WERF Completes Innovative Sewer Rehabilitation Research

Flood Grouting for Infiltration Reduction on Private Side Sewers – Making the Business Case in a Seattle Neighborhood

Seattle’s Broadview neighborhood has a 1950s-era concrete pipe sanitary sewer system that has experienced repeated wet weather sanitary sewer overflows from both manholes and into basements during extreme storm events. Over time, the pipes have deteriorated, allowing excessive amounts of infiltration through separated joints, fractures, and other defects. As a result, Seattle Public Utilities (SPU) undertook a project to address two poorly understood aspects of sewer rehabilitation:

  1. An intensive asset management-based evaluation process known as a business case evaluatin (BCE).
  2. Evaluation of the cost effectiveness of an innovative sewer rehabilitation process on a large targeted area.

Simulations revealed that the project reduced the peak hour flow. SPU intends to continue the use of this technology in select locations.

Map of Broadview sewer basins.

As sewer collection systems age and deteriorate, infiltration of groundwater into mainlines as well as lateral or side sewers increases. Municipalities interested in reducing wet weather flows have limited options; primarily, these efforts are limited to structural repairs or replacement. Often the pipes are in satisfactory structural condition, yet only structural rehabilitation methods are available. Point grouting of isolated points has been conducted for some time with varying degrees of success. SPU looked to an innovative nonstructural solution known as flood grouting.

This BCE was conducted to identify a preferred alternative and to validate the technology. The selection process identified four leading alternatives for the infiltration reduction project: flood grouting, joint grouting, pipe bursting, and curedin- place pipe (CIPP) lining. The much higher cost and disruptive nature of open cut pipe replacement eliminated it from more detailed analysis. Cost estimates for each of the remaining methods were developed and then compared to the benefits of infiltration reduction. Some of the benefits included reduced insurance claims, reduced storage costs at a regional wet weather treatment facility, reduced conveyance and treatment costs, the cost of installing cleanouts on side sewers, and inspecting privately held sewer assets. The BCE identified flood grouting as having the greatest benefit to cost ratio of all the options. It was therefore selected as the preferred alternative for the infiltration reduction.

For the pilot project to maximize infiltration reduction, it was important to include the maximum length of side sewers possible. To achieve that objective, SPU had to obtain permission to enter private property and to work on the privately owned side sewers.

SPU conducted an extensive education campaign. This included holding several community meetings, mailing informational flyers, developing a website, following up with telephone calls, and an onsite meeting with the contractor to show the expected equipment to be used during the construction process. This work resulted in a 95% signup rate from the affected residents. The remaining 5% of the houses had a cleanout installed on the side sewers within SPU’s right-of-way and the portion of the sewer within the right-of-way was rehabilitated.

Sanipor application.

The Sanipor flood grouting technology was applied to a 31-acre residential sub-basin in the Broadview neighborhood of Seattle. All of the manholes and mainlines were sealed; however, only 30% of the total side sewer length could be accessed for sealing due to several reasons, including multiple side sewer branches on each house, landscaping, elevation differences, and homeowner approval of the cleanout location. Approximately 56% of the entire sewer basin was sealed. Based on measured exfiltration rates of the flood grouting chemicals, the sections that were sealed had a 99% improvement in their exfiltration rates. The average total construction cost per foot of sewer sealed was $77 for this pilot project.

SPU maintains a network of flow meters and rain gauges in the Broadview area that was augmented with additional flow meters to capture before and after information. The recorded depth, velocity, and flow rate was used to calibrate two sewer models, one for before the project and one for after the project. Long-term simulations that were conducted reveal that the project reduced the peak hour flow rate coming out of the pilot basin by 41% and reduced the storm event volumes by 66%. SPU intends to continue the use of this technology in select locations where sewer infiltration has been determined to contribute significantly to wet weather flow issues (WERF project INFR5R11).