La niña vs el niño - slabs on clay

So anyone have some insight into how waffle pod slabs perform over time when laid during uncharacteristically wet vs uncharacteristically dry periods on highly reactive clay?

All engineering requirements and drainage being conisddered as met of course.

Have you read the linked thread and watched the video?

viewtopic.php?f=31&t=102622&p=1892575#p1892575

Thanks mate. I'm familiar with the aspects discussed etc. I haven't watched the video yet. Though before I do, does it answer the question?

I have an decent understanding of the common issus affecting waffle slabs and dont want to embark on 45 minutes on rehashing common knowledge if that's all the video is. My question is effectively this:

For a reactive site, all things being equal, is there any difference in laying a slab on an moist clay or a bone dry clay. Assume the surrounding moisture conditions remain consistant.

Your question is probably best asked on Matt's thread.

During the millenniun drought, most houses were build on dry soil and there was a high incidence during that time of established houses cracking over vast areas of Australia. In Adelaide metro for example, it was estimated that 70% of all houses suffered some degree of cracking. I have detailed State suburb by suburb statistics still stored somewhere.

When the drought broke, there were large numbers of houses built on reactive soils that suffered slab heave and this came as no surprise to the knowing.

I said "most" earlier because some engineers required the soil to be rehydrated during the drought to achieve predetermined moisture levels before laying a slab. I actually (naively) thought that this would become the norm, particularly with KDRs but time has proven otherwise.

Moisture distrubition needs to be consistent, there is plenty of information available online about a soil's matrix and matric suction.

Matt discusses many things on the video including important observations of possible/probable drainage implications when developers level blocks pre sale.

i think im good on the drainage aspect. Builder is on notice, seems to be a little more cognisant than others I've used and other sites of theirs I've visited seem to have the engineers notes taken into consideration (grading away from slab, no pooling, really really secure temp down pipes - no the ones made from cellophane lol which seem to be mechanically attached to teh stormwater etc). Ive also had a surface drainage system installed at the bottom of the grading to catch the water. Will be there during the build and will be part of the concrete perimeter when I get to that after hand over.

I think your comments where some engineers required the soil to be rehydrated during the drought to achieve predetermined moisture levels before laying a slab answers my question somewhat and the insight i was looking for. I figured it would be better for there to be some level of moisture (normal, not aqbnormal) as a slab is laid rather than bone dry.

Good video.

I think the waffle/raft debate is way oversimplified and people get confused. Some slabs are designed to be supported by direct contact with the soil underneath, and some are designed to be supported by piers. Some piers are sunk with the base supporting the slab and some are sunk using the earth suction to support the pier that supports the slab. Some pier holes are made just to provide weight against uplift. I mean if we wanted to stop slab heave by maintaining the moisture content of the soil underneath, then you could just get a d9 dozer to rip a 1.5m tench and you could bury a plastic skirt that would stop moisture transition from the soil under your slab to the soil outside your slab. Maybe I'm just simple actually.

Sounds like you've taken all the right steps to manage/minimise the risk of issues and you're well informed Follow the build up with landscaping that also follows recommendations and fingers crossed you'll be a-ok.

Fingers crossed isn't great though and it's surprising how much difference there can be in the moisture levels from one end of a house to the other over time; and that's the problem - change is fine, even some movement is fine (and expected), but significant and UNEVEN change is not fine.

I'm in Innovation and Tech and my new business SLABsense aims to uncross those fingers and replace it with actual data. Not a measurement every 6 months...lots of it. We're starting off with soil moisture sensors placed around your home and have tilt sensors in testing. Data is uploaded several times a day over several months/years and is displayed on a simple online dashboard and mapped against your local rainfall data.

Tracking changes (or ideally a lack of) over time & seasons is hugely beneficial to addressing significant changes early on and to help diagnose (or potentially prove) issues or fault. Several times a day and against rainfall also means you can be specific enough to correlate significant changes to something as targeted as a neighbour over watering a lawn (2pm every day over 30 degrees etc) or a broken pipe on one side of the house.

Don't guess and stress, be smarter and use data

Sorry for the spiel, if you've gone to all those lengths to best protect your home, adding some ongoing monitoring for less than the cost of your front fence is likely well worth it. Happy to chat anytime.

Cheers, Rob.
SLABsense Founder.

I can tell you the 3 things require for significant slab heave are 1.initial dry clay 2.highly or extremely reactive clay and then 3.a source of water.
I have seen hundreds of slab heave cases and I can guess what year they were constructed and practical all of them were built in dry soil moisture conditions.