Here’s a question about waffle slabs on Class