What’s a soil test? Why do you need one before renovating or building your new home? And what are the 3 mistakes I see homeowners make that can cause extra cost, time delays and loads of stress?
Watch this video to find out.
And here’s the transcript, if you prefer reading to watching …
I wanted to quickly share with you three common mistakes that I see homeowners make when it comes to getting soil tests done for their project.
Okay, so what’s a soil test?
Well, a soil test is where a geotechnical engineer will come out to your site, and they will take samples of your site. Basically, they’ll core down, and they’ll see what the soil is doing at different levels under your site. And from that, they’ll be able to see how reactive the soil is on your site. The reactivity is how much of an expand or move and what type of structural foundation and support it will provide to whatever you’re putting on it. And that information is really an important thing for the structural engineer to be able to design the structure of your house, the foundations, the footing, the slab design. Okay. So once they get that information, they then classify your soil, There’s different levels of classification. And then, that is then important information that the structural engineer needs.
Okay, now, there are three mistakes that I see homeowners make when it comes to getting soil tests done for their site. The first one is that they only get one core done. So as I said, geotechnical engineer comes out and they take this sample. And what happens is that in one site, you can, even in a suburban site that have 600 square meters, you can have very different types of soil conditions across that one site, and when you’re just getting one soil sample done, then that will give one piece of information that then the whole structure is designed. And you can say that you may then start excavation work and find that the soil is actually different in a different location and requires different things. And then that can create some headaches on site. Okay, so definitely get at least two done, if possibly, not more. And then that leads onto my second mistake that I see homeowners make and that’s that they don’t do any design work first. So in terms of getting some design work done first, what that does is show exactly where you’re going to be building. And then you can get soil tests done in those areas. Had a recent example where a homeowner had got some soil tests done, and then got their design work done. And what happened was that their site was actually, it was at the bottom of an old valley that had been filled. And there was one edge of this site, which was just slop and mud. And that’s where footings were going. But the soil tests have been done elsewhere on the site and the structural design done for the soil in that area. What happened was the builders started excavating and couldn’t find anything decent to actually put the footings for the extension interns so they’ve had to do it through piles. And that’s just unexpected information that hadn’t been budgeted for and is being discovered on site and just has to be dealt with on the fly which can cause a lot of stress and headaches and of course, extra costs. So get some design work done first and then you can give that to the geotechnical engineer and say this is where we want soil test done, this is where footings are going to be and foundations are going to be and we need information about. Okay. And then the third mistake that I see homeowners make is that they don’t get professional support in actually commissioning, the geotechnical engineer. You know, professionals speak to professionals in the industry in different ways, then you might expect you’d be able to speak to them and give them different information and different briefing about how to get the soil test done. Okay. And so it’s, it’s really good to get professional support from your designer in terms of asking the geotechnical engineer the right questions, and giving them the right briefing to make sure that they’re getting the best information for your site and that they only need to come out once. Okay.
So, those three mistakes again is one is only getting one soil sample done at the site. The second is not doing any design work first to give the geotechnical engineer information about where to do the soil samples. And the third is not getting any professional support in commissioning the geotechnical engineer
Soil samples are really important. And you can imagine when the equipment comes in to do those courses sometimes they can’t get access to exactly where you need those soil tests done. So just take that into account. Really get some support and help in making that happen for your project. You might find too that if you’re doing a renovation that involves demolishing some of your existing house, in order to extend that you will have to get two soil tests done; one before the demolition and one after the demolition. I hope you found that helpful. I’ll see you later. Have a great week.
What’s soil test?
A soil test is where a geotechnical engineer will come out to your site, and they will take samples of your site. Basically they’ll core down and they see what the soil is doing at different levels under your site. From that they’ll be able to see how “reactive” the soil is on your site.
The reactivity is how much it will expand, or move, and what type of structural foundation and support it will provide to whatever you’re putting on it. That information is really important then for the structural engineer to be able to design the structure for your house, the foundations, the footing, the slab design.
Once they get that information they then classify your soil. There’s different levels of classification. That is important information that the structural engineer needs to do their structural design.
There’s three mistakes that I see homeowners make when it comes to getting soil tests done for their site.
The first one is they only get one core done.
As I said, a geotechnical engineer comes out and they take this sample. What happens is that in one site, even in a suburban site of six hundred square meters, you can have very different types of soil conditions across that one site.
When you’re just getting one soil sample done then that will give one piece of information that then the whole structure is designed. You can see that you may then start excavation work and find that the soil is actually different in a different location and requires different things.
That could create some headaches on site. Definitely get at least two done, if possibly not more.
