Use of flexible joints in drains

Issue

NSW Fair Trading is aware of a building approval condition issued by Subsidence Advisory NSW in the Menangle Park area for the use of flexible joints in drains.

This advisory note has been prepared for industry outlining the requirements for the installation of flexible joints where the soil classification outlined in AS2870 is of a highly or extremely reactive classification and to clarify the information on unstable ground provided in AS/NZS 3500.2.

It is the responsibility of the licenced plumber drainer to ascertain whether the site is subject to a building approval condition, or a soil classification that requires flexible joints to be installed on the sanitary drainage system.

Detail

AS2870:2011 - Residential slabs and footings

Clause 5.6.4 and Clause 6.6 prescribe the use of flexible joints for drains on sites where the soil classification has been determined to be highly (H1 or H2) or extremely (E) reactive.

AS/NZS 3500.2:2021 - Sanitary Plumbing and Drainage

Clause 5.6 Drains in other than stable ground

Where drains are to be laid in filled*, unstable or water-charged ground, methods of support and bedding shall be designed to withstand the ground conditions.

Note 1: See Appendix G for information on the installation of plastic pipe drainage systems in unstable soils with Australia.
Note 2: AS2870 provides special design considerations for drains associated with residential slab or footing systems on moderately, highly or extremely reactive soils.
Note 3: Clause 3.8.2 provides special design considerations for drains in close proximity to footings.
Note 4: For proclaimed mine subsidence or landslip districts, the appropriate authority should be referred to for advice on subsidence or landslip design parameters for proposed drainage systems.
Note 5: In Australia, acceptable evidence that a design may be deemed suitable is provided for in the Plumbing Code of Australia.

*See the Fair Trading Technical Advisory Note 12 on Filled Ground.

AS/NZS 3500.2:2021 - Sanitary Plumbing and Drainage

Appendix G Drains in unstable soils (Informative)

Appendix G is of an informative and guidance nature only and is not a mandatory requirement of the standard.

Appendix G does not provide design advice. Advice from suitably qualified engineer is recommend to ensure that the solution proposed is appropriate for the conditions of the site.

Appendix G provides guidance on how to install products intended to reduce the risk of soil or ground movement effecting the sanitary drainage system.

Regulatory Commentary

Clause 5.6 of AS/NZS 3500.2 prescribes that where drains are to be installed in unstable ground it is to be accompanied with a design. The associated notes are of an informative and guidance nature only.

AS2870 is currently not prescribed as a normative reference document in the Plumbing Code of Australia or the AS/NZS 3500.2

Appendix G of AS/NZS 3500.2:2021 is for information and guidance only. The information provided recommends design advice from a suitably qualified engineer.

As there is no normative Deemed-to-Satisfy requirements to install a drain in unstable soil, the installation would be subject to the Performance Requirements of the Plumbing Code of Australia 2022 requiring a Performance Solution to be submitted to the Plumbing Regulator.

Requirement

Where the Responsible Person is aware of the site soil classification being highly or extremely reactive, or a condition placed on the site (example Subsidence Advisory NSW condition in Menangle Park) they are to submit a Performance Solution via the MyInspections Gateway prior to starting work.

The following documents are to be submitted:

• Covering Letter with property information and Evidence of Suitability Statement,
• Notice of Work with Sanitary Plumbing & Drainage to be indicated as “Combined”,
• Letter from the Owner,
• Letter from the Network Utility Operator,
• Detailed design of the proposed drainage system for the property including location of flexible joints from a suitably qualified engineer.

The Evidence of Suitability Statement is to be provided from an engineer or recognised expert certifying:

“The sanitary drainage design consists of a Performance Solution for the use of flexible joints to be used to accommodate unstable soil and potential ground movement. The design meets the Performance Requirements set out in Part C2 and specifically C2P7(1) of the Plumbing Code of Australia 2022 and is the equivalent to the level of health, safety and amenity provided by the Deemed-to-Satisfy Requirements.

All other elements of the Sanitary Drainage system not listed above are to be installed in accordance with the Deemed-to-Satisfy Requirements of the Plumbing Code of Australia 2022”.

Fair Trading will assess the Performance Solution information provided, and if all requirements have been satisfied a Letter of Acknowledgement will be issued to the Responsible Person with details of the inspection requirements, including details of inspection bookings.

For more information on Performance Solutions see Guideline for plumbing and drainage: Installing Code Compliant Work and Performance Solutions.

Note: Information contained within this document is current as of May 2023 and refers to codes and standards current at the time of writing.

NSW Fair Trading disclaims any liability (including for negligence) to any person in respect of anything done, or not done, by that person in whole or partial reliance on any of the information in this advisory note.