Drains in other than stable ground

Issue

To clarify the requirements when drainage pipework has been installed internal or external to a building in ground considered to be “other than stable ground”.

Regulatory summary

In NSW some installations may not be required to comply with Clause C2P7(1) of the Plumbing Code of Australia when avoiding damage to a sanitary drainage system from ground movement by meeting the Deemed-to-Satisfy provisions set out in Schedule 3 of the Plumbing and Drainage Regulation 2017.

Schedule 3 prescribes:

• Flexible Joints must be used in a drain where it has been laid in ground that is:
  • Filled, or
  • Unstable, or
  • Water-charged, or
  • Soil class H1, or
  • Soil Class H2, or
  • Soil Class E, and
  • In the opinion of a recognised expert is likely to move and the movement would be likely to cause damage to the drain and surrounding structures.
• A design must be prepared for a sanitary drainage system by a recognised expert.
• Recognised expert qualifications and education.
• The sanitary drainage system must be installed in accordance with the design.
• The design must be supplied to the plumbing regulator or their delegate with a Notice of Work.

For more information, see the technical advisory note on the use of flexible joints in drains.

Regulatory commentary and requirements

Filled, unstable¹ or water-charged ground requires a suitably qualified expert to confirm the suitability of the drains to be installed in the ground considered to be other than stable via the below options:

• Note ¹ - Unstable ground although not defined in the AS/NZS 3500.0 Glossary, can be reasonably assumed to mean instances of highly reactive type soils, ground prone to slippage, or where relevant authorities such as Subsidence NSW have conditions in place relevant to the installation of drainage systems (i.e. flexible joints in drains).

Filled ground

If the filled ground has been compacted and a suitably qualified expert has deemed it be adequate to continuously support the drain, and any movement is not expected to damage the drain or surrounding structures, a letter is to be supplied to the regulator. The letter must contain:

1. an outline of the expert's qualifications and,
2. a statement verifying that: “The filled ground in the immediate area of and/or under the sanitary drainage pipework will continuously support the drain and accommodate the loads of the surrounding ground and is considered stable. Any movement that may occur is not expected to damage the drain or surrounding structures”.

Unstable ground

If the unstable ground has been assessed, and a suitably qualified expert has deemed it to be adequate to continuously support the drain, and any movement is not expected to damage the drain or surrounding structures, a letter is be supplied to the regulator. The letter must contain:

1. an outline of the expert's qualifications and,
2. a statement verifying that: “The ground in the immediate area of/under the sanitary drainage pipework will continuously support the drain and accommodate the loads of the surrounding ground and is considered stable. Any movement that may occur is not expected to damage the drain or surrounding structures and requires no provisions such as flexible joints, or similar to accommodate movement."

Water-charged ground

If the water-charged ground has assessed, and a suitably qualified expert has deemed it to be adequate to continuously support the drain, and any movement is not expected to damage the drain or surrounding structures, a letter is to be supplied to the regulator. The letter must contain:

1. an outline of the expert's qualifications and,
2. a statement verifying that: “The water-charged ground in the immediate area of / under the sanitary drainage pipework will continuously support the drain and accommodate the loads of the surrounding ground and is considered stable. Any movement that may occur is not expected to damage the drain or surrounding structures”.

Designs for ground conditions

If the filled, unstable or water-charged ground has been assessed by a suitably qualified expert, and deemed unsuitable to support the pipework or damage to a drain or structure is expected to be caused by ground movement, a method to support the drainage is required to be determined by a recognised expert.

Any design to counteract or accommodate unstable ground or reactive soil conditions using flexible joints must be given to the local plumbing regulator before the work is carried out. For more information, see the technical advisory note on the use of flexible joints in drains.

Any engineered solution to support the pipework that does not include flexible joints must be submitted as a Performance Solution. For more information, see the Guideline for plumbing and drainage: Installing Code Compliant Work and Performance Solutions.