Geotechnical Information and Frequently Asked Questions


Geotechnical Engineering refers to a branch of engineering that applies geological science in a practical manner.

A geotechnical site investigation is the assessment of the subsoil strata at a proposed development site. It is essential to know what the underlying soil profile contains so a suitable slab or footing system for a building can be designed appropriately.

Site Classification

Before any construction can commence on a proposed development, Local governments require all residential and commercial developments to have a site classification in accordance with Australian Standard AS2870. This allows structural engineers to design a suitable footing system to support the proposed development.

The site classification is a measure of the potential reactivity of the foundation soil below the slabs and footings of you proposed development. Reactivity refers to a soils potential to move due to seasonal moisture variation which can cause damage to buildings that don’t have suitable footings, hence t is essential to get the footing or slab correct to provide stability to the structure and minimise movement.

Effluent Disposal

Building out of town? If so your proposed house will not be serviced by reticulated sewers and you will require an Effluent Disposal Assessment and Report.

Effluent Disposal Reports provide you with a suitable designed wastewater treatment and disposal method that complies with your Local and State Government requirements. This includes suitable disposal methods and setback distances from waterways, boundaries and buildings. Our effluent disposal investigation involves a subsoil assessment by recording the soil profile and taking samples to determine suitable methods for effluent disposal. There are several types of disposal systems and the suitability of the systems depends on the local site restraints mentioned above and the subsoil conditions.

The most well-known and widely used system is a septic tank in conjunction with absorption trenches. The septic tank treats the wastewater to a standard known as Primary Treatment. This means the effluent (treated wastewater) has to be disposed at least 300mm below the surface, typically into an absorption bed. This system relies on gravity (i.e. the absorption beds have to be located at a lower elevation than the septic tank) and the permeability of the soil to dispose of the effluent. As the effluent percolates into the soil, shallow bedrock or low permeability soils are not suitable for this type of disposal method as the disposal area will eventually surcharge. A full list of approved septic tanks can be found here.

The other main type of system is the AWTS or Aerated Wastewater Treatment System. This system treats the wastewater to a higher standard by passing it through an aeration chamber and clarifier before pumping it to the disposal area which is typically a subsurface irrigation field comprising drip lines 150mm below the ground. Due to the power requirements to run the aeration chamber and pump as well as the requirement to have the system serviced 4 times a year there are ongoing costs with this system, however it is the only suitable methods of disposing of effluent on some sites, however it does have some benefits in the sense that the disposal area can be located uphill of the tank as it is a pumped system and does treat the effluent to a higher standard which is preferred by our more environmentally conscious customers. A full list of NSW Health approved AWTS systems can be found here.

Other systems commonly used are a Worm Farm system which can either be disposed in trenches or LPED (low pressure effluent distribution) which is distribution through a 25-30mm pipe in 200x200mm trenches filled with 10-15mm aggregate. A full list of wet composting toilet systems can be found here. Bear in mind with these systems that they have ongoing costs due to electricity costs and pump maintenance and the system functions best with continual use as opposed to intermittent use as long periods of rest can kill the microorganisms.

After conducting our site and soil analysis, one of our engineers compiles a report to submit to your local Council that complies with all relevant Australian Standards as well as Local and State Government requirements which vary from Council to Council. This includes a specification of the system type, disposal area size and a site plan with all relevant buffer distances off


My report specifies an AWTS, is an AWTS the only suitable system for my site?

Yes. Typically (unless requested otherwise) we specify a Septic Tank in conjunction with absorption bed if the soil is suitable for this application. If you have an AWTS specified it will most likely be due to the low permeability of the soil or shallow bedrock.

Why hasn’t a brand of Septic Tank or AWTS been specified in the report?

There are many different brands that can be used that comprise either concrete tanks or polymer tanks. We typically leave this at the discretion of the installer as different plumber have different preferences and site constraints may mean using a lighter polymer tank as opposed to a concrete tank if access is limited. A full list of NSW Health approved systems can be found at the links below:

Septic Tanks


Wet Composting Systems (Worm Farms)

We also do not specify the final location of Septic/AWTS tank as the installer will determine this with consideration given to the drainage plan of the house and site limitations to ensure all plumbing meets the required minimum grades specified in AS3500.2.

Can I irrigate greywater?

In short, no. NSW Health requires that all effluent undergoes treatment prior to disposal and this includes greywater. In order to use effluent for surface irrigation it must undergo tertiary treatment (including treatment twice and then disinfection with chlorine tablets or U.V. light. This can be achieved through an AWTS or a NSW Health approved greywater treatment system which can be found here. Note if you intend to irrigate greywater you will need to submit a Section 68 (Local Approval to Install, Alter and/or Operate a System of Sewage Management).

Can I complete the work myself?

Only a licensed plumber can complete the works as all work has to be compliant with AS/NZS 3500 and typically the local approving authority will require an inspection of the works by either one of their Building and Health Surveyors or a certificate of inspection from a private Building and Health Surveyor. Prior to completing any of the work an application in the form of a Section 68 must be submitted with an accompanying report completed by a competent Civil Engineer.

The Section 68 for various Council can be found at the links below:

Bathurst Regional Council

Lithgow City Council

Oberon Council

Blayney Shire Council

Orange City Council

Cabonne Council

Forbes Shire Council

Mid-Western Regional Council

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