What lurks beneath the surface: Why you need a Geotechnical Report before you build

Let’s just state the obvious – before you even start designing your home get to know the plot of dirt you are building on! To the naked and usually untrained eye, a plot of land is pretty much all about the size, shape, slope, and what vegetation grows on it, right? But what lies below the surface and why does it matter?

It’s easy to forget that when building a property or even extending your home, you are creating a structure that is firmly anchored into the soil. So why not just start digging? Well, it’s because the type of design and how you may want to engineer your home will be strongly influenced by the conditions of the land on which you are building. It will also greatly impact on your budget. The last thing you want is nasty surprises, and there can be many lurking below the surface.

Now the only way to really know what you are in for is to get a geotechnical report for your property. You can do this whether you have a clear block of land or an existing house already on site.


What is a geotechnical report?

 

 

A Geotechnical Report or Geotech Report is an analysis of the soil, rocks, bedrock properties, chemical composition, and water conditions of a parcel of land. Think of it as an important tool to communicate site conditions, which will influence the design and construction of any structure you are looking to build.

The reports are created by specialist Geotechnical Engineers will usually also include design and construction recommendations relating to earthworks, footings, and drainage. They will also flag any potential issues around how the soil on site is likely to move, expand or contract, which in turn could have serious long-term repercussions for your home or property; think cracks in the walls, water damage, flooring issues etc.

 

 


When will I need a geotechnical report?

Many councils require a Site / Soil Classification Report or Geotechnical Report when submitting building consent applications or when subdividing a property. Jump online or give your local council a call to see what their requirements are. Even if not compulsory, you may still want to consider getting one done for peace of mind and to arm yourself with all the info you and your builder will need.

You will want to get this done at the same time as you are starting to look at design options; not after you have the perfect plans drawn up!


What are the soil classifications in Australia?

Soil classifications are intrinsically linked with foundation classifications stipulated by the Australian Standards for Residential Slabs and Footings.

Your parcel of land will fall into one of the following 7 categories, with A class being the best result and P class requiring the most cost:

  • Class A – Stable, non-reactive: possibility of very little or no ground movement because of moisture change (sand, gravel, and rock sites).
  • Class S – Possibility of slight ground movement (often gravel / sand / clay / silt mixed sites).
  • Class M – Possibility of moderate ground movement (clay or silt mixed with some sand sites).
  • Class H – Possibility of high ground movement (clay, organic silt (peaty) sites, with little sand or gravel).
  • Class E – Possibility of extreme ground movement (getting nearly to pure clay and organic silts/peats).
  • Class P – Problem sites: ground movement as a result of moisture change may be very severe or unknown (uncontrolled fill). You will need to consult a geo-engineer and structural before building a new house.

Who can I get to do this for me?

It’s easy to organize yourself. Simply search for a Geotechnical Engineer or Geotechnical Consultant who operates in your area. Most list the range of services they provide on their websites.

Reach out to a few different providers, asking each for a quote and details as to what investigations they will undertake, what will be included in their report, how much it will cost and in what timeframe can they deliver the report. This way you will be able to compare apples with apples.

To provide you with a quote they will ask you for the address of the property to be investigated and any drawings or plans you have for the property eg water/sewerage plans, any existing building plans etc.

Many builders will no doubt also be able to organize a Geotech Report for you, but it’s better to get this done upfront before you even decide on a builder and in any event, they would subcontract this work out, so why not simply go direct to the source? This way you also avoid paying a margin that the builder may charge you to organise the Geotech Report.


What will I learn from the report?

You can expect a Geotech Report to include the following information:

  • a clear description of the parcel of land and information about the ground conditions;
  • comments about any geotechnical hazards or risks;
  • an overview of how the ground conditions may impact your proposed building project; and
  • recommendations for the design and construction based on their professional experience and research.

Speak to them about the type of building you have in mind as the extent of investigations a Geotech Engineer can carry out will range from a desktop analysis of the area your property is located in, to a visual walkover mapping the site features, to what they call “subsurface investigations” eg drilling boreholes, inspecting existing excavations and test pits for sampling and testing.


How much should it cost me?

The cost will depend on the scope of works you are asking for and the complexity of the investigations required. A heavily sloping block of land with lots of boulders or significant geographic features will be more complex to investigate than a flat, cleared block that is part of a well-established residential area.

 

A simple soil test should cost you around $500. If you are looking for more detailed investigations and contour surveying this can cost between $500 and $1,500. More complex investigations can creep up to between $2,000 to $4,000. Ultimately it will always depend on the complexity of the local geography, local council regulations and what you are intending to erect (e.g. a granny flat is much simpler than a massive 3 level house).

Remember though, it costs nothing to organise 3 quotes to compare what your choices are, and in the long run keep the cost of your Geotechnical Report in perspective to the overall project costs. After all, spending $500+ for a new building project worth hundreds of thousands of dollars and a property that is being built to last decades, really makes sense.

Investigate now, don’t pay later!

 

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