Why a geotechnical report is a good idea

What is geotechnical work?

Not quite geology and not quite geo-engineering, nor structural engineering. Think of it a little like a less exciting version of the hit TV show, CSI. However, rather than gruesome crimes scenes, we examine your potential building site to figure out your design, based on the following sub fields of geology;

  • Biology (we look for certain tree and plant species which can give precious clues away on the above…surprisingly reliable!)
  • Climatology (how the climate has affected the landscape over the last several million years – related to the above points)
  • Geography (the terrain; both human-made and natural)
  • Hydrology (the water table and how it effects the local landscape)
  • Geomorphology (science of how landscapes change over time)
  • Mineralogy (the type of minerals and rocks around)

And my favourite, and probably the quickest way to clear a dinner party; Sedimentology (what the soil is made of)

Much like our more glamorous cousins in CSI, we geo-techs use evidence, logic, laboratory testing (we have lab coats too!), historical documentation and often heavy machinery to piece together the puzzle of what lies beneath to calculate the safest and most economical design for your project; literally from the ground up.

What it the purpose of it?

When designing a project it is very normal to not spare a thought to the invisible stuff that lies underneath your site. It is one of those things that we can’t really see and if it looks okay on the surface then what’s the big deal anyway?

The issue comes down to time and the Earth has had lots of it; about 4.5 billion birthdays worth! When rock surfaces, our environment begins to break them down into boulders > pebbles > gravels > sands > silts / peats and clays.

All from the action of chemical erosion, as well as the breakdown of organics and mechanical and organic erosion (think of rocks tumbling in a washing machine slowly wearing down and tree roots breaking up large outcrops).

The definitions of different sediments are largely about their grain size and composition. Clays and peats are by far the most relevant when structural design comes into play. The reason for this is they are what we call “reactive” sediments, as in they react to differing moisture contents ie. In winter they soak up water and expand, and in summer they contract as they dry out. This effectively means the ground is in constant slow-motion movement throughout the year, in all directions; up, down, north, south, east and west. We just don’t notice until something breaks.

The record amount of movement we had in my previous Lab (from memory) was recorded at 157mm (this is huge!). Imagine plonking a building down on a surface that can move 157mm per 10 lineal meters of wall in any direction – that is enough to quite literally tear a building in half if the footings and sand pad are not calculated to absorb this movement.

Over time climate changes, rivers change course (often multiple times), floods occur, sea levels rise and fall, rocks get weathered and sediment gets deposited. Things can change very quickly from nice, easy sand to highly reactive clay and you would never know without taking a peep. With some knowledge and experience, this can be calculated and preventative measures can be put in place.


The F-Word

Humans like to dig holes and fill them in with stuff, we have since the dawn of time and we are really good at it.

We call this un-natural sediment “uncontrolled fill” and it can be a HUGE problem when building and potentially add thousands of dollars to your build due to poor quality fill. Even worse, it may contain contamination which in turn may affect your and your family’s health. Think asbestos, chemicals, un-exploded ordinance etc. It’s more common than you would think.

Fill is unpredictable and causes big headaches, often resulting in the entire site needing to be scraped back and needing to start over, costing upwards of $30K in nasty situations for a typical residential block.


When would you need one?

A Geotechnical investigation is always recommended if you are in a known clay area or a very rocky area, or even if you would prefer to be cautious and sleep better at night.

These typically cost about $1,500-$4,000 (in extreme cases) and the price is usually to do with;

  • The complexity of your design
  • Local councils’ regulations ie. drainage regulations vs water table.
  • The complexity of the local geology ie. if your project is located in an earthquake zone vs on clean natural sand near the beach.

If your project is located within known areas of clay, swamp (peat), intrusive (volcanoes that never erupted) or if known sinkholes are around. I would recommend taking your engineer’s advice, every time, as this is something that could potentially ruin your dream home or renovation if ignored.

On the bright side, it is said that for every $1 you spend on a geotechnical investigation you will save $50, either in the design savings or long-term maintenance on your property.

Surprisingly – you do not always need a full geological investigation. Depending on where you live, about 50% of the time a Geotechnical report’s younger cousin called a “site inspection” can suffice. This is basically a mini-geo investigation where a driller comes to site, logs the soil profile, checks for clay/peat and takes a few samples if required and classifies your site.

If the samples are sand / non-reactive material it will usually cost you around $300-$500 if you live near a metropolitan area, and if lucky, can take less than a week.

Otherwise, if clay/peat is present, you will need several lab tests done to determine the surface movement in order to calculate what footings are best for your project. With lab tests and a check over by a Geo-Engineer, this will usually cost around $1,000-$1,500 and can take 3-8 weeks, depending if the lab testing is in-house or outsourced.


The verdict is in – soil classifications for your site

Following your geo report / site classification, your building site will fall into one of the following 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 as a result 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 PProblem 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.


Most builders are pretty good at identifying the nasty stuff and will let you know if they have any concerns and will possibly advise you to undertake a proper geo-investigation. Yet it is always worth asking before you commence as site works and concrete costs can easily end up blowing out your budget.

Ultimately it is the engineer’s call, but if you look closely you will see some clues you can use to gauge an idea of what is happening underneath your site are and help you figure out roughly how much it may cost before you begin. Otherwise, you can always ask your neighbours if they have had a geo report and hear what they have to say.


About our guest contributor:

Our thanks to Tom Young, CEO of Udrew for sharing his insights and thoughts with us this month.

Tom was recently the overall winner of the 2018 WA Innovator of the Year Award for his interactive building and instant approval system. Congratulations Tom!

Udrew is a multiple award-winning cloud platform for homeowners and contractors without any technical skill to design, engineer and manage their own custom-building project from start to finish, for 70% less cost and receive potential instant council approval for their small construction projects.




All designs are checked and assessed against all local and national building regulations in real time while materials are quantified as they are drawn, meaning you can order every component with the click of a button. This means you can begin construction of your own project in minutes – not months. See udrew.com.au for more details.