Does bored piers stops slab heave?

Hi,
Noticed that many new houses have many bored piers for waffle slab.

Does it stop slab heave?

Nope. Just means the piers are to adequate bearing pressure depth for their home or piers are needed because of sewer, pools etc

Slab heave is the slab deflecting upwards due to volumetric expansion of the underlying soil (due to an increase in moisture content in the soil relative to when the slab was originally built. And since waffle slabs simply sit on top of the piers, as pointed out, they do not mitigate the chances of slab heave. So far as I am aware, the only things that will mitigate slab heave are thorough and accurate soil tests, good engineering, builders who follow what the engineers require, excellent drainage right from when the slab is first built right through the life of the home and as-constant-as-possible soil conditions under and immediately surrounding the slab (which follows on from the drainage). Piers would significantly reduce the incidence of settlement though (which is one of their main reasons for being).

Thanks all. I reasked the same question on an existing threas in general discussion.

So all goos.

Interesting question

My builder has just put in around 30 bored piers and 16 screw piles (at the back where filled in swimming pool was) for my build, with a waffle slab to go on top. I was interested to see that it seem there will be no bond between the slab and the piers/piles. I had assumed there would some steel protruding that would be cast into the slab for this very reason. But maybe this is against the principle of the the waffle slab??

That's right, slab on ground means exactly that. They will put gravel and plastic under your slab on top of the piers.

Was thinking about this, as it has been wet here and my slab is about to be poured. I have 40+ piers (class P), so seems slab heave is more of a concern than settlement? So maybe it is better to pour the slab when it has been wet rather than dry so that if there is any change in the soil it will be volume decrease rather than increase ??

I tend towards that belief myself.

What about differential settlement? Can crack a slab in half just as easily as heave.
Consistent moisture levels through the soil is all you can hope for, so at worst the slab moves up and down evenly.

The whole point about building on highly reactive soils is to stabilise and keep moisture content even. That is why special care is required during construction and good maintenance after.

Regardless, any dwelling on a reactive soil will be at risk of an "event" (leaking drains, overflowing gutters as Save H2O talks about, burst water hoses, forgetting to turn external tap off, site flash flooding etc etc)

I think what all of us above agree on is "abnormal moisture conditions".

As building-expert points out above, its generally a problem with "Abnormal" not "normal" moisture conditions. Problem is, there is currently not enough education/engagement from builders/gov/engineers to engage with clients to inform them that "leaking drains, overflowing gutters" cause abnormal moisture conditions etc. I am sure any home owner with an investment that is their home, will engage and diligently care for their property when informed with the right information.

The builder also has a major role in this in ensuring that "abnormal" moisture doesn't occur during construction, correct grade around the dwelling, temporary down pipes, etc. The builder can not also shy away in regards to providing the owner with the relevant information to ensure after hand owner maintenance is maintained. That's also a case of whether the builder has done the same during construction, which is often not the case.

The fact that most of the eastern seaboard is currently in drought, I do firmly believe we will start to see many cases similar to VIC in regards to slab heave over the next few years.

I do know "insider" may have something to add to this in regards soil moisture conditions in VIC.

Regards Geo1

Only just this week on my daily walk I saw a new house being built in an area that has had only a few days of rain in the last four months. They had laid a standard waffle slab, going within half a metre of the boundary. The plumbers had dug vertically down right at the very edge of the slab to lay the drainage lines and then re-filled the trench with un-compacted clay.

I realise that when you are building to the boundary you don't get the luxury of maintaining a 45 degree angle of repose but even so, this slab was laid on bone dry soil and then a drainage trench dug right around the immediate perimeter and then only with the original soil tossed back in.

OK, so I am not a builder and have no qualifications, but if this was my house I'd be quite concerned about all of that.

With the home I am building I got into a dialogue very early about this whole heave / drainage /soil condition business. The builder I went with will actually deliberately introduce moisture to dry soil and won't go building over soil that is 100% dry. They told me they did this for two reasons - one is that it is much easier to compact the soil down versus when it is 100% dry and the second is to limit further soil expansion once the home is fuly completed and the paths built. As soon as the formwork is off the slab they build up soil around it to make the correct grade. They get the frame up as soon as they possibly can (after 6 days) and then the roof, guttering and temporary downpipes within a week after that.

I know that my building process was not under ideal conditions because as you pointed out, Australia has had drought conditions more or less everywhere over the last year. Every new house I see going up near where I live (which is about 700 kms from where I am actually building) is having it's waffle slab poured on absolutely bone dry soil. And I haven't seen one single temporary downpipe anywhere. I suppose they feel they can get away with this given the drought but my own builder is finicky about all of this and wants them on straight away regardless.

Add to that poorly maintained neighbouring swimming pools and tree planting without understanding the possible ramifications.

Hi mate, my slab was poured at the same time as peers so they are connected. Is that all good?