented block sewers are 70 years or older and like most communities have had minimal inspection and cleaning over the years. To add to the challenge, over the past several years the City has experienced a couple of collapsed sewers within their segmented block sewer system with the most recent collapse occurring in 2012, requiring an emergency repair to the sewer. Both collapse ended up with cars in the hole with passengers. Fortunately no one was hurt. These occurrences elevated the need for the city to understand not only the condition but what changes in their O&M should take place to extend the useful life of their segmented block combined sewer system. BACKGROUND In 2014 Black & Veatch in cooperation with Terra Contracting Services undertook the task of inspecting approximately 40,000 feet of mainline segmented block sewers ranging in size from 24-inch to 102-inches and laterals. The inspection areas were identified by the City in maps provided to Black & Veatch and included only segmented block sewers. These segmented block sewers were divided into thirteen (13) inspection areas for planning of the work and are shown on Figure 1. 462
Figure 1: Inspection Areas 463
These combined sewers react quickly to storm conditions and therefore fill quickly once a storm event occurs. The impact time is minimal and during storm events portions of this combined sewer are surcharged creating internally forces on the sewer system. Segmented block sewers were used for construction during the transition from brick to pre-cast pipe and were made of terra cotta and approximately 18 x 9 in size. Segmented block sewers are double walled with the exterior wall serving as the structural support for the sewer. These tiles are staggered and interlocked and failure typical occurs at the spring line of the pipe. When failure occurs in a segmented block sewer, typically it is not one tile that fails but many tiles that fall allowing the external wall to be exposed. These older systems have lateral connections and many were cut in during construction. This construction methodology weakens these points within the system and often creates points of failure. When performing a condition assessment of these sewers the NASSCo coding system was applied but some interpretation was required due to the type of materials used. INSPECTION OF COMBINED SEWER AND LATERALS The inspection was conducted by technicians certified by the National Association of Sewer Service Companies (NASSCO). The technicians used the NASSCO standard rating of structural defects and operation and maintenance defects using the Pipeline Assessment Certification Program (PACP). The code for PACP identifies defects in two categories, structural and operation and maintenance (O&M). The structural defects include cracks, broken pipe, surface damage and holes and are identified with an S followed by a rating score. The O&M defects include deposits, infiltration/inflow, roots, obstacles and vermin and are identified with an M followed by a rating score. The rating scores are based on a scale of 1 to 5 and are summarized in Table 1. The term failure is considered to mean the pipe is no longer able to furnish the expected level of service. These guidelines are general in nature and give the technician leeway to interpret the results based on their experience and provide a basis for consistency within the inspection. Table 1: Overview of NASSCO Grading Pipeline Grade Classification General Description Guideline Relating to Failure 5 Immediate Attention Defects requiring Has failed or will likely fail immediate attention within 5 years 4 Poor Severe defects that will Pipe will probably fail in 5 to become grade 5 within the 10 years foreseeable future 3 Fair Moderate defects that will Pipe may fail in 10 to 20 years continue to deteriorate 2 Good Defects that have not Pipe unlikely to fail for at 1 Excellent begun to deteriorate Minor defects least 20 years Failure unlikely in the foreseeable future 464
INSPECTION METHODOLOGY AND PHOTOS Pipelines less than 72-inches were inspected with CCTV mounted on a robotic platform, shown in Photo 1. This is an IBAK CCTV unit with an Orpheus Camera and 1,000 feet of cable. Pipe diameters 72-inch and larger were inspected with an IBak Man- Cam system. The laterals on pipe with a diameter less than 42-inches were inspected by a side launch CCTV unit and laterals 42-inch and larger were inspected by manned entry into the pipeline and pictures taken of the connection and inside the lateral as described above. Photos 2 and 3 below are representative of condition observed in the mainline sewer and Photos 4 and 5 are those of the lateral connections. Photo 1: IBAK Man -CAM Photo 2: Missing Tiles at Spring Line of Main Line Sewer Photo 3: Missing Tiles at Spring Line of Main Line Sewer Photo 4: Collapsed Lateral Connection Photo 5: Collapsed Lateral Connection 465
