Position Statement on CIRIA C737

Structural and geotechnical design of modular geocellular drainage systems

The BPF Pipes Group welcomes the publication of CIRIA C737. There has been a significant increase in the use of plastic geocellular drainage systems over recent years and we are pleased that the practical knowledge gained from this growth is reflected in this revised document.

These systems are essential components in delivering the changes needed in the UK to address flood resilience. They can be complex geotechnical structures which are increasingly being used as part of holistic SuDS solutions across a wide range of construction industry sectors.

C737 recognises that good design, testing and installation is essential to deliver long term, whole life performance. It aligns with the Eurocodes and acknowledges the ongoing development of European test standards. This document improves upon the previous edition (C680) by increased emphasis on long term testing and a more scientific approach to risk management. Whilst it provides a general introduction to systems, materials and applications, additional guidance will be required to enable practitioners to apply the principles in practice.

With this in mind, the BPF Pipes Group would like to emphasise that:

  1. These products have been in use for over twenty years and, as Section 2.5.1 of the document recognises, failures of installed systems are rare. Investigation has shown that previous failures could have been further reduced by a more rigorous approach to design, preparation and installation.
  2. The process for evaluating products against the overall performance parameters in this document is not yet fully defined and this may lead to differences in interpretation.
  3. At the time of drafting, a target date for manufacturers to comply with testing requirements was discussed (Section 1.7). It is not now possible to meet this due to the time taken to finalise the report and the ongoing work to develop appropriate test methods and a consistent approach.
  4. The document provides a lot of general information on materials which means that the specific guidance on design of geotechnical structures is difficult to indentify. The practical application of the design methodology is difficult to use as it is overly rooted in theory.

To move this forward, the BPF Pipes Group are committed to:

  1. Remaining actively involved in the development of European standards referenced in this document and working with 3rd party certification bodies to ensure consistency of approach.
  2. Developing practical guidance targeted at designers and specifiers.

It is acknowledged that this will take time, and until this work is complete the BPF Pipes Group members firmly believe that C680 (Structural design of modular geocellular drainage tanks, 2008) continues to provide an effective and conservative approach to designing and installing plastic modular geocellular drainage systems.