Finding cracks in your home can be scary.
I know because I hear this all the time.
I have concerns about the structural integrity of my house which may be the result of movement of the concrete slab or footings.
I don’t know whether I should be talking to a structural engineer or someone specialising in underpinning or slab stabilisation.
Scary stuff right?
We put so much love into our homes. So much trust. And we want our homes to love us – protect us – and keep us safe.
So when our houses move and crack, it is only natural for us to try and stabilize that movement as quickly as possible.
As structural engineers trusted by many to help fix their houses, we have a few tools to help fix your home and stop it moving.
In our toolbox, underpinning is the big tool. The game-changer. The winning blow. But it’s also expensive and can make a mess of your home if done badly.
Depenidng on your circumstances and why your house is moving, underpinning just might be the perfect tool to fix your home. Or it might not be.
First, let’s get back to basics.
What is Underpinning?
Underpinning a house, a residential dwelling or even a commercial building is the construction process of building additional concrete or steel footings to stabilise or strengthen or lift the existing footings.
When structural engineers specify underpinning in houses it is normally to support a house that is suffering from subsidence and needs to be supported on deeper footings.
When Does a House need Underpinning?
House footings move for different reasons and not all footing movement requires underpinning.
So when is underpinning needed?
Underpinning is useful when the footings for a house or set of units have subsided or dropped for a reason that is unlikely to reverse.
Examples of footing movement that is unlikely to reverse are when a footing that is constructed on loose soil or uncompacted fill.
Footing movement that might reverse is caused by changes in soil moisture content in clayey soils (also known as slab heave). Underpinning is not recommended for solving slab heave.
How Is Concrete Underpinning Performed?
Concrete underpinning a house normally involves digging new bored pier footings beside a house at 2m to 3m centers around the outside of the house. Sometimes concrete underpins are also required inside the house. The underpin footings have to go deep enough to found into the soil that is strong enough to support the weight of the building.
Next, the deeper footings have to be joined to the existing building. Often this is achieved by digging under the existing footing (undermining the footing) to create a strong prop for the existing footing.
Then each pier and prop arrangement is reinforced with steel reinforcement bars and filled with concrete.
Clever underpinning contractors leave enough space between the underpin and the existing footing so that once the concrete is strong enough, the underpin can be used to jack the house up and make it level again.
Sometimes the jacking procedure will close up the settlement cracks that were in a house, but because the cracks sometimes dislodge unevenly the cracks won’t close up every time.
Is Underpinning Only Done on Outside Footings?
No. The footings in the middle of a house can be underpinned too. The underpinning contractor has to dig through the slab to access the ground under the house, but then the underpinning work can continue as if the footing was on the outside of the house.
How Long Does Underpinning Take?
How long the underpinning process takes depends on the number of underpins specified by the engineer.
Fur a house needing up to 10 underpins the work will be done over approximately two weeks.
If your whole house needs underpinning, then you might be out of your house for about a month.
How to get your house underpinned in 5 easy steps
Step 1: The Structural Engineer’s Assessment
You’re going to need a qualified structural engineer to assess your home. The structural engineer should be:
- experienced in assessing and repairing damage
- have local knowledge,
- be registered to practice in your area
- and be easy to speak to.
Arrange for an inspection first – and preferably do not pre-empt the whole conversation by stating you need underpinning – leave that assessment to the structural engineer.
A decent, experienced engineer’s inspection and report might cost somewhere between AU$1,500 and AU$2,000, but determining the cause of the movement and cracking is the essential first step in getting your house underpinned.
Your house may not even need underpinning. Speak to your engineer about your options for stabilising your home with these alternatives:
- Soil moisture stabilization
- Adjustable stumps
- Flexible joints in walls and ceilings
Step 2: The Soil Test
Your underpinning contractor can’t quote your underpinning job until the soil conditions on your site have been assessed.
Enter the soil tester.
A soil test is needed to see how deep the firm soil is. That helps determine how deep your underpins will need to be.
The soil test in preparation for underpinning is not the same as a site classification for building a new home.
A soil test in preparation for underpinning goes much deeper (between 6m and 8 m deep) and sometimes also involves recording soil moisture conditions at 500mm depth increments.
Use a soil tester that knows your area, can get a digging machine into the area for the soil test.