That leads on to my second mistake that I see homeowners make …
… and that’s that they don’t do any design work first. In terms of getting some design work done first, what that does is show exactly where you’re going to be building and then you can get soil tests done in those areas.
I have a recent example where a homeowner had got some soil tests done and then got their design work done.
What happened was that their site was at the bottom of an old valley that had been filled, and there was one edge of their site which was just slop and mud, and that was where footings were going.
But the soil test had been done elsewhere on the site and the structure design done for the soil in that area.
What happened was the builder started excavating and you couldn’t find anything decent to actually put the footings for the extension into, so they had to do screw piles.
That’s just unexpected information that hadn’t been budgeted for, and is being discovered on site, and just has to be dealt with on the fly. Which can cause a lot of stress and headaches, and of course extra costs.
Get some design work done first, and then you can give that to the geotechnical engineer, and say this is where we want soil tests done, this is where footings are going to be, and foundations are going to be, and we need information about.
The third mistake that I see homeowners make is that they don’t get professional support in actually commissioning the geotechnical engineer.
Professionals speak to professionals in the industry in different ways than you might be able to speak to them. And give them different information and different briefing about how to get the soil test done.
It’s really good to get professional support from you designer in terms of asking the geotechnical engineer the right questions and giving them the right briefing to make sure that they’re getting the best information for your site, and that they only need to come out once.
Those three mistakes again.
- Only getting one soil sample done of the site.
- Not doing any design work first, to give the geotechnical engineer information about where to do the soil samples.
- Not getting any professional support in commissioning the geotechnical engineer.
Soil samples are really important, and you can imagine when the equipment comes in to do those cores, sometimes they can’t get access to exactly where you need those soil test done. Just take that into account, Really get some support and help in making that happen or you project.
You may find too that if you’re doing an renovation that involves demolishing some of your existing house in order to extent that you will have to get two soil tests done, one before the demolition and one after the demolition.
I hope you found that helpful. I’ll see you later.
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With over 30 years industry experience, Amelia Lee founded Undercover Architect in 2014 as an award-winning online resource to help and teach you how to get it right when designing, building or renovating your home. You are the key to unlocking what’s possible for your home. Undercover Architect is your secret ally
Great post! When we were putting in our foundations we discovered TWO old mineshafts! That made for interesting conversations with our engineers I can tell you. Local knowledge now tells us that the entire site is pocked with old shafts and that we were lucky to have only come across two! Knowing the full extent of our building might have saved us some creative engineering LOL
Hi Cate,
Oh wow … that would have made for “interesting” conversations and decision-making!! (Interesting probably isn’t the word that got used at the time!!) Thank you for sharing your experience,
– Amelia, UA x
Hi Amelia, I am soon to start building a pool. The engineer wants to take soil samples of the pool site. Soil samples were taken around our property for the new OSSMS. Is it necessary to pay for new samples or can we re-use the old ones? Nothing has changed (in terms of soil structure). Your thoughts would be appreciated. PS I love your Insta & websites, blogs and podcasts. Keep up the great work!
Hi Simone,
I’d discuss with your engineer why they need to take soil samples again, if you believe nothing has changed. However, ultimately they’re making an assessment and providing advice based on information at hand, and have to professionally feel they have sufficient information to do so. They may feel that (risk and liability-wise), they want a fresh one taken so their structural engineering advice is provided, relying on the most current information (that they’ve also been involved in commissioning). Hope that helps,
– Amelia, UA x
The contractor we hired to build our home failed the soil compaction test for several feet of fill dirt required to fill our stemwall. The contractor went ahead and built the house without telling us about the failure and we only recently discovered it since it was a condition for getting the certificate of occupancy, which we don’t have. What can be done now?
Hey Polly
Wow, that’s really challenging. I would suggest speaking to the overseeing body for your contractor, which will be based on where you are, and also review the agreement you had with your contractor (and what insurances there were in place, etc), so you understand what recourse you have for the contractor not completing this properly. I would also suggest speaking to the person / organisation / authority that grants the certificate of occupancy to see what they’ll allow you to do retrospectively. In regards to the contractor not fulfilling their responsibilities, you may also need to seek legal advice.
Best wishes for a good outcome,
Amelia, UA
We’re in the due diligence period on a property, and won’t have design work done in time to get exact spots. Do you still recommend doing multiple samples now? We know the general area, just not specifics since the land hasn’t been excavated yet.
Hi Brad,
It can be worthwhile, however I would discuss with your team who may understand the nuances and characteristics of your site and area more closely.
