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

Technology Selection Criteria
Aside from selecting renewal technologies on the basis of their fit to the force main’s operating conditions
(e.g., pressure, burial depth, etc.), other site-specific parameters must be considered in the selection
process. The life-cycle cost of the renewal method and its impact on extending the life of the asset are
often the primary concerns in technology selection. As discussed in this report, studies in the US and UK
have shown the relative cost benefits of rehabilitation versus open cut replacement in urban environments.
Other site-specific factors that should be taken into consideration include post-renewal capacity needs,
accessibility, future operations and maintenance (O&M) requirements, the condition of the host pipe, and
the consequence of its failure (criticality).
All rehabilitation options will result in a reduced cross-sectional area. Spray-on linings and close-fit
liners will least impact flow capacity, while sliplining will have the greatest impact. Loss of capacity is
mitigated somewhat by improved friction factors. If capacity restraints exist, then replacement may be
the only feasible option.
Other items that can impact the selection of a renewal method are accessibility of the force main,
maintenance crews familiarity with the liner system and repair methods, and the criticality of the force
main. The higher the consequences associated with a failure, the more conservative the approach will be
towards renewal of the main. A partially deteriorated force main that has an extremely high consequence
of failure would be one that would probably be treated as fully deteriorated from the perspective of the
design of the rehabilitation system.
Design and Quality Assurance/Quality Control
The design of a rehabilitation product to renew the life of a distressed sewer force main ranges from an
inner corrosion barrier to an outright structural replacement. The factors that will control the design are
the condition of the existing main, including its expected remaining life if further deterioration is arrested,
and the operating conditions under which that main is used. The degree of deterioration is typically
broken down into one of two categories: (1) partially deteriorated, where the existing pipe is expected to
support all external loads (soil, live, surcharge), or (2) fully deteriorated where the existing pipe is not
structurally sound.
Design methods currently employed are for either interactive or independent liners and depend upon the
condition of the existing (host) pipe. Interactive liners are generally thin liners, in direct contact with the
inside wall of the existing pipe, with a lower ring tensile stiffness than the existing pipe. Interactive liners
should not be used in sewer force mains where the existing pipe has deteriorated to a point where it is not
expected to be able to carry the full internal pressure over the renewal design life. An independent liner is
one that is designed to carry the full internal working pressure and surge pressure itself independent of
any contribution from the host pipe.
Few rehabilitation products are designed specifically for use in a sewer force main. Therefore, design and
Quality Assurance/Quality Control (QA/QC) requirements and best practices may need to be adapted
from relevant American Water Works Association (AWWA) and American Society for Testing and
Materials (ASTM) standards.
The AWWA M28 Manual on Rehabilitation of Water Mains has established four classes of design: nonstructural
(Class I), semi-structural (Class II and III), and fully structural (Class IV). Class I liners only
act as corrosion barriers, Class II/III liners are designed to bridge over small holes or gaps in the host
pipe, while Class IV liners will carry the full internal pressure without support from the host pipe.
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