Void under slab

stanks · Mar 02, 2024 12:24 pm

My house is about 17 months old. So far, all is well. Except just now when I discovered a void has appeared under a section of the slab:

https://imgur.com/a/Jtmd37f

Here, the NBN conduit and water pipes runs up the wall and on the opposite side of the gate, the conduits for electricity. From what I can tell, the plastic moisture barrier seems to have been pulled very tight and I'm not totally convinced the concrete is as thick as what it should be. Either that, or the loose backfill they used after putting the pipes and conduit here have settled. So far, there is no cracking on the brick work, slab or internally.
So, what am I dealing with here? I've logged a call with the builder and I'm worried that it'll be fobbed off as 'not my problem'.

groundzero · Mar 04, 2024 6:53 pm

Looks like the fill is settling may be localised hopefully. Do you have the original soil test? One thing I noticed in your photos is you have a tap without a stormwater drain to catch excess water from the tap, excess water can accelerate fill settlement. I would get some floor levels done and and even a sensor to constantly monitor movement from a company called slabsense.

stanks · Mar 04, 2024 8:14 pm

groundzero

Looks like the fill is settling may be localised hopefully. Do you have the original soil test? One thing I noticed in your photos is you have a tap without a stormwater drain to catch excess water from the tap, excess water can accelerate fill settlement. I would get some floor levels done and and even a sensor to constantly monitor movement from a company called slabsense.

Yeah, that tap has only ever been opened once or twice. In one of the photos taken prior to the slab going down, the soil doesn't look compacted or prepared enough (with big lumps of dirt by the looks of it), it kind of correlates to the edge beam not being reasonable square, but rather, rounded and not really uniform which you can kind of make out in the photos. The rest of the site looks fine in terms of preparation, but that one area where the void is, doesn't look so. Also, the underside of the concrete in that area has dirt stuck to it, so I can only assume the vapour barrier wasn't laid correctly when the slab was poured. In general, that section of slab looks strange.

How likely is the builder to pass-the-buck with 'just one of those things'? I mean, I know they've recommended putting a concrete apron, but I guess if I had done that, I never would have come across the void and I don't know how long that void has been there for.

groundzero · Mar 04, 2024 8:35 pm

It may be the trench for the pipe under there that wasn't properly backfilled and compacted. How wide is the settlement area ? The builder will shake it off. Slabs are designed to span certain soft spots or settlement areas.Two things I would keep monitoring is that the area doesn't expand further and stays localised and that the pipe under the slab doesn't disconnect or start leaking.
If the settlement slows down or stop then you could get some resin injection to lift the settled soil.Take some floor levels then you can always double check the slab is not being effected in the future.

stanks · Mar 05, 2024 6:47 am

groundzero

It may be the trench for the pipe under there that wasn't properly backfilled and compacted. How wide is the settlement area ? The builder will shake it off. Slabs are designed to span certain soft spots or settlement areas.Two things I would keep monitoring is that the area doesn't expand further and stays localised and that the pipe under the slab doesn't disconnect or start leaking.
If the settlement slows down or stop then you could get some resin injection to lift the settled soil.Take some floor levels then you can always double check the slab is not being effected in the future.

To me, that's what it looks like: poor backfilling and/or compaction. The width is about 1000mm maybe. That conduit in the photo is for the NBN. The water is about 500mm over to the right and I didn't notice any subsidence that way. It seems to be localised mostly around the NBN conduit and just behind the fence/gate towards the electricity.

I did my deck not long ago where I had to Chemset some anchors into the slab. That part of the slab had a nice 200mm edge to it (from the bottom of the brick, to the bottom of the slab). The bottom edge of the slab was straight. Compare that with where my void has appeared, the slab edge looks messy and the vapour barrier hardly extends beyond the slab.