– Amelia, UA
Thanks for this info Amelia, When you say “Okay, so definitely get at least two done, if possibly, not more.”… Did you mean to say, get more than 2? Does time of year make a difference? We took 1 sample in October in Maine, pretty dry at that point. We are testing hydrology for removing land from Natural Resources Protection.
Hi Mary
It depends a lot on your site and what you’re planning, (and the amount of site you’ll be covering) … but yes, I’d suggest that, in most cases, one core hole will not necessarily give you a good understanding of the soil conditions on the WHOLE of your site. So two, or more, would be good to do. I’m not sure about time of year making a difference, unless there’s been some significant changes in the meantime, but I’d discuss with local geotechnical engineers (who assess the soil core information), what their advice is. You may find given the conditions in Maine, and specifically why you’re testing, may require you to demonstrate specific things that will drive how and when soil tests need to be done.
Hope that helps!
– Amelia, UA
Hi amelia. Great blog. Thx for all the useful info. We designed a house with an architect for 2.5 yrs. We started our build 2 months ago.
We were advised a soil geotechnical test was not required as no basement or swimming pool was being constructed. This is despite me asking if a soil test was needed. Instead a site investigation will be completed during construction by the builder. The engineer’s designs will be based on their assumptions of the soil conditions for the local area with disclosure on their drawings that this will be subject to an on-site investigation during construction.
We have now discovered that there is a layer of iron stone and the actual bedrock is 1.7m below ground. As a result the slab on ground design has to be changed to incorporate 50 concrete piers with steel cages and pier casings. The cost of this change is substantial.
We feel really disappointed with this outcome as we dont have the extra money. Do you think a soil test should have been performed or is the design and build team correct in their assertion that this is done once construction starts?
What do you advise we do now?
Thx
Hi Jay,
Thanks for your message and your feedback.
Firstly, I’m terribly sorry to hear this has been your experience. The experience you’re having right now is the very reason it’s best to get a soil test done early in the process. It’s why it’s one of my suggested Pre-Design checks in HOME Method, it’s what I see many designers do early in the process, it’s what I’ve always done in my projects, and I’m always surprised when an architect recommends against it and defers it to construction. Because I don’t understand how you can help a homeowner manage their risk, design to a budget, or work with consultants without that critical piece of information about the site conditions.
I can imagine how disappointed you must be. I would suggest getting the builder, architect and engineer together to strategise the next best steps for your project, and having some frank conversations with them. You may also wish to talk with the architect’s and engineer’s overseeing bodies / registration boards to see if you have any recourse (I’m not sure whether you will, but at least you’ll be informed).
Hope that helps,
Amelia, UA
I was planning on building a commercial property on the Gold Coast, and the soil tests (about 6 over the property approx 1 200m2) found that there was an old spring on the site, right in the middle of where we wanted to build. The engineers were able to design a floor, much like bridge spans over the old spring, making the floor thicker in that area, tying this into the foundation piers around the edge of the building, and we were able to build knowing there would be not problems with floors cracking or subsiding down the track. All budgeted and engineered; it’s the only way to go. ALWAYS do the soil tests, grid them over the property, depending on size of the land. While each core test costs – it can save you more money in the long term.
Hey Linda
Thank you so much for sharing your experience – it’s so helpful for others to learn from. Sounds like you avoided something that could have totally derailed your project, thanks to getting those soil tests done early in the process!
– Team UA
Our contractor poured our foundation before the soil was inspected by the building inspector. We had to hire a geotechnical engineer to do soil testing. The soil test was 2.5 weeks ago. How long does it take to get the results?
Hi Andrea,
Thanks for your question. It will totally depend on how busy the geotechnical engineer is, and how their business is structured. I’d always suggest, when hiring someone, to ask “when can we expect to hear back from you / receive the information?” so you then know when to expect information back. I’d suggest just calling the engineer to ask them when you can expect to hear from them … you never know, they may have sent it on email already and it’s in your junk mail!
I’m understanding that a soil test is what I need to determine if I need a Perc test? Do I leave this up to the contractor to do or do I hire someone on my own? Does my contractor handle everything involved in building my house, even putting in the septic tank?
It’s my suggestion that you get your contractor involved early in your project process (as you’re commencing design work, and working collaboratively with your designer). Then, they can also conduct all the necessary due diligence and testing as part of their involvement. It’ll depend on the contractor as to whether they’ll do the septic set up as well, and some contractors may sub-contract it out to a company as part of the process. Have the conversation with contractors you’re talking to, to see how it gets handled, and also what that means for who is managing the work (and responsible for ensuring it is done properly). If you’re not getting your contractor involved until right before construction, then you’ll need to organise the testing. Hope that helps.