Step 3: The Engineering Design
This is why you used a structural engineer to assess your house in the first place – the engineering design for underpinning should be completed by a registered professional structural engineer local to your area.
The structural engineer will use the information contained in the soil report to determine the best location, spacing and depth of the underpins that will be used to stabilise your home.
The engineer didn’t originally know your house had to be underpinned when they fist quoted your job, so expect to pay for the underpinning design.
Be wary if a salesman for an underpinning or resin injection company offers to pay for your design – you might end up with more underpins than necessary.
Step 4: Engaging an Underpinning Contractor
Get at least two quotes from underpinning contractors that service your area.
The underpinning quotes should explain what is included, what is excluded and a basic indication of what costs could vary.
For example, a good underpinning contractor will include prices for removing and replacing concrete slabs, getting the plumbing checked after the work is complete, engineer’s inspections during construction, council fees and workers’ compensation fees.
If the quotes are too hard to compare, also ask for references and speak to the people who have previously used the underpinning contractor.
- Did the workers make a mess and refuse to clean it up?
- Were they courteous and friendly?
- Did they do the job they said they would do?
- Were there any unexpected price variations?
Step 5: Construction of the Underpins
The underpinning contractor will run the project now.
They will construct the underpins in stages if there are quite a few underpins. They’ll arrange for the structural engineer to attend prior to each concrete pour to ensure the underpin footings have been excavated to the correct depth.
They’ll place and pour the underpin concrete. Then days later they’ll come back to jack up and stabilise the house.
Your underpinning contractor will finalize all the paperwork, give you a copy of the final floor levels and submit a final invoice.
Does Underpinning Require Building Approval in Queensland?
Yes. If you are undertaking an underpinning project in Queensland you will need building approval.
To get building approval you will need a set of structural engineer’s drawings. So you can get a good set of engineer’s drawings you are going to need a soil test.
So I recommend you start with an engineer’s inspection to determine if underpinning is the best way to fix your house.
Then, if needed, the structural engineer will provide a scope for the soil tester to follow in the soil test. Soil testing in preparation for underpinning is not a normal site classification. Normally the soil test for underpinning has to go deeper.
When we arrange soil tests we also ask the soil tester to measure the soil moisture at 500mm depth increments to determine how wet the soil is at the test locations.
Once you have a soil test and an engineer’s for residential foundation underpinning you are ready to speak to some underpinning contractors to get prices for your underpinning.
The underpinning contractor will normally engage the building certifier to arrange building approval.
How to Arrange Underpinning
Most underpinning contractors recommend an inspection and report by a structural engineer first. This is because it is important to identify and diagnose footing movement that could reverse with a change in soil moisture conditions.
Structural engineers experienced in the diagnosis of slab heave and slab settlement are a suitable choice for this investigation.
The structural engineer should recommend independent soil tests before designing a foundation underpin solution.
The structural engineer can specify the depth, diameter and spacing of the underpins based on the type of construction of the building, the strength of the soil, the type of soil and the amount of footing movement.
Then you’ll need a contractor to arrange building approval and then to install the underpins. (Yes – building approval IS required for the installation of underpinning).
Underpinning isn’t technically difficult but an experienced underpinning contractor will have a better feel for managing difficult site conditions and safe house jacking procedures.
The excavations should be inspected by a structural engineer before they are filled with concrete.
Concrete vs Resin Injection Underpinning
Ha. Glad you asked. Resin injection is done by contractors that pump a resin into the ground. The resin expands and hardens. The theory is your house in then stable.
But if the resin injection technique doesn’t work and your house keeps moving, then, unfortunately, we can’t help you. The process can’t be reversed and it becomes really, really hard to stabilize and re-level your house.
Before you engage a contractor to inject resin under your home ensure a structural engineer (not the contractor’s salesperson) inspects and diagnoses the issues in your home, provides you with a report, arranges a soil test and confirms in writing that your soils are loose and need to be strengthened by resin injection.
Underpinning in Queensland
Cornell Engineers have worked with a number of competent underpinning contractors in Queensland.
We’d be happy to inspect and diagnose your foundation movement, prepare an underpinning specification if relevant and introduce you to an underpinning contractor that suits your project.
We have previously worked with these underpinning contractors in Brisbane. Contact them for a quote.
Underpinning Your House
Got some problems with your house and think you might need underpinning? We’d love to help.
Send us a message or call Matt Cornell for a chat. I am sure we can help.