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rock on my building site



You may be looking at buying a block of land or you already own it, and you want a new home on it but are wondering, “what happens if there’s a lot of rock on my building site?”


And that’s a valid question, because every home has foundations which involve digging into the ground, and if there’s a heap of rock in the way then that could be a real nuisance. Then again, it might not be as much of a nuisance as you think – the old adage talks about building your home on the rock instead of the sand! But in practice, it depends on a few things – which we’ll discuss in this blog.


Step 1 is to determine the ins and outs of the rock. Theoretically this can be done by a geotechnical engineer, but a series of simple questions will help you as the landowner to come up with the right answers.


  1. Is it a series of individual rocks and boulders? Or is it ‘sheet’ or continuous rock?
  2. If it’s a series of individual rocks, then is it made up of lots of smaller rocks & boulders (100 to 200mm)? Or is it a group of much larger rocks (1mtr+)? Smaller rocks can be annoying but generally don’t pose any problems to construction. But larger boulders (350mm+) demand attention – we’ll discuss this later in the blog.
  3. If it’s sheet rock, then this can throw up some challenges as well, but see the next point.
  4. What type of rock is it? Is it really hard granite? Or is it softer basalt? Or even limestone? There’s no simple way to identify this, but a tip is to take some photos and then search on Google – each different type of rock has different characteristics and properties and can be identified that way.
  5. How widespread is the rock? This involves working out if the rock completely covers the whole area of where you wish to build or just part of it. And is the rock right at the surface or 500mm down? Driving a thin steel stake into the ground at various points will help you determine a lot of this.


Once you’ve got a clear picture of the extent of the rock, then try and draw it out on a page.  You could even screenshot a close up of the land from Google Maps and then make notes on it of the types of rock, where it is on the site and the depths.


This information will be invaluable when you approach your builder, because the next step involves the structural engineering aspect of what happens if there’s a lot of rock on your site.

Scenario 1

Let’s say you’ve determined that your site has numerous groups of large granite boulders – well over a metre in diameter, some of them several metres. But you’ve also identified that the group’s of rock are patchy and in some areas of the footprint of the home the rock doesn’t start until a metre or 2 down into the ground, but at other sections it’s protruding out like a bald head, and there are actually a few huge boulders lying around on top like a giant left his marbles lying around.


So you give this info to the builder, and for a suspended floor system, their engineer comes up with something like this:


  1. Bring in an excavator to remove the boulders that are not deeply embedded into the ground. This means the rocks lying around on top that are in the way, but also the smaller rocks (under 2 or 3mtrs diameter) that are only half submerged.
  2. Fill in the areas where rocks have been removed with compacted fill.
  3. Drill holes for the house footings 350mm round to 1mtr deep in the locations where the rock doesn’t start until 1mtr or deeper + drill holes 350mm round down onto the rock for the locations where the rock is less than 1mtr but more than 300mm below the surface + bolt some hold-down plates directly onto the surface of any exposed rock by injecting chemically cured glue into 4 holes drilled into the rock that have bolts in them.


This scenario relies on the size of the boulders and hardness of the rock to get the foundation strength for the home. And it’s also a classic example of creativity to work around the real-life situation and end up with an excellent result. The cost of this sort of situation would be in the vicinity of an extra $3,000 to $15,000 depending on the size of the boulders, the extent of them, and how many stumps are being bolted to rock instead of a normal footing.


There’s no such thing as some houses having better footings than others…


One thing we want to point out here – this is not an inferior result. There’s no such thing as some houses having better footings than others – the building code dictates that all homes must have an equal strength supporting the home for its size and layout, and the engineer will do the calculations to achieve this.


Scenario 2

A completely different example might be a site with massive sheets or layers of rock. These layers seem to show up at the surface in some spots like a chocolate wafer poking out of the icecream, and then the layers taper away deep under the ground. And you’ve determined that there’s rock less than 1mtr below the surface right across the footprint of where you want to build. But you’ve also worked out that it’s basalt rock, which is considerably less hard than granite.


The simple answer here is to run with normal footings into the ground (based on a suspended floor) and engage a contractor with a rock boring drill to go straight through the sheet rock. These contractors have the necessary machines and boring bits to do this, although expect to pay an extra $2,000 to $4,000 depending on the size of the home, the thickness of the rock and how hard the basalt actually is.


Scenario 3

A 3rd scenario might involve lots of smaller rocks. The site might look a disaster – hundreds of grey rocks as big as basket balls lying around everywhere and you’ve also worked out that they’re under the ground too. But as long as the general maximum size of these rocks is not more than a basket ball size, then for a suspended floor, these rocks just get pushed out of the way by a standard hole boring machine.


No special equipment is needed and no special engineering, but it’s highly likely there’ll be some additional costs for concrete because some of the holes blow out in size a bit due to some small boulders being pulled out of the sides of the holes, and also some extra time for the hole boring contractor – nuisance factor. Could be an extra $1,000 to $3,000.


Every site requires a soil test to be conducted by a geotechnical engineer, and this report would normally spell out the details of any rock encountered by the engineer when they test drill, but although it’s a useful report, it won’t provide a comprehensive report of all rock across the area you want to build – this is something that you and the builder need to thrash out in the early stages.


At the end of the day, there’s generally a way around the rock, especially with a suspended floor system. Of course, a concrete slab is still possible, but may involve bringing in more fill to raise the base up above any protruding rocks, and more rock removal / rock breaking work. But the structural engineer that the builder uses will determine exactly what needs to be done to get the best result for the least upheaval.


Getting rid of the rock is not necessarily the best option – getting top advice is always the best option!


We trust this blog has helped you towards answering that question, “what if there is a lot of rock on my building site?”.

Keen to find out more?

Reach out to us. We’re here to help.

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