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Building near sewer mains

If you are planning on building near, over or adjacent to sewer lines you need to be aware that there are design and construction standards that apply in order to protect the sewer line and enable access to these lines for maintenance and repairs.

The following requirements (as per AUS-PEC Standard D12:09) applies:

General requirements

  • All footings (see definition below of what a footing is) to structures within the zone of influence of a sewer main shall be designed and/or certified by a Registered Structural Engineer who will include a schedule on the drawings certifying that the design is in accordance with Council's AUS-SPEC Design Specification - Building over or adjacent to sewers.
  • Footings founded on rock may generally be considered to be outside of the zone of influence however, the advice of a Registered Structural Engineer should be obtained.
  • Footings for residential like buildings shall be designed according to the Australian Standard AS2870-1996 - Residential Slabs and Footings. Footings for other structures and buildings shall be designed under the Australian Standard AS3600 - Concrete Structures Code and Australian Standard - AS2159 - Piling Code.
  • No part of a footing is to be closer than 300mm to the edge of the sewer trench.
  • Sewer junctions are not allowed under buildings.
  • Buildings or structures may be constructed over sewer mains except for carrier mains (ie 225mm diameter or greater) or any main conveying a pumped flow.
  • Tanks or stands are not to be located over or adjacent to sewer mains.
  • Any proposal to build over or adjacent to a sewer main is to include a CCTV inspection of the sewer main at the applicant's/property owner's cost. Council’s Water & Sewer Services will review the footage and if it is decided that the sewer main needs replacing, this will be done at the applicant's/property owner's cost.

Design requirements

When designing footings to support the building loads the following design parameters need to be taken into consideration:

  • Footings shall be designed to take into account the probability that the disturbed and/or uncontrolled fill in the sewer trench may not provide adequate support for the adjacent footing loads.
  • Footings shall be designed or located so that the additional forces within the soil due to the footing load, that are transferred to the sewer main along with the existing soil loads, do not exceed the allowable loads on the sewer main.
  • Consideration should be given to the construction material and condition of the sewer main. Reference should be made to manufacturer’s information for allowable loads on sewer pipes.
  • In all cases, except sewer junctions, it can be assumed that Council would not excavate to access the sewer or that if they did, Council would take the necessary action to maintain support for the building foundations. 

Definitions

The following definitions will apply with regard to building over or near sewer mains:

  • Clay  = fine grained soil with plastic properties when wet. Includes gravelly, sandy or silty clays.
  • Footing = construction which transfers the load from the building to the foundation.
  • Foundation = ground which supports the building.
  • Registered Engineer = has the same meaning as set out in Council’s Policy for the registration of Professional Engineers.
  • Sand = granular soil that may contain a small proportion of fines including silt or clay. The amount of fines may be assessed as small be a visual inspection or if the amount that passes a 75 micron sieve is 15% or less, material with a higher proportion of fines shall be treated as silt or clay.
  • Silt = fine grained soil that is non-cohesive and non-plastic when wet and can include some sand and clay.
  • Rock = strong material including shaley material and strongly cemented sand or gravel that does not soften in water or collapse under the combination of loading and wetting. Material that cannot readily be excavated with a backhoe may be taken as rock.
  • Zone of Influence = a footing would be determined to be within the zone of influence where the edge of the sewer trench is below a line drawn at 45-degrees in clay, or 30- degrees in sand or silt, from the underside of the building (see AUS-SPEC Standard Drawing ASD 406 - below).

For further advice contact Council's Water and Sewer section on PH: 6581 8111 or council@pmhc.nsw.gov.au

This page was last updated on: 23 June 2016