Tips & Tricks

Guidelines for Diagnosing Heave, Subsidence and Settlement

We spend a lot of time working around Brisbane diagnosing slab heave, settlement and subsidence. They’re not all the same thing and sometimes working out which way a building is moving can be confusing.

We’re always trying to improve our knowledge so that we can help you better but last week I came across a document that could help improve YOUR knowledge, especially if you are a structural engineer involved in this sort of work.

It’s all about diagnosing heave, subsidence and settlement and it has some handy definitions and guidelines.

It’s written using American terminology and standards. Notwithstanding it is an excellent reference guide.

Read Guidelines for Diagnosing Heave, Subsidence and Settlement

Have a good week.
Matt Cornell
Cornell Engineers

Tips & Tricks

What is a Cut Off Trench

A cut-off trench is my favourite, last-chance weapon against slab heave. Also known as the cut-off wall, a cut-off trench is a powerful weapon of last resort.

What is a Cut-Off Trench?

A cut-off trench is a deep trench around the outside of an existing house that is filled with impermeable material to isolate the soil under a house from external soil moisture sources.

Try Everything Else First

All of the other ways of fixing slab heave must be tried and exhausted first. Check our other pages on slab heave first!

The reason this is your last chance to fix slab heave is that the cut of trench is very expensive and very disruptive. It must be installed by a very careful, qualified contractor as the work involves digging deep trenches right beside the house.

What it Looks Like

Cut off trench for houses suffering slab heave
Cut Off Trench Detail

What is a Cut-Off Trench

The features of a cut-off trench are as follows:

  • Dig a deep trench right around your house. Position it about 1.5m from the house.
  • The depth of the trench is normally dug to the Hs (the depth of soil that contributes to ground surface movement).  Ask your soil tester for this number.
  • The trench is lined with a vertical layer of polyethylene (black plastic) or is fully concrete filled. I like using root barrier plastic.  It’s thicker and a little harder to place, but tougher and more resistant to tears. The plastic runs from the bottom of the trench to the top of the trench and then across to the house.
  • At the house the black plastic is parged to the foundation concrete to seal it.
  • The vertical laps are taped.
  • The pipe penetrations are covered with another square of plastic and taped.
  • The bottom of the trench is sealed with bentonite clay.
  • The trench is back-filled with concrete or a soil and cement mix.
  • A concrete path is poured around the house to protect the plastic where it runs across to the house.

Isolation Achieved

That’s it. The ground under the house is now fully isolated from external moisture sources.

Any moisture trapped under the house slowly stabilises across the width of the house. The stable moisture content across the building is now consistent like it was before the house was built.

In time the house will stop moving and you can go ahead and fix the damage.

A Cut of Trench is Not A Drainage Trench

Installing a drainage trench around your house (as recommended by some experts) is one of the worst things you can do to treat slab heave on a flat site.

A porous drainage trench allows water to flow into and sit in the trench and soak into the ground right around your house. A drainage trench is exactly what I would use if I was TRYING to cause slab heave in a house!

However, if you are experiencing slab heave on a sloping site and the bottom of a drainage trench can be designed to discharge water AWAY from a building, then a drainage trench might have merit.

Contact us to see if a cut-off trench or a drainage trench are the right solution for your slab heave problems.

Tips & Tricks

Slab Heave Explainer Video

What is slab heave and how does it affect houses?

Hi. I’m structural engineer Matt Cornell and this is my explainer video on slab heave.

Tips & Tricks

Slab Heave Hits Melbourne – and now Sydney

Slab Heave in Melbourne

A recent article by Simon Yohanson in The Age, Melbourne has highlighted the ongoing problem faced by home owners in new Melbourne subdivisions – slab heave.

“Thousands of suburban home owners facing financial ruin

Read more:

Yet, if you’re a Melbournite affected by slab heave, it is of little comfort to know that you are not alone.

Slab Heave in Sydney

There’s been a spike in users from Sydney hitting our website looking for information about slab heave. Welcome – and don’t panic! There’s plenty of information on the web about slab heave and how you can minimise its effect. I hope we can help provide some of that information for you.

What is Slab Heave?

See our post: What is slab heave?

Who is Responsible?

Melbourne, Sydney and Brisbane homeowners – if your house is within the builder’s warranty period, the builder and the developer are responsible for fixing slab heave – but they will try to re-assign the blame. For your best chance of having your house fixed, you MUST be prepared to eliminate conditions that could be considered to contribute to the problem.

If your home is outside the builder warranty period, it is your responsibility to fix slab heave. There’s plenty of information on this site on how to improve conditions around your house. If you need more help or a recommendation for an engineer please ask. I’ll try to find an experienced residential engineer that can help you.

The Claim Process

If you are within the builder’s warranty period, this is the claim and fixing process:

  1. Monitor the damage. See my post about how to add some science to you monitoring and writing a ‘crack diary’. This will improve your chances of getting a good outcome.
  2. Improve the drainage conditions around your house. This is your responsibility as a homeowner and you can not get out of this responsibility. Read this post and follow the recommendations in “Do this First”.
  3. Make yourself aware of the insurance claim process for your state. Knowledge is your best weapon against slab heave.
  4. Make a written complaint to the builder outlining the damage, when it occurred and that you want the builder to fix it. Do not mention legal action. At this stage, you want to work with the builder – not against the builder. Your complaint MUST be in writing and you MUST keep a copy as evidence. Send it by email and request a read receipt; or send it by registered post, or send it by facsimile. Include a history of the damage (including photos you took and a copy of your crack diary).
  5. Allow the builder fair access your property to assess the damage. A clever builder will engage a residential structural engineer almost immediately. Have your crack diary available for inspection or for copying but do not give your crack diary away.
  6. The builder should, but doesn’t have to, give you a copy of the engineer’s report. If any aspect of the engineer’s report is unclear, ask the builder if you can speak to the engineer directly. If you disagree with any aspect of the engineer’s report, consider getting an independent interpretation of the report, or better still, an independent engineer’s inspection. Read here about what to ask any engineer that enters your property.
  7. Follow the recommendations of the engineer’s report to the letter.
  8. Allow the builder access to the property to make any repairs required in accordance with the engineer’s report.
  9. Continue to monitor the cracks and keep your crack diary up to date.


Warning 1: Fixing slab heave is a slow process. You will not see improvements in crack width or floor slope occur very quickly. It takes a long time for heavy clay soils covered by a building to stabilise.

Warning 2: Bumping this problem up to litigation will slow the resolution process down infinitely. Most builders take the hands-off approach when lawyers get involved.

Warning 3: If you haven’t built your house yet – and your builder is using a waffle slab, carefully read and implement the builder’s and engineer’s advice on foundation maintenance (management of soil moisture around and under your house).

The Forums Might Help

We contribute to the HomeOne forum from time to time. Check out this long discussion on slab heave.


Slab heave can be avoided and it can be fixed. You have to be patient and you have to improve the site conditions around your property.

If you’re from Sydney or Melbourne and your house is suffering slab heave, I know your pain. I wish you all the luck in the world.

By Matt Cornell
A Structural Engineering Blog