A new method for the structural design of flexible liners for gravity pipes of egg-shaped cross section: Theoretical considerations and formulation of the problem

• Problems with existing methods for egg-shaped liner design are identified.
• A new method is then developed which eliminates these difficulties.
• The method provides for a safety factor against either buckling or material failure.
• The method is non-linear and shown to be expressible as a simple computer app.
• Example applications demonstrate the design capability of the new method.

The majority of gravity pipes with a non-circular cross section have an egg-shaped profile and an inner surface that can be geometrically delineated by three interconnecting circles on each side of a vertical axis of symmetry. Renovation of these pipes can be undertaken at low cost and with minimal surface disruption by installation of a close-fitting polymeric liner, most commonly by the ‘cured-in-place pipe’ technique.There are two pre-existing methodologies for the structural design of such liners. Although these procedures have served the international wastewater industry well over an extended period, detailed consideration of both methods suggests that an improved methodology which is straightforward to apply and addresses all relevant issues in a consistent manner would be both technically and financially beneficial. Accordingly a new design method is here proposed, which is based on an assumed displacement distribution in a manner consistent with previous work on close-fitting liners of circular cross section. The new design procedure addresses all the relevant issues in a rational manner, and is readily implemented as a small computer software simulation mountable on any current generation personal computer.