Water sitting near a waffle slab

Water Around a Waffle Slab

 

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.

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 H 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 only the ground around the perimeter of the slab swells and lifts. The house footings and slab directly above the swollen ground lift too.

The dry ground under the middle of the house doesn’t swell and the slab height there doesn’t change.

Water near a waffle slab
Poor site drainage can lead to slab heave

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.

Water next to a slab
Remove water from around a slab to stop water entering under a house

Now the clayey soil under the slab can get swell and lift too.

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.

Water sitting next to a slab
Poor drainage around a slab can cause brickwork cracking

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 H site (because you read the www.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.

Why Me?

Did the builder do a good job or not?

No.

The site drainage has to be maintained from the start of site preparation, all through construction and until the end of time.

Water sitting near a waffle slab
Control site drainage from the start until the end of construction

Do not let water sit against the slab of your newly built house slab.

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?

  • If you can, raise the pad height with compacted clayey fill so that the underlying site is free-draining.
  • Use spoon drains on the upslope side of a house to direct stormwater around a house.
  • 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.
  • 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 them but not connected to them, an increase in soil moisture will cause the house 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 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. 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.

 

7 thoughts on “Water Around a Waffle Slab”

  1. Great information – thank you! Our build has just been completed and thankfully our waffle slab was poured in summer when it was as dry as a bone. However, we are wondering what is best to do now to prevent water getting in to the slab. We have heard advice that we should concrete around the perimeter of the house. To do this though we have to get a retaining wall made though because our slab is raised above ground level. We have had some quotes done and it seems to be a very costly exercise.. one that we can’t afford to do at the moment. Can you suggest any other options?

  2. Hi,
    Your website is very interesting an informative 😁
    Our home is nearly complete. It’s a waffle pod 400mm high on screw piles 1.4m deep. It sits high on our block which is good for drainage.
    How far away should we be keeping the ponding? Water is collecting about 1.5m away where the storm water pipe trenches have not been filled and compacted.
    Many thanks
    Justin

    1. Hi Justin
      Thanks for your question. So long as storm water can’t drain into the plumbing trenches and backflow under your house (yes that is a thing) then having the ground fall away from the house for the first 1.5m is a good distance. To prevent storm water filling the trenches and flowing back under your house the Australian standard recommends a tamped clay ‘plug’ in the trench. I think that the trenches should also be free-draining away from the dwelling.
      Enjoy your new home!
      Matt Cornell
      Cornell Engineers

  3. My house is built on clay and has sunk at on side were tbe previous owner put concrete up to the footings ,I have changed the roof to tin from concrete to reduce the weight and cut away the concrete I have been told to add water slowly and it should level back out ,here’s hoping the other side of the house is garden bed so I am thinking this is very relevant

    1. Hi Ken
      I hope you’re right. If that doesn’t work try opting for a dry regime. Remove the garden, improve drainage away from the building in all sides and allow the ground to dry out all around the house. This could take up to six months so you have to be patient. Best of luck.
      Matt Cornell

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