RECOMMENDATON FOR FUTURE PREVENTATIVE OPERATION AND MAINTENANCE Based on our physical inspection and review of the results of the CCTV inspection, we can see a correlation between the structural deficiencies and the build-up of debris in the segments and break in installation of taps for the laterals. The pipe is typically failing along the spring line of the pipe and in many cases is initiated at the break in taps for the laterals. As the pipe wall fails large pieces of segmented block become obstacles that collect debris creating a dam affect in the flow. Due to the age of the pipe and the amount of debris build up in the segmented block sewer system Black & Veatch recommended a rigorous cleaning and inspection schedule to be performed over the next three years to track any changes to the infrastructure and to lengthen the useful life through preventative maintenance. RISK ASSESSMENT EVALUATIONS Risk-based capital spending prioritization is one of the core components of good asset management practice. By evaluating the consequence and likelihood of failure for each pipe section, limited available capital and operational budgets can be directed to those projects and activities that will give the greatest reduction in risk for each dollar spent. (CoF): The failure of large diameter sewers like those inspected as part of this project can have serious negative impacts on customers and lead to expensive emergency operations and reputational damage for Cities. The relative severity of these consequences is an important variable considered when prioritizing capital spending and can be estimated using pipe attributes, operational information, and geographic information. Likelihood of Failure (LoF): This analysis examines the likelihood of two modes of failure for inspected pipe segments. Structural pipe failures may result in pipe collapses which can, in turn, result in subsidence, backups, and overflows. Structural failures are typically caused by deterioration of the pipe wall material, changing environmental stresses, and construction strikes. Operational pipe failures are defined as blockages that reduce pipe capacity cause conveyance system problems like surcharging, overflows, and basement backups. These blockages are caused by debris or fats, roots, and grease (FOG) deposits. 466
Likelihood of Failure Likelihood of Failure WEF Collection Systems Conference 2017 RESULTS After the scores are calculated, they are plotted on the two-dimensional risk matrix below with likelihood of failure represented on one axis and consequence of failure represented on the other. Structural: Structural failures can range from broken blocks to a collapse of a road. Based on the below assessments approximately 8.4% of the City s segmented block sewers inspected as a part of this study are in need of immediate attention. A quick overview of the results based on 35,946 LF inspected is summarized as follows: Immediate (Red) High Risk (Orange) Medium Risk (Yellow) Low Risk/Maintenance (Green) Not Inspected (Due to Issues) - 8.4% of segmented block sewers - 5.1% of segmented block sewers - 14.2% of segmented block sewers - 69.1% of segmented block sewers - 3.2% of segmented block sewers 100% of segmented block sewers Tables 2 and 3 show the breakdown of the Structural Risk Profile and Risk Matrix: Table 2: Structural Failure Risk Profile - Segments 5 0 0 0 5 2 4 0 0 0 3 0 3 0 0 0 4 0 2 0 0 0 8 2 1 0 0 0 77 8 1 2 3 4 5 Table 3: Structural Failure Risk Matrix 5.00 4.00 3.00 2.00 1.00 1.55 0.00 0.00 1.00 2.00 3.00 4.00 5.00 467
Likelihood of Failure Likelihood of Failure WEF Collection Systems Conference 2017 O&M: O&M failures can range from debris build up to FOG causing a reduction of flow through the pipe with the potential to cause backups and potential overflows. Based on the below assessments approximately 9.9% of the City s segmented block sewers inspected as part of this report are in need of immediate attention. Some of these sewers are also identified as immediate for structural at risk. A quick over of the results based on the 34,804 LF inspected: Immediate (Red) High Risk (Orange) Medium Risk (Yellow) Low Risk/Maintenance (Green) Not Inspected (Due to Issues) - 9.9 % of segmented block sewers - 4.5 % of segmented block sewers - 59.3 % of segmented block sewers - 23.1 % of segmented block sewers - 3.2 % of segmented block sewers 100 % of segmented block sewers Tables 4, 5 and 6 show a breakdown of the O&M Risk Profile and Risk Matrix: Table 4: O&M Failure Risk Profile - Length of Pipe 5 0 0 0 2,788 0 4 0 0 0 1,605 782 3 0 0 0 12,714 0 2 0 0 0 7,187 1,431 1 0 0 0 7,188 1,109 1 2 3 4 5 Table 5: O&M Failure Risk Profile - Segments 5 0 0 0 5 0 4 0 0 0 5 2 3 0 0 0 29 0 2 0 0 0 24 5 1 0 0 0 34 5-1 1 2 3 4 5 468
Likelihood of Failure Likelihood of Failure WEF Collection Systems Conference 2017 Table 6: O&M Failure Risk Matrix 5.00 4.00 3.00 2.00 1.00 0.00 0.00 1.00 2.00 3.00 4.00 5.00 PRIORITIZATION AND RECOMMENDATIONS Of the sewers evaluated, 3,035-feet received the highest ranking of combined structural risk and consequence of failure, which should be addressed immediately. The second highest priority sewers are considered high risk and are comprised of 1,817-feet, followed by 5,089-feet of medium risk, and nearly 25,000-feet of low risk sewers. Table 7: Structural Failure Risk Profile Length of Pipe 5 0 0 0 2,303 732 4 0 0 0 1,817 0 3 0 0 0 1,252 0 2 0 0 0 2,985 852 1 0 0 0 23,125 1,738 1 2 3 4 5 Recommended projects were developed by referencing the structural likelihood of failure ranking, consequence of failure, and resulting Business Risk Exposure (BRE) for each segment. However project limits cannot be defined using only a numerical ranking system. Detailed inspection reports compliment the numerical rankings of each segment, along with rating characteristics of surrounding sewers. Project limits and project priority were also developed considering total length, pipe diameter, cost, and construction conditions, such as minimizing the number of mobilizations required for each project. 469