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I am a first home owner based in Sydney. I have water pooling in my crawlspace so I was advised by a structural professional to lay a french drain around the perimetre of my home. I watched hours of french drain videos from Apple Drains on YouTube and thought a foundation drain would be a good idea.

So I dug a trench immediately next to the foundation wall so that I can expose the wall that was below grade / buried where I can apply some waterproof liquid rubber.

When I dug though, I was expecting the brick wall to go down deep (maybe 2 feet) which would be sitting on a concrete footing protruding away from the house. Basically all videos I have watched made me expect the wall to be buried quite deep.

However, I only discovered three layers of bricks below grade and thought surely this isn't it? This isn't the foundation footing that I see on videos? Its just 3 layers of brick cobbled together in a messy fashion. I dug deeper to see if there was more wall underneath and discovered there isn't any so it turns out it is the foundation footing.

The problem - and my mistake

While digging though through the clay, bits of clay right underneath the footing were removed along with the clay next to it. I didn't think this was a big problem at the time nor that I thought I was undermining the foundation footing.

enter image description here

The red circle is the undermined bit. So far, I have backfilled the space immediately below the footing (lowest course of brick) with the clay that I dug out and compacted it as much as I can with my hands (basically punching the clay into the space so that it can be molded in).

This isn't complete, of course, as the area that supports the footing is 45 degrees down and away from the house.

So I guess my question is:

  1. how badly have I messed up; and
  2. what do I do now to rectify the situation?

I have been planning to backfill the trench with "Crusher Dust" and trying my best to compact it as well as I can about 2-3 inches to form the base of the french drain. Then adding in the non-woven geofabric, then backfilling about 2 inches with 20mm gravel for drainage to form the base of the french drain pipe and then backfilling the rest with the same 20mm gravel.

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    I wonder if the small foundation is consistent all around the perimeter of the house. I lived in a house once that had a normal foundation for most of the building, but there was one room that had been converted from an old porch. That had the skimpy sort of foundation you described.
    – aparente001
    Commented May 23, 2022 at 5:16
  • I cant tell for other sides of the house as they are covered with concrete paths. But for this side of the house (around 15 metres (50 feet), its consistently like this.
    – Gil
    Commented May 23, 2022 at 6:54
  • "I am a new home owner based in Sydney" - I read this as this is a new build which you have bought and moved into. If this is the case, contact the building company and have them resolve it, at their cost. Don't dig any more exploratory trenches, or they'll blame you for causing the issue.
    – spikey_richie
    Commented May 23, 2022 at 12:08
  • @spikey_richie thats bad wording from me sorry. I am a first home owner is more correct and Ill edit it. This house is built in 1970.
    – Gil
    Commented May 23, 2022 at 12:20
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    @aparente001 yes you are right the water problem still remains but has taken a step back now that I seem to have a bigger problem. I have in fact actually bought a sump pump to place in the crawlspace per your recommendation. THank you
    – Gil
    Commented May 24, 2022 at 8:28

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Well, you wouldn't want to leave it like that, but it sounds like you are planning to remediate the excess excavation, and like this might be a perfectly suitable foundation for your mild-climate, non-freezing area.

The undermining is minor in scope.

A small heavy hammer (hand sledge, or perhaps a dead blow mallet) might be more effective than "punching it with your hands" for packing back under there, without requiring the room to work that a larger one would.

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  • thanks for the advice. I am absolutely wanting to remediate it - just I dont know if backfilling it alone is enough (even with perfect compacting). I have heard others say that it may need underpinning for the soil that I have disturbed and the undermining that resulted. If you think backfilling is sufficient, do you recommend any type of material in particular? I have been recommended pretty much all material under the sun.
    – Gil
    Commented May 23, 2022 at 13:27
  • I have been recommended fatty brick layers sand, paving crusher dust (doesnt actually look like dust), road base, fine gravel, scoria, crushed stone as backfilling material.
    – Gil
    Commented May 23, 2022 at 13:30
  • I'd choose road base or crusher dust (trick being both HAVE the "dusty" fines in them) because they pack well (due, in part to the dust.) Crushed stone (without dust, or "fines") does not pack well. Unless you did a lot more undermining than you show, I would not get too worried, but if you want to consult a local engineer to be sure your approach is suitable, the peace of mind and having a definite plan might be worth it.
    – Ecnerwal
    Commented May 23, 2022 at 13:41
  • thanks heaps! The photo is the worst part with the most amount of undermining where it goes deeper towards the house with the front half inch of the brick sitting on air. The rest of the footing has less undermining - most have the house side trench wall vertically flush with the lowest course of the brick (which is still an undermnine as soil suport needs 45 degree angle).
    – Gil
    Commented May 23, 2022 at 14:01
  • See also: diy.stackexchange.com/a/45020/18078
    – Ecnerwal
    Commented May 23, 2022 at 14:25

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