Water ponding around a waffle slab house being built on a reactive clay site is a problem and it’s not an easy fix.
That’s why the Australian standard for footings and slabs AS2870 says that the surface drainage of a construction site shall be controlled from the start of site preparation until the end of construction.
Why? Because it is easier to get good drainage around a house at the time of construction than after the house is built.
Adding Water to Clay Makes it Swell
When water soaks into the ground, the reactive clay in the soil absorbs the moisture and the clay swells.
As the clay swells, it expands and the ground surface lifts.
For a site classified as a reactive clay site (Class M, Class H1, H2 or Class E soils) the lifting forces are large enough to lift a house noticeably.
Water Around a Slab
When the water is only around the perimeter of the house slab the ground around the perimeter of the slab swells and lifts.
The house footings and slab directly above the swollen ground lift too when the ground surface lifts.
The dry ground under the middle of the house doesn’t swell and the slab height there doesn’t change.
High edges and low interior slab is known as a dished slab or edge heave.
If the ground is only wet on one side of the house then only the side will swell and the slab will look like it has tipped slightly.
If the whole under-slab area on a house under construction was totally saturated during construction the whole slab would lift semi-evenly.
Water Beside a Waffle Slab
When water sits against a waffle slab it isn’t actually just sitting there; the water can and probably is seeping in under the house – especially when it sits near your patio area (which is set lower into the ground than the rest of your house).
Now the clayey soil under the slab can get swell and lift too.
However, the amount of lifting is still uneven. The ground where the water is soaking in lifts but the ground that stays dry does not lift.
Now Let’s Build
The slab has now deflected due to the soil swelling and lifting.
The carpenter arrives to build the walls and the bricklayer arrives to lay the bricks.
Both the carpenter and bricklayer take great care to build walls that are straight and plumb.
They notice variations in slab height that they assume were caused by concreter failing to pour a dead flat slab. They adjust their work to suit and build walls on a deflected slab are nice and straight and true.
The walls get plastered and finished and you move in.
And the Movement Starts
You know not to water around a Class H1 and H2 site (because you read the cornellengineers.com.au website – ps yay you!) so you take great care not to water around the house, not to plant trees against the house and to make sure the site is well-drained.
The ground around the house dries out over the next 12 months and the slab surface drops back down to its original height.
Your brickwork cracks and your plasterboard walls crack and you panic.
You’ve done everything right and yet you still have cracks in your house.
The builder sends an engineer out to check the slab levels and they look pretty good. The slab is nice and flat – but there is damage everywhere.
Did the builder do a good job or not?
The site drainage has to be maintained from the start of site preparation, all through construction and until the end of time.
Do not let water sit against the slab of your newly built house slab. Speak to your builder, point out the engineer’s requirement for the builder to maintain site drainage. Ask the builder to stop work and repair the site drainage immediately.
Do not let water sit against the slab of your newly bricked up house.
Do not let water sit against the side of your fully complete, ready to live in house.
The effect of water sitting next to a house during construction may not be permanent until it is.
Ask your builder to maintain excellent site drainage right from the start of construction.
If You’re a Builder
If you’re a builder reading this (well, welcome!) some sites are hard to maintain site drainage. Yes, it’s true. But do you really want to keep going back to a house that is suffering from slab heave?
The National Construction Code 2019 Volume 2 says this:
I’m sorry if you think this is optional. It is not.
It doesn’t matter whether you are on a flat site or a sloping site with the ground falling towards your building. The external finished surface surrounding the slab must be drained to move surface water away from the building and graded to give a slope of not less than 50mm over the first 1m.
If you can, raise the pad height with compacted clayey fill so that the whole site is free-draining and water drains off and falls away from the building pad.
- Use spoon drains on the upslope side of a house to direct stormwater around a house.
- Keep an eye on your plumber. If the plumbing trenches that run under your building are backfilled with sand, these tranches could be a conduit for water under your slab. Get the plumber to observe the rules in AS2870 as specified (very carefully) by your engineer.
- Keep an eye on your contractors. Don’t allow bog holes to form and not get filled. Fill bog holes with clayey fill. Turn off taps. Connect downpipes asap.
- Find a way to drain water from under slab setdowns like verandahs and patios. Speak to your engineer about this because this is a major fault with waffle pod slabs.
- Read the engineer’s plans with respect to site maintenance. Pass this information on to your sub-contractors and then the owners.
- Ensure pipes are articulated if they need to be.
- Keep subsurface drains away from buildings.
Waffle Slabs on Piers
Say your waffle slab footing is on piers.
If a house has piers that go down to bedrock things change slightly – but it depends on how the piers were built.
If the piers were poured first and the waffle slab was poured on top of the piers but not connected to them, an increase in soil moisture will cause the waffle slab footing and slab to lift off the piers.
If dirt gets between the piers and the slab then the house can’t go back down when the soil dries out again. Depending on the amount of dirt that gets in between the piers and the slab might stabilise but end up permanently out of level. Bad.
If the piers are connected to the slab and reinforced then the heaving forces will be somewhat resisted by the friction between the piers and the ground – especially if the piers go very deep. The amount of slab movement will be moderated. Good.
Waffle Slabs and Water
Waffle slab footings allow water to penetrate under a house. If water sits next to a waffle slab it is probably draining under a waffle slab. Don’t allow water to pond next to a waffle slab. It’s just not swell.