Designing footings

This information will help designers and structural engineers to design footings and structural support that meet our protection of services guidelines.

The certifying structural engineer for your project is responsible for designing suitable footings and structural support for the proposed work. The designer is responsible for ensuring ground conditions are acceptable for the final design.

The guidelines below will help you to design the footings and structural support suitable for your structure.

Cantilevered footings rely on the soil above them for support, they should be designed to allow for removal of soil within the zone of influence.

A compressible foamed plastic insert or similar is normally acceptable as a means of filling the space between the pipe and the building or structure.

Sufficient clearance should be allowed for settlement to occur without damage to our service. Clearances of 50mm–80mm are usually allowed, however actual dimensions can only be determined based on the structural engineer's assessment of long-term building settlement.

In some cases involving major construction across all or most of a site, our gravity sewer may be slung from the underside of a basement ceiling, for example in an underground car park. In these cases the designer of the building is also responsible for the design of support straps and integrity of the sewer.

The sewer should be supported at each joint and at intervals that ensure maintenance of the required grade without deflection, even where the pipe is full (eg. during blockages). The spacing of pipe supports should not exceed the pipe manufacturer’s recommendation.

This may occur where a retaining wall, swimming pool or basement to a proposed structure is taken to a level below our sewer invert.  

The builder should be aware of the need to maintain access to the sewer and the prospect of undermining the sewer during construction.  Soil anchors (soil nails) are not to be installed over our services, and they are not to rely on any soil within the zone of influence.

Underpinning of structures may be necessary where construction has commenced without provision of footings to the required depth. Underpinning of existing structures may also be necessary during construction of the proposed works.

In these cases:

  • a structural engineer needs to ensure that appropriate remedial action is taken where construction has already commenced without provision of footings to the required depth
  • structural support meeting the structural engineer’s requirements is to be taken at least to the required level
  • the requirements of our footings guidelines, especially maintaining the nominated clearance from the centre-line of the sewer, or alternatively providing an access tunnel, need to be met
  • all costs are the responsibility of the owner or developer
  • a plan needs to be submitted to us for examination.

Above ground floor slabs may be cantilevered to form balconies or verandas. Alternatively columns or posts may support the slab projection. In cases involving verandahs over footpaths, water mains and service connections may be affected.

POS requirements should be determined based on the location of building walls and posts/columns relative to our services.

An above ground floor slab may only be cantilevered over our service where the:

  • service is of 300mm or less in diameter
  • required 2.4m headroom (distance between finished ground level and underside of slab) is maintained at all times over the service.

Cantilevering of ground floor slabs is permitted up to, but no further than, the sewer centre-line and only if all other alternatives have been exhausted.

The cantilevering of ground floor slabs must be supported by a continuous wall supported on a strip footing to at least 300mm below the sewer invert level. A ground floor slab may only be cantilevered over our service where:

  • The service is of 300mm or less in diameter
  • At least 1.2m of headroom (distance between pipe invert and underside of slab) is maintained at all times

This is to allow ensuites, walk-in-robes, bedrooms and similar domestic areas to be cantilevered. It is not the intention to allow cantilevering for the full length of a building.

In all cases, other than exempt structures, piles or footings must extend at least 300mm below the zone of influence or to refusal in unexcavated solid rock.

In some situations, continuous strip footings supporting a structural wall may be required.

You can find graphs and tables to help you determine the footing depth for your structure in our Protection of Services Manual.

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