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.

3 replies on “What is a Cut Off Trench”

Matt, I know you can’t give a quote in a place like this but when you say cut off walls are “expensive” what are we talking ball park?

I’ve contacted you before and you were kind enough to offer some advice. I have a small four bedroom house (190 sq m) on a 600 sq m block that is suffering from slab heave and has extensive internal damage.

When you say expensive are we talking $20k, $50k, $100k? More? Just asking to get an idea of what I’d be looking at if I went down this path and if it’s viable for me.

Thank you.

Hi Adam
An underpinning contractor might have a better feel for the cost of cut-off walls than me – we rarely get to see the quotes when they come in. Expect to need 2.5m deep cut-off walls x 200mm wide as a starting point.
Matt Cornell

It’s great to know that we should really avoid digging trenches next to our home because that can allow water to get into our foundation. My wife and I want to do some relandscaping and add a barn to our property this summer. We should hire some professionals to tackle this project to ensure things are done properly.

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