Who is a geotechnical practitioner?
Most authorities in Australia require geotechnical reports, assessments and related field investigations to be done by, or under the effective supervision of, a “suitably qualified and experienced geotechnical practitioner” who also has appropriate professional indemnity insurance. “Suitably qualified” in Australia means a specialist geotechnical engineer or engineering geologist. In some jurisdictions, the geotechnical practitioner will be required to formally declare and verify his experience and qualifications. Some geotechnical investigations may require the involvement of other scientific specialists.
What investigations should a geotechnical practitioner undertake?
Site investigations for geotechnical assessments might involve surface inspection (eg mapping, photography), sub-surface work (eg drilling, excavator test pitting, hand or mechanical augering), sampling of materials (eg soil and water sampling), and laboratory testing (eg shrink-swell or slope stability testing of clayey soils). In any event, they should be done in accordance with Australian Standard 1726-1993 Geotechnical Site Investigations.
Before conducting sub-surface investigations, the site should be checked by suitably qualified personnel for underground services, which might include electricity, telephone, mains water, stormwater, sewerage or gas lines.
In some instances, your local Council may also require a house site soil test or a domestic wastewater assessment, or both. Because all of these tests involve similar types of investigations (eg. test pitting, augering or drilling, and soil testing) it saves money if they are done at the same time.
What does “geotechnical” mean?
Geotechnical issues arise when humans bump into geology. We bump in a big way with major projects like highways, ports, airports, cities, railways and mines, and the disciplines of engineering geology and geomechanical engineering are fundamental inputs to safe development.
Regulators are well aware of the adverse effects (actual and potential) of geotechnical issues (“geohazards”). Landslides in their various forms are a common group of geohazards, but there are others. Here’s a typical list of geotechnical questions you might ask yourself if you plan any development:
- Is the land steep?
- Is it in a proclaimed landslide zone, or is it at risk of landslides? View a landslide in action.
- Does it have clay soils?
- Are the soils of adequate strength to support a building?
- Are there varying soil or geological conditions?
- Does it have dispersive soils and tunnel erosion, or other drainage problems? View a PDF on Dispersive soils and their management.
- Has fill been placed on it?
- Is it likely to flood?
- Has it been contaminated by past activities?
- Will sea level rise, storm surge or coastal erosion be problems?
- Is it likely to be approved for domestic wastewater disposal?
(Incidentally, all of these issues also ought to be addressed in reports for soil tests for houses).
Are you buying land to develop it?
If you plan to buy land and develop it, your conveyancing lawyer ought to include in the contract for sale a condition precedent that states that you will purchase subject to a satisfactory geotechnical assessment and report. The report should specifically address your intended use of the land.
If you are making a development application (eg building a house) to you local council, the Tasmanian Landslide Code in the Interim Planning Scheme may apply to you. Use this flowchart I’ve compiled to help you determine whether your development may be exempt from the Code (PDF Document 69kB). You may need professional advice from someone like me.
In Tasmania, state government policy makers have developed principles and guidelines to inform public policy (for example, on Landslide Planning) and help regulators like local councils incorporate geohazards management into planning schemes. This in turn means that if you are intending to buy or subdivide land for any form of development, or extend a house or building, you may be required to demonstrate that what you propose can be done at an acceptably low risk to yourself, other people and property. You will require the services of a suitably qualified person — a geotechnical practitioner — whose geotechnical assessment and report must be done in accordance with accepted procedures. In Australia, some commonly quoted guiding documents listed by regulators are:
- AS1726: 1993. Geotechnical Site Investigations
- AS/NZS4360: 2004. Risk Management
- AS2870: 2011. Residential Slabs and Footings
- Australian Geomechanics Society, 2007. Landslide Risk Management
The Australian Geomechanics Society (AGS) has published a valuable series of Geoguides for public education, and the 2006 Landslide Hazards Handbook produced by the Australian Building Codes Board provides guidance on building sites prone to landslide hazards.
In Tasmania, Mineral Resources Tasmania is working hard on categorising areas at risk of slope instability. Its landslide database is continually upgraded. Helpful brochures describe Landslides in Tasmania, and the Tasmanian Landslide Map Series. The series of landslide hazard maps for all major population centres in the state is freely available, as is a comprehensive guide to their interpretation and use, namely the Tasmanian Landslide Map Series: User Guide and Technical Methodology.
It’s all about risk (what can happen and how bad will it be?)
Risk is a combination of likelihood and consequences. Risk of geohazards can be acceptable, tolerable, or unacceptable. A risk that is acceptable to one person might be tolerable or unacceptable to others.
I typically include in my geotechnical assessments, and in my soil test for houses reports, a table of geotechnical issues which subjectively summarises the risks of various geotechnical issues in relation to development, and where appropriate, suggests risk management procedures to reduce the risks.