In September 2013 a workshop was held in Delft, the Netherlands, to establish the current state of knowledge on the structural design of sewer renovation systems. The workshop was instigated by Working Groups WG13 (Renovation) and WG22 (Drains and sewers outside buildings) and was part of the European Standards Technical Committee CEN/TC165 Waste Water Engineering.
The purpose of the seminar was to build on the success of Working Group WG12 (Structural design of buried pipelines), a group that had managed to gain consensus from a divergent set of views to establish a unified process for the design of new buried pipeline systems.
Current standards
The current set of European standards developed for pipe renovation systems range from EN13566.1 to EN13566.7 and cover requirements for materials, fabrication, installation and testing. When first released, these standards intentionally avoided any reference to design; with each member country holding a distinctly different design approach, consensus on these standards would have been difficult to achieve and protracted delays of the release of the standards was expected.
By gathering leading lining design practitioners at the one workshop in Delft, it was anticipated that it would be possible to identify a way forward to establish an agreed basis for the standardisation of pipe lining design methods.
Participants of the workshop included John Gumbel and Don Ridgers of the United Kingdom, Francois Clemens and Kees Snaterse from the Netherlands, Bernhard Falter and Ulrich Bohle from Germany, Olivier Thepot from France and Ian Moore of Canada. Over thirty other regular practitioners specifically interested in the design process for linings also attended and contributed to the workshop discussions.
During the workshop, participants agreed that although design procedures developed over 20 years ago for lining – such as the WRC method and enhanced buckling in ASTM F1216 – had served the Trenchless Technology industry well, they did not sufficiently represent the actual performance of linings. In most cases, the methods were overly and unnecessarily conservative. In other cases, they were too liberal or were not appropriate at all (such as irregular geometries).
FEM is A-OK
With the recent development and availability of finite element modelling (FEM) techniques, it has been possible to simulate the interaction of the host pipe, surrounding ground, and lining under imposed loading and deformations with great accuracy.
Although there are still some areas that require further investigation and review, FEM was considered by the workshop to be most universally applicable method to provide a lining design that best matched actual conditions.
Although FEM would be appropriate for critical cases such as large diameter, heavily loaded and/or irregular geometry pipes, it was also agreed that simplified methods could still be suitable for the more normal range of applications.
This agreement avoided the need for establishing an elaborate FEM for straightforward non-critical applications and allowed the continued use of locally developed methods – provided these methods could be demonstrated to deliver conservative results compared to an equivalent rigorous FEM technique.
Two-tier: the way forward
The expected way forward for the European standard on lining design would be to establish a two-tier system for design. The first tier, “÷Tier One’, would be based on FEM techniques with the standard stipulating minimum requirements for the establishment of the model in terms of areas such as material characteristics, loading assumptions, safety factors, soil structure interaction, assumed lining geometry and impact of any pipe or lining discontinuity.
The second tier’s methods could then include local design methods used by the various member countries. These “÷Tier Two’ methods may also include tabulated minimum lining thicknesses for standardised loading cases and cover depths.
The expectation is that Tier Two methods are validated by a Tier One method before use. The Tier One method would also become the default process for design in critical cases, or cases not covered by the Tier Two methods.
Lining design in an Australian context
The presentations and discussions at the workshop that focused on current lining design thinking revealed some interesting differences between Europe and common Australian practices.
In Australia, rather than implementing separate methods for intact pipes (lining that resists external hydrostatic pressure only) and deteriorated pipes (lining that resists all applied loads), a unified method is adopted. For this method, the existing pipe is considered to exhibit different levels of partial deterioration where loadings and design parameters are adjusted to match the level of observed deterioration (deformation).
Comments at the workshop on the current process used in Australia for deteriorated pipes, where the lining was designed as if it was a new buried flexible pipe (i.e. AS2566.1), reflected the viewpoint of the method being overly conservative and one that did not represent the actual performance of the lining in its surroundings.
By contrast, further enhancements are planned to current methods in European countries such as France and Germany. These enhancements will come in the form of additional design criteria relating to limiting strains or stresses in linings and are intended to cover cases of linings in severely deteriorated pipes.
The various design methods presented did not cover all the design cases typically occurring in Australia. Two cases in particular that were not covered were the impact of grouting on the buckling of profiled spiral lining in essentially intact pipes and the design of linings in pipes that are experiencing extensive deterioration and loss of structural integrity from gas-related attack (but have not experienced any noticeable cracking or deformation).
A call for a unified approach
Although the workshop identified that there were additional areas that require further review and investigation, it seems apparent that eventually a unified approach will be developed using FEM techniques for lining design.
Australia will be in a position to use these techniques, but it is possible they may not cover all the cases that are typically experienced locally. It is expected that some work will be required to both adapt the FEM techniques and modify or develop “÷Tier Two’ methods to suit range of available lining products and the pipe deterioration and loading conditions more typically experienced in Australia.
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