What is Underpinning in Australia

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Video Transcript

Has someone told you your house in Australia needs underpinning? What is underpinning? Why do we need it for houses?

I’m Matt Cornell from Cornell Engineers and today we’re talking about underpinning a residential house.

What is Underpinning in Australia

So underpinning in Australia is a little bit different to underpinning in other parts of the world.

In Australia when we’re talking about underpinning we’re talking about extra footings that are being built on an existing house to help stabilize that house because it’s moving.

Underpin footing with steel reinforcement

When do we use Underpinning to fix Houses?

Now, typically we only use underpinning when there are soft soils or collapsing soils or one-way movement – downward movement – of a building that can’t be stabilized using other means.

For example, if the movement in a house is due to uneven soil moisture conditions or trees sucking moisture out of the ground then we’re going to try and solve those problems using our other tools of stabilizing soil moisture conditions.

But if a house has been built on soft or loose fill and the soil has consolidated under the weight of the house and the house has started to move downwards in those areas where the fill is loose and hasn’t moved so much where the fill is firm or in places where the house is founded on cut or on natural soil then there’s that unevenness.

Part of it is moving downwards on the soft loose soil and part of it is fairly stable and you get that big disparity in the amount of movement.

In that situation, we’re going to focus on supporting that part of the house that is moving downwards because it doesn’t have enough support underneath its footings.

So underpinning is the stabilization of a house at its most basic level but it can also be used to re-level the house and bring it back to a little bit flatter – not necessarily all the way to dead flat because we’re only talking about stabilizing that bit of the house that has moved downwards and the stabilization is only achieved where those new footings are installed.

It depends to some extent on how strong the existing house foundations are, how they’re reinforced, how strong they are, and how they’re spaced. 

Details

When we look at a construction detail you’ll see that there’s this deep shaft of concrete and steel reinforcement, a pocket in underneath 
a house that is used to support the house, and then after this concrete has been poured the underpinning contractor uses this space to install jacks which are like carjacks except a lot stronger and then in careful, careful stages they are pumped up and the house is brought back towards being more level.

Underpinning jacks
Underpinning jacks in a wheelbarrow

Once the underpinning works and stabilization works are complete then the house has to be set up to be permanently supported by the underpins.

You can’t leave jacks in underneath the house permanently so the underpinning contractor will then fill in that space where the jack is with steel plates and concrete so that the house is permanently supported by the underpinned concrete.

Summary of Underpinning in Australia

In Australia, underpinning is that deep footing that goes down beside a house at regular centers in a location where the house has dropped and is unlikely to come back by natural repair of the foundation material where the soil was soft or loose and won’t be coming back.

It’s used to support that house maybe bring it back to a little bit more level and to give it a good firm foundation. 

I’m Matt Cornell from Cornell Engineers and this has been our chat about what is underpinning.

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