How to repair a concrete void under a garage?

[kalvinder_bains]

Hiya, We have a concrete void that is running down two sides of my garage, as the soil has erodes over the years.  I'm after some advice on a DIY repair please?  Please let me know if I need to get professionals in to undertake this repair too, if its too complex for a DIY repair.  Much appreciated. concrete voidconcrete void whole length of garage on two sides

[JacobZ]

Hi @kalvinder_bains,

Welcome to the Bunnings Workshop community, it is wonderful to have you with us.

The issue isn't so much with your concrete, nor does the concrete itself really require a repair. The issue is that the soil is eroding out from underneath it, which means surface drainage is the main problem.

The first step is to stop the cause of erosion. Look for things like roof downpipes discharging near the garage, surface water running along the edge of the slab during downpours, or ground sloping toward the slab. Any of these things can contribute to drainage issues, so redirecting downpipes to extend them away from the garage, and regrading soil so water flows away, not toward, the concrete are good steps to take. If you have a good look at how water acts in this area during the next downpour, this will give you a good idea of what you need to look at.

Once drainage is improved, as the void is from the soil eroding, not the concrete eroding, underneath the slab can be filled with a compactable material such as road base. Push the material as far under the slab as possible using a length of timber or steel rod, building it up in layers. Lightly dampen and compact as you go, so it locks together.

After filling, cap the edge to prevent future washout. This can be done by building up a road base sloped away from the slab or using concrete edging along the slab edge. The goal is to seal the edge so water cannot easily get underneath again.

Allow me to tag our helpful members @Nailbag, @R4addZ and @AlanM52 to see what they think and if they have any ideas.

Let me know what you think, and if you have further questions, please don't hesitate to ask.

Jacob

[AlanM52]

Hi @kalvinder_bains,

As @JacobZ suggested very likely a drainage issue so we need more photos of the whole area from different angles which will also help to judge the lay of the land - thanks.

Cheers

[Nailbag]

Hi @kalvinder_bains 

@JacobZ has already supplied some very helpful advice. To confirm his most likely cause being water running under the slab, is to observe that area during the next heavy rain. If thats the only time water comes out, then have the downpipes and guttering repaired/replaced as required by a plumber. Then observe again during the next downpour. If no water comes out, then the cause has been rectified. Then your right to compact the road-base material as Jacob has suggested. You could use something like a fencing bar to help push the road-base deep in to the hole and compact it.

Nailbag

[kalvinder_bains]

Dear @JacobZ, AlanM52, Nailbag, thank you all for your responses and your suggestions.  They're very much appreciated.  

Over the years, I have never noted any water discharging from the drain pipes or storm water pipes attached the garden or near the garage.  I also haven't observed any wet areas inside or around the garage area either.  However, I will most certainly be observing this next time it rains.

I wonder whether the issue for the erosion of road base is possibly due to slopes to the rear and side of garage as the garage slab is sitting higher than the earth surrounding it on the two sides where the erosions has occurred.   Additionally, I'm currently building an extension and many heavy vehicles with supplies have used the side side entrance next to the garage to gain access to the extension. This extension has been ongoing for 11 months. Previously this side of the garden was fenced with grass and very rarely used.  Could a combination no grass, slope, heavy vehicles accessing passing close to the garage and compacting the soil have contributed to the erosion of road base?  

I have attached more photo's that may help with providing further advice.  Also, I am thinking about installing that water tank you can see in the pictures to the garage gutter and will need to build a water tank base.  Any ideas on how to do this safely would be very much appreciated :). Thank You. 

garden

[kalvinder_bains]

Thank you Nailbag for your suggestions.  I will most certainly observe the next downpour to see if there is any water egress.  

[JacobZ]

Hi @kalvinder_bains,

Thank you for the additional photos; they certainly help to give a broader picture of the issue, and it makes me more certain that surface-level drainage is the issue. Seeing how water moves around the slab is going to give you the best idea of what needs to be done, so it is definitely worth it to observe and take note.

When a slab sits higher than the surrounding ground, like in your case, any surface water will naturally run away from the slab edges. If the soil alongside the slab has no grass cover and has been repeatedly trafficked by heavy vehicles, it is going to become compacted. Compacted soil sheds water rather than absorbing it, which increases runoff. That runoff can then track along the edge of the slab and slowly wash out the road base underneath. The prolonged construction period and vehicle access you mentioned would very likely have accelerated this process.

Things like regrading the ground so that the bottom edge of the concrete is no longer exposed and establishing a lawn will help with the drainage issues. By building up the soil along the edge of the slab, water will more naturally flow away from the slab. Adding grass will also help to stabilise the ground and improve the soil's ability to soak up water so that it naturally drains away without taking the soil with it.

If you wanted to, you could also put in a basic retaining wall using a single row of sleepers. This would help to keep the soil where you want it to be.

For the water tank, given the history of erosion, it would be wise to prepare a stable base using either pavers or a concrete pad. Check out How To Lay a Base For Pavers and How To Pour A Small Concrete Slab for some guidance.

Let me know what you think.

Jacob

[kalvinder_bains]

Thank you JacobZ for the advice, which I certainly agree with.  I'm hoping that the building works will finish in Feb 2026 and we can start the process of building up the soil as you've suggested.  We'll then set about using road-base to fill the cavity as suggested by Nailbag, including using a fencing bar to push the road-base deep inside the cavity to maxmimise compaction.   

Would foam be another solution, or is road base a better solution? We'll then re-grass the whole of the garden, probably by using seed, because its a huge area.

Thank you for your support and advice  

Kalvinder 

[JacobZ]

Hi @kalvinder_bains,

Specialised expanding foam is sometimes used to jack up slabs by drilling holes through it and spraying the foam through these holes. This technique is usually used in situations where there is no way to access the underside of the slab. As you have access to the underside of the slab, road base is a better and substantially cheaper solution. 

Once the construction has finished, establishing grass is definitely going to help. Check out How To Grow Grass From Lawn Seed for some guidance on the process.

Let me know if there is anything else I can assist with.

Jacob

[AlanM52]

G'day @kalvinder_bains 

After watching Making a Murderer last night I'm back to approach your situation as a forensic investigation, to look for more clues - seriously mate.

At this point my thoughts may not be in logical order.

With @JacobZ suggestions I mind how is it possible for any of that to cause that amount of 'localised' erosion?

More clues.

The concrete pour is not to specification (BlueScope Steel Limited) for a COLORBOND/ZINCALUME/TRUECORE garage.

Final slab inspection (if there was one) should have picked up...

The not-to-spec drip edge has already caused bottom edge corrosion and that will continue up the sheet.

That lackluster approach makes me wonder what else could have happened under that area.

Does something else run through there maybe stormwater/irrigation and does the garage sit on a clay base.

Cheers