In 2014 and 2015, the Metropolitan St. Louis Sewer District (MSD) responded to, and made emergency repairs on multiple force mains in its service area. Force mains are pressurized pipes that move material using pumps. Their repair and maintenance is a top priority of MSD Project Clear. All pipes degrade over time and eventually need repair, rehabilitation, or replacement, among other activities; therefore, MSD Project Clear is focusing efforts on force main inspection and repair. MSD recently accelerated the first phase of a two-phase project to repair the Caulks Creek force main in Chesterfield, replacing 6,600 linear feet of sewer line that have experienced the most recent breaks.

About Force Mains

A force main sewer is a sewer line through which wastewater is propelled by the force of pump stations located along its path. Pump stations are sometimes necessary to overcome gravity and allow for the transport of wastewater to area treatment plants. In contrast to “gravity sewers”, which slope downhill to transport material, force mains are used in areas where the pipes are either too low to rely on gravity or they reach a barrier of some sort. This requires electric pumps to move material toward a treatment plant.

Force mains move material with pumps instead of gravity; ergo, they don’t necessarily follow a straight line. Force main alignments can vary by several feet up or down or side to side between access points, which are called pump stations. This makes determining the exact location for maintenance and repairs a challenge. In some cases, force mains were not built exactly to plan and in other cases original plans did not survive the transition to MSD administration years ago.

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Today, larger force mains are typically constructed of iron pipes lined with concrete. They can last anywhere from 25 to 50 years, depending upon the amount of flow the line handles and the corrosive properties of the wastewater. MSD’s current design practice includes lining the pipe with an epoxy coating to protect it from the corrosive impact of the wastewater flowing through.

Why Pipes Fail

One of the primary reasons a force main would need repair is due to corrosion. Corrosion in force mains occurs differently based upon pipe material, age, pipe diameter, and other factors. MSD prioritizes pipe repairs, rehabilitation, and replacements based upon a risk-based, asset management strategy that balances the probability of failure of a pipe versus the consequence of a failure to the service area (i.e., size of community impact). Those areas with the greatest need where repairs would have the greatest impact are where MSD efforts are focused.

Visual Inspections

Force mains are classified according to their risk level, which determines how frequently the pipes are inspected by MSD staff. Based upon their categorization these pipes are then placed on a visual inspection cycle. There are three categories of risk that determine how often a visual inspection is conducted. MSD then prioritizes repairs to its force mains based on the results of these inspections.

  1. High-risk โ€“ once every year
  2. Medium-risk โ€“ once every two years
  3. Low risk โ€“ once every five years

A real-life example: The Caulks Creek force main rates in the high-risk category because of its age (built in 1983), its diameter, and its material (iron pipe).

Technology Identifies Risks

In addition to the visual inspections, MSD also conducts testing to identify potential weak spots within pipes using a variety of sophisticated, specialized, in-pipe, condition assessment technologies. Testing, such as smart-ball testing, occurs only in pipes that are classified as high or medium risk due to the cost and complexity of such condition assessment.

Smart-ball testing sends an acoustically sensitive, GPS-traceable device through a force main pipe to locate existing leaks or failures. The incoming data highlights potential “hot spots” in the force main, where a higher probability of failure exists. This triggers continued monitoring or fast-tracked repairs, allowing MSD to repair or rehabilitate/replace the pipe before a failure occurs, or mitigate the consequences of failure if they do occur.

Smart ball enters pipe.
Smart ball enters pipe.
Smart ball is extracted from pipe following inspection.
Smart ball is extracted from pipe following inspection.

The inspection techniques employed help MSD identify potential trouble spots in these pipes, enabling staff to conduct small repairs rather than complete replacement. Identifying and responding to trouble spots using technology means a significant cost-savings, benefiting the region as a whole.

Force Main Management

MSD utilizes the results of its inspections to prioritize their repair, rehabilitation, and replacement efforts. Proactively repairing sections facing possible failure instead of replacing the entire force main means MSD can spend what is necessary to keep the sewer system functioning at the optimal level of service. This management strategy results in saving money and a potential decrease in the number of future emergency repairs needed.

In the first three years of visual inspections and testing, MSD has spent approximately $1,000,000 to accurately identify the condition of its extensive network of sewers, which includes approximately 125 miles of force main. The anticipated future benefit is enormous. A single force main emergency can cost between $250,000 and $500,000 to repair. In other words, preventing just two large-scale emergency repairs will immediately return the investment.

This program represents MSD’s commitment to effectively, proactively, and assertively manage maintenance needs in its service area. However, the process is ongoing, and MSD acknowledges other emergency repair events are likely to occur during the next several years as the program is fully implemented and its benefits fully realized. The end result of this program will be a reduction in force main emergency repairs, a savings to our rate payers, and a reduced impact to the environment.

MSD has already seen a drop in the number of emergency repairs based solely on improving the accuracy of force main mapping. Accidental contractor strikes, where a contractor breaks a force main that was not recorded, occur more frequently where MSD has incomplete or inaccurate records of the existing force mains. In response to this, in the past three years, MSD has undertaken an extensive and sophisticated GPS mapping process to fill the gaps in their records and to increase the accuracy of field records.

MSD is also committed to achieving the maximum possible cost-savings and efficiency when performing emergency repairs, and has developed pre-packaged emergency repair kits for each type of force main. These kits are pre-staged ahead of time, saving time and money.