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

It was reported at the EPA-sponsored International Forum on Rehabilitation in Edison, NJ in September
2008 that a new hydrostatic buckling model, with imperfections quantified, is being prepared for ASTM
F1216. ASTM F1216 is described below.
5.4.1 Design of Pressure Systems. As outlined above, deteriorated pipelines are classified as
either partially or fully deteriorated based on their ability to continue to resist internal pressure and
external load. A partially deteriorated pressure pipe would have some evidence of pinhole leaks or
leaking joints, but would be capable of withstanding the internal operating pressure and any external loads
(soil, live, and hydrostatic groundwater). In this case, the renewal liner is designed to be a close-fit to the
existing pipe and the hoop tensile stresses are carried by the host pipe. However, the liner is designed to
support any external hydrostatic pressure (only a factor when the line is not under pressure) and to be able
to span over any holes or gaps in the host pipe.
A fully deteriorated pressure pipe is one with large holes or gaps (most probably caused by severe internal
or external corrosion) and is unable to withstand the operating pressure of the system. A partially
deteriorated pipe, which is expected to continue to corrode and deteriorate over its remaining design life,
would also be considered a fully deteriorated pressure pipe in terms of the design of a renewal liner. Here
the renewal liner needs to be designed to carry not only the full internal pressure, but also any external
loads, including soil, live, and hydrostatic groundwater. In essence, the liner is a replacement for the
existing pipe.
As is clear from the introduction to this section, Appendix X1 in ASTM F1216 is the most commonly
accepted design method for CIPP and deformed/reformed thermoplastic pipes. It is interesting to note
that the design appendix in ASTM F1216 is not mandatory, so in theory there is no obligation to use this
design method when using CIPP products.
5.4.1.1 Partially Deteriorated Case. In the case of a partially deteriorated pipe, two basic equations
are used in the design. The hydrostatic buckling resistance of the liner is calculated by the following
formula:
where:
P = groundwater load, psi
K = enhancement factor of the soil and existing pipe adjacent to the new pipe (a
minimum value of 7 is recommended for design)
EL = long-term (time corrected) flexural modulus of elasticity for the liner, psi
µ = Poisson’s ratio (0.3 assumed)
DR = dimension ratio (diameter/thickness) (D/t)
C = ovality reduction factor
N = factor of safety (2.0 is suggested)
The above equation is patterned after the classic Timoshenko elastic buckling formula for an infinitely
long cylinder subjected to uniform external hydrostatic pressure. Modifications include the addition of
the “enhancement factor” K and the ovality reduction factor. The enhancement factor is based mainly on
hydrostatic buckling experiments published by Aggarwal and Cooper (1984) where buckling
enhancement from the existing host pipe could range from 5 to 20 times greater than the free-standing
liner. The ovality factor compensates for ovoid pipes and conservatively reduces the liner’s resistance
(refer to ASTM F1216 for the equation for C).
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