Protecting your Slab

Filthyfil
Here is a list (in no particular order) of precautions for those about to build to minimise the chances of slab heave.These are things that have contributed to slab heave in the past.

Developers should avoid using highly reactive clay fill. The fill should be not over compacted and not dry of optimum moisture.
This is obviously beyond the control of the home builder but
check your soil report to see if you have fill over highly reactive
clay .If you do then you are more susceptible to heave.

The ground around the proposed slab area should be graded away to allow water to flow away from the footings. The ground should be scraped and not be built up to get this gradient, as water will flow through the built up fill and onto the foundation.

Properly constructed agi/cut off drains must be installed the agi drain must be dug into the natural clay. The finishing depth must not be in the fill or topsoil diagram 1. Soil that is graded away from the footing or correctly sloping concrete should channel water into the agi/cut off drain or storm water inlet. A cut off drain will also prevent water ingress under the slab from neighbouring properties. See below
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Concrete apron should be constructed around the perimeter of the house. This concrete apron must have a correctly sloping surface away from the house and into storm water inlets or spoon drain so that all surface water is collected and carried away in storm water pipes. The paving concreter must not do the plumbing; a qualified plumber should do all the necessary pipes and connections. The paving concreter must ensure that no existing pipes are crushed or punctured during concrete construction especially when using star pickets and when using machinery like bobcats.

You should have eaves to protect your house from the weather and prevent rain from directly hitting your walls and running down onto your foundations.

Your builder should ensure that the carpenters do not hard nail the plates between the top plate of your internal walls and the roof truss to allow for movement see below.
http://i1028.photobucket.com/albums/y342/jmfieldwork1/IMG_0835_zps9151fef0.jpg
The plumber should ensure that the pipe trenches slope is correct and the gradient complies with the Australian standards. The plumbing design should try to minimise the amount of plumbing under the slab and make sure that a clay plug or bentonite is used to prevent water from the outside travelling back up the plumbing trenches and under the slab.

Flexible plumbing must be used on highly reactive soils. Before the house slab has been poured a full plumbing test should be performed video and pressure check
and preferably again just before hand over.

There must be appropriate number of downpipes to prevent overflow during storms especially if there are no eaves.

During construction temporary down pipes must be used .

Hot water unit must have overflow discharging into stormwater inlet. Air conditioners should have a connection to the storm water system. See below
http://i1028.photobucket.com/albums/y342/jmfieldwork1/IMG_3862_zps829a1aae.jpg

The house slab concreter must not allow over pour or blowout of the footings during slab construction this causes the load on the soil to be greatly reduced making the footings more susceptible to heave.

All exposed decks/timber porches must have a roof or be cover to prevent water run between timber slates and ponding under the deck.
The exposed deck will allow water in and prevent evaporation creating a wet area.

Garden bed areas and veggie patches should be well away from the perimeter of the house.

Garden sheds and covered small storage areas should be away from the perimeter of the house or have proper gutters and storm water connections.
All connections for water tanks must be carried out by a qualified plumber and tested. Tank over flow must be connected to the storm water system.

Discharged recycled water should not be used near the perimeter of your house. The effects of this type of discharge to the behaviour of the soil are not known yet.

No sprinkler system should be close to your house including soakage hoses.

Do not use landscaping quartz pebbles or cobbles around the perimeter of the house unless there is plastic sloping away from the house underneath it. Pebbles and cobbles allow the ground to be come soaked during the rain and prevent evaporation. See below.
http://i1028.photobucket.com/albums/y342/jmfieldwork1/IMG_3860_zps4a097c32.jpg

Raft slabs do heave but as you say the one thing in their favour is they can sometimes restrict water flow under the slab.
You can still have slab heave even with piers.What is the reason for piers?

Yes they will stop settlement but not heave.You only have a thin layer of clay under your footings so
p type based on a m type soil should be ok, the less clay under footing the less reactive the site will be.What area are you in.

Yes well that is a highly reactive area.The rock may be floaters and not actual bedrock.
By the way what is KDR

Thanks BE
Good advice about pergolas in fact it maybe the only way to possibly reverse slab heave by allowing the ground to dry out and protect the slab from weather.Risk assesment is also a good suggestion.
In Queensland during the soil test they take samples for lab testing which is the way we should be heading.
I've been working on a assessment I'll see if I can find it.

For what we pay for a soil test you'd think they were lab testing.

Knock Down & Rebuild.


I have to admit that just about every waffle pod slab I've seen pre-pour just doesn't look strong enough despite all the arguments to the contrary.
Luckily the engineer I use and who will be looking after our build is old school too. We are in a different boat though as 99% of our build will be over rock ( which brings its own problems too in some ways ).

Stewie