Underpinning is the construction of additional, stiffer footings to stabilise or strengthen an existing property. When we specify underpinning in residential properties it is normally to support a house that is suffering from subsidence that needs to be supported on deeper footings.
When is Underpinning Needed
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 is when a footing that is constructed on loose soil or uncompacted fill.
How Is Underpinning Performed?
Underpinning a house normally involves digging new bored pier footings beside a house at 2m to 3m centres. The footings have to go deep enough to found into 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.
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 diagnosis of slab heave and slab settlement are a suitable choice for this investigation.
The structural engineer should recommend independent soil tests before designing an 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.
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.
Underpinning in Brisbane
We have previously worked with these underpinning contractors in Brisbane. Contact them for a quote.