State of Technology Report for Force Main Rehabilitation, Final ...

Introduction
EXECUTIVE SUMMARY
Force mains that carry sewage flows under pressure represent a special set of challenges for sewer
rehabilitation. Force mains represent about 7.5% of the wastewater system and they typically use
materials that are not commonly used in gravity sewer systems. Ductile iron (DI), cast iron (CI), steel,
and concrete pressure pipe are all material types that are frequently used for sewer force mains, especially
in larger diameters. All of these materials are susceptible to both internal corrosion from the sewage flow
(liquid and gaseous states), as well as external corrosion due to the environment in which the pipe is
buried.
Historically, the most common renewal technology employed has been to replace the main using open cut
construction. Part of the reason for that choice has been a lack of rehabilitation technologies appropriate
for sewer force mains. There is a wealth of technologies available for gravity sewers, but the field has
been limited for pressurized systems. Fortunately, that situation is changing as more vendors recognize
the growing opportunity in sewer force main rehabilitation. The other reason for replacement is that
sewer force mains tend to have a fairly high consequence of failure. A rupture of a sewer force main
could release millions of gallons of raw sewage into the environment posing significant health risks to the
general public. Cleanup costs can be staggering.
As some of the newer rehabilitation technologies develop a positive track record of use in sewer force
mains and confidence in their design approach and installation process strengthens, more utilities will be
willing to consider these trenchless technologies as potential renewal solutions. Trenchless methods have
proven themselves to be cost-effective for gravity sewer mains, especially when both direct and indirect
costs associated with a replacement program are considered. A similar outcome is expected for sewer
force mains once data on the effectiveness and longevity of these technologies and materials and lifecycle
costs become more readily available.
Recognizing that there would be some cross-over amongst the various rehabilitation technologies
common to water mains and gravity sewers, a series of technology-specific datasheets were created for
each identified rehabilitation technology deemed relevant to sewer force mains and are included as
Appendix A of this report. An effort was made to collect representative cost information, but often only
limited cost data were available.
Characteristics of Force Mains
The approximate length of the force main system in the US is 60,000 miles (96561 km). The Water
Environment Research Foundation (WERF) recently published a report titled “Guidelines for the
Inspection of Force Mains” (WERF, 2009). The data from this WERF survey are presented and
characterize the types of pipe materials used, diameter ranges, ages, location accessibility, and failure
modes and mechanisms for force mains. Ferrous pipe materials (i.e., CI, DI, and steel) represent on
average 63.4% of all pipes used for force mains. Over 91% of sewer force mains are between 4 and 36
inches (100 and 900 mm) in diameter, which are within the non-man entry size range and only 2% are
beyond 50 years in age. Over 91% of the force main pipes are buried. Only 5% of the total number of
force mains surveyed for the WERF project had some built-in redundancy.
Internal corrosion was rated as being responsible for ferrous force main failures 26.2% of the time, ahead
of all other known causes. External corrosion at 19.2% and third-party damage at 19.4% are the next
most common causes of failures in ferrous mains. The most common single cause of failure in non-
iv