State of Technology Report for Force Main Rehabilitation, Final ...
State of Technology Report for Force Main Rehabilitation, Final ...
State of Technology Report for Force Main Rehabilitation, Final ...
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thickness? It can still support the overburden soil and live loads, yet does not have an acceptable factor <strong>of</strong><br />
safety on pressure. According to the ASTM F1216 definition, this is a partially deteriorated pipe, while<br />
most engineers might claim fully deteriorated based on its pressure capability.<br />
The ASTM standards <strong>for</strong> new products coming into the market place designed to reconstruct pressurized<br />
mains need to be modified to reflect a better understanding on what constitutes the various degrees <strong>of</strong><br />
deterioration that a pressurized main can undergo without failure. This will be important <strong>for</strong> sewer <strong>for</strong>ce<br />
main renewal, as well as water main renewal.<br />
5.2.2 Interactive vs. Independent. Liners <strong>for</strong> sewer <strong>for</strong>ce main renewal fall into two categories:<br />
interactive or independent. Interactive liners are generally thin liners, in direct contact with the inside<br />
wall <strong>of</strong> the existing pipe, and which have a much lower ring tensile stiffness than the existing pipe. When<br />
pressurized, the existing pipe with the higher hoop stiffness will carry a proportionately higher percentage<br />
<strong>of</strong> the tensile ring load. Consequently, interactive liners should not be used in sewer <strong>for</strong>ce mains where<br />
the existing pipe has deteriorated to a point where it is not expected to be able to carry the full internal<br />
pressure over the renewal design life. Examples <strong>of</strong> this might be a ferrous main that has suffered<br />
extensive external corrosion with no protection, and has through-hole pitting over a significant portion <strong>of</strong><br />
the system. Placing a liner on the interior will arrest any internal corrosion and can be designed to bridge<br />
over small holes and gaps, but will do nothing to stop any further external corrosion, which will<br />
eventually lead to failure. In this case, the independent liner would be preferred.<br />
An independent liner is one that is designed to carry the full internal working pressure and surge pressure<br />
itself independent <strong>of</strong> any contribution from the host pipe. Slipliners are, by design, independent liners as<br />
they are not in direct contact with the existing pipe (unless grouted in place) and will expand<br />
circumferentially under pressure, but not transfer hoop load to the existing pipe. Some liner products start<br />
<strong>of</strong>f as interactive, as they will be close-fit, but have sufficient inherent strength to carry the full internal<br />
pressure should the host pipe fail. These would then be considered independent liners.<br />
The AWWA M28 Manual <strong>Rehabilitation</strong> <strong>of</strong> Water <strong>Main</strong>s has established four classes <strong>of</strong> design <strong>for</strong><br />
rehabilitation, ranging from non-structural to fully structural (AWWA, 2001). These definitions or<br />
classes are more relevant to the design <strong>of</strong> sewer <strong>for</strong>ce mains than those found in the current ASTM<br />
standards. The four classes are described below:<br />
Non-Structural<br />
• Class I – provides no structural support, only acts as an internal corrosion barrier and<br />
improves water quality.<br />
Semi-Structural<br />
• Class II – resists external hydrostatic pressure from groundwater, bridges over holes and gaps<br />
in the host pipe, but not able to carry the full internal pressure independently, adheres to the<br />
interior surface <strong>of</strong> the host pipe.<br />
• Class III – same as II except not dependent on adherence to the host pipe wall.<br />
Full Structural<br />
• Class IV – independently capable <strong>of</strong> resisting external hydrostatic pressure from<br />
groundwater, and can handle the full internal pressure without support from the host pipe.<br />
5.2.3 Design Loads. Depending on the state <strong>of</strong> deterioration <strong>of</strong> the existing main, the renewal liner<br />
can be designed to be either a corrosion barrier or an outright pipe replacement. Some <strong>of</strong> the loads that<br />
must be considered depend on the state <strong>of</strong> distress in the existing pipe, and include:<br />
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