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
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.
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.
Have you just noticed some cracks in your house? I’ll bet you’re worried. Why did they appear? How do you fix the cracks?
First thing you should know: You’re probably going to need help to fix cracks in your house. They’re not normally the sort of house damage you can fix yourself unless you:
diagnose the problem
solve the problem
and can repair the damage.
So don’t be afraid to schedule an inspection of cracks or damage with a structural engineer. An engineer experienced in diagnosing and fixing the damage in buildings can offer invaluable advice and get you on the road towards satisfactory building performance.
Take Immediate Action
If you think your building is unstable – stay clear of the area and arrange for a qualified and registered builder to stabilise the area straight away. Then arrange for an inspection by an experienced structural engineer to design and document a permanent solution.
Start a Crack Diary
Depending on the nature of the damage, I sometimes recommend monitoring the damage.
Be a little scientific about this process because it will help later. I suggest that a crack diary records:
where the cracks are; and
how long the cracks are; and
how wide the cracks are.
Write a new entry every couple of weeks or when you notice new damage. “No change” is as important as “some change”, so write it down!
Here is the link to our handy crack monitoring kit that might help you keep track of the cracks in your house.
Digital photos are time-stamped so your photos and the diary should correspond if possible.
This is an excellent example of a monitoring photo. It clearly shows the crack and a piece of photocopied ruler has been bluetacked to the wall to help monitor the width of the crack.
Use a Water Level
You won’t be able to tell whether your slab is level just by walking on it. I use a professional digital water level. They’re expensive but make life easier if you do a lot of testing. Here’s a video I made about the water level I use:
You can make your own using material from around the house. See how to make your own water level. Rolling a marble on the floor will tell you a little bit but it won’t work on carpet. That’s why a water level is better.
Turn the Levels into a Contour Plan
Contour plans help structural engineers diagnose the cause of movement and damage in a house. The process is quite complicated so you will need an engineer for this part.
Our Youtube video talks about how we assess slab contour plans
Ask for Help
Get the damage checked out by a structural engineer. Do this BEFORE you decide to sell your house and BEFORE a building and pest inspection and if there are signs of collapse BEFORE any part if the structure starts to fail.
Do Cracks in Walls Indicate a Structural Problem?
Cracks in walls tend to indicate some sort of movement in a wall – but they don’t necessarily indicate a structural problem.
Depending on the width of the crack they might not indicate anything at all.
Most times a hairline crack can be monitored without any further action.
A crack wider than 5mm wide or a group of cracks 3mm wide could indicate structural movement and that it is time to contact a structural engineer to investigate the cause of the damage.
Why is My House Cracked?
I’m a structural engineer. When I take calls about cracks in houses, this is what I want to know and why:
How old is the house?
If the house is within the statutory warranty period, you might be covered by the builder’s insurance policy.
The age of the house also tells me a little bit about the type of construction to expect and puts into perspective the age of the damage.
A 10-year-old house and a 10-year-old crack that hasn’t changed in 10 years is a different scenario to a 3-year-old house and a 1-year-old crack that is developing rapidly.
Did the damage occur as the result of an insurable event?
If yes or maybe, refer the damage to your insurance company. See Lodge a Claim below. Don’t forget to stay clear of any unstable areas. Your insurance company may arrange a temporary ‘make safe’ visit by a builder.
What sort of house is it?
Is your house single storey? Is it double-storey? Is it on a concrete slab or is it on pole footings? Is it brick veneer or concrete block or is it timber frame? Is the roof tiles or metal sheet or something else? This tells me about the type of construction but also starts to give clues on what could be causing the damage.
Has something changed recently?
Cracks in houses rarely just happen.
Is there a new retaining wall near the house?
Has the building flooded recently (see Building Insurance below)?
Has the structure been changed recently?
Have you planted or removed trees or shrubs recently?
I’m looking for clues as to the cause of the damage. A recent change might give me ideas on how to solve the problem.
Get your pipes tested!
If I can’t link the house damage to a recent change or an insurable event, I will suggest you get your house pipes checked by a qualified plumber.
Leaks in pipes can cause a lot of damage to houses.
Engage a plumber to check the integrity of your sewer, stormwater and water supply pipes.
Use a plumber with a drain camera if possible because a camera is a quick, easy way to identify the location of a leak. However, to be certain there are no ‘invisible’ breaks, ask the plumber to flood your pipes and check to see if the water level drops (also called hydrostatic testing).
Get any damage repaired immediately!
Schedule an Engineers Inspection
This is where a structural engineer comes to visit your house in person. This is my preferred process:
Tour of the House
Take me for a walk around the house. Show me where the damage first appeared and how it progressed. I’ll only need 10 minutes of your time, but where the damage started is critical in helping me identify the cause of the damage.
The Site Inspection
I take photos of the outside 4 elevations of the house first – before I forget. I use these photos to confirm the locations of crack photos later.
I start at the front left-hand corner of the house and work around the outside of the house in a clockwise direction. I record the location, width and nature of any damage I see. I take a photo of any ‘good’ size cracks for the report.
I record sources of uneven soil moisture around the outside of the house including poor drainage, dripping taps, gardens located against the building and hot water systems that discharge water against the footing.
I categorise all damage on the inside of the house by starting at the front left-hand side of the front door and working through the house room by room by keeping the wall on my left-hand side. I record the location and size of cracks, gaps in cornices and gaps under walls in a schedule cross-referenced to a floor plan with the defect location.
I use a digital water level to record the floor level in each room. Levels are taken in easily reproducible locations such as at doorways and beside windows. That way we can compare levels in the coming months to determine if the slab levels are getting better or getting worse.
The causes of house damage I see the most of are:
Slab heave – an uneven movement of the house foundations caused by changes in soil moisture conditions. This damage is often identified as diagonal cracks starting at doors and windows, doors and windows uneven in frames, gaps under internal walls, broken cornices.
Consolidation of soft soils – downward movement of the house slab and footings one to two years after the house was constructed. This damage is sometimes caused by soft or loose soil becoming more compact under the weight of a house. Sometimes this damage is related to poorly compacted backfill over council sewer pipes and other underground infrastructure.
Slope failure – instability caused by building too close to a slope without sufficient footing protection or footing depth.
Poor workmanship – not as common a fault, but sometimes the plasterboard hasn’t been properly nailed or back blocked. This damage is often characterised by seemingly unrelated straight cracks in plasterboard ceilings or walls which follow plasterboard joints.
How can you tell if a crack is structural?
The direction of the cracks helps me work out which part of your building is moving and which part isn’t. If the whole house was moving uniformly, you probably wouldn’t have any cracks!
I also ask if you have your crack diary. An inspection of a crack at a single point in time can only be used to deduce a limited amount of information. If you are paying for a structural engineer, you may as well get some bang for your buck.
Lots of information is required to solve the problem of cracks in houses. The more you can tell me about the history of the crack the better!
The Slab Heave Investigation
As part of my desktop investigation and assessment I like to analyse and assess as much of the following sources of information as possible:
Results of the plumbing test that you commissioned.
Original footing and slab plan.
Original soil test.
Independent new soil test to current standards with soil moisture contents at 500mm centres.
Previously recorded slab levels.
Photos of cracks from your crack diary.
Bureau of Meteorology rainfall reports at the time your house was built.
The Slab Heave Report
If the damage doesn’t indicate impending structural collapse and the cracks aren’t very wide and you have only just noticed them I might recommend more monitoring.
Otherwise, my report includes my assessment of the severity of the damage, my opinion on the cause of the damage, and my opinion on how to solve the problem.
Note that I use the term “opinion”. An inspection by an engineer can only tell so much. Much of the building is covered with cladding (the structure) and dirt (the footings). So I am using clues and EXPERIENCE to solve problems and provide solutions.
But I can’t see through walls. Can you?
I have inspected many houses and repaired lots of damage, and occasionally homeowners place a higher priority on beautiful gardens against the house, or extra green grass or a large tree than repairing damage in a house.
That’s fine. The next owner of your property could still find the report useful!
Fixing Cracks in Your House
The most cost-effective solution to cracks in houses is to find the cause of the damage, remove the cause and watch the cracks close up by themselves. No expensive chemicals. No extra footings. Just some minor repairs when the building stops moving.
A Word of Caution
Regardless of the age of the building or the age of the crack, if you’re in a rush and you decide not to ‘solve the problem’ before fixing the damage please DO NOT repair any cracks in your house or allow your builder to repair any damage without first taking photos and recording the locations of the cracks.
If you haven’t solved the problem before patching the cracks new cracks WILL occur. When they do, and if you engage a structural engineer at this stage, they will want to see the original damage as well as the new damage.
A Note on Statutory insurance
Is your house still within the builder’s statutory warranty period? Even if the builder no longer builds or the company has collapsed this is relevant.
Statutory insurance can be used to solve the problems and repair the damage. If the age of the house is getting close to the warranty expiring, act BEFORE it expires.
Lodging an insurance claim with the statutory insurance provider gets you on their books before the expiration of the policy. In Queensland, this is the Queensland Building and Construction Commission.
The QBCC has a strict procedure that you need to follow and normally it involves working with your original builder first where possible.
Keep the lines of communication open. Be honest and as friendly as possible. Nobody likes cracks in houses and it can be a very stressful time for homeowners. so try to limit the stress by working with the people that can help you wherever you can.
A Note on Building Insurance
If the house is outside the statutory warranty period, you still might be covered by your building insurance.
Is the damage linked to an ‘insurable event’? If you had a burst pipe or an accidental impact or flood water through your house then the repair of the damage might be covered.
Note that the CAUSE of the damage won’t be covered. You have to fix the burst pipe yourself.
Each insurance policy is different, so read your policy before you make a claim. Knowing your rights is the best way to ensure you receive a fair go when fixing cracks in your house.
Who to call for cracks in your house?
Call Cornell Engineers for some plain advice, a quote or a recommendation on what to do next to fix cracks in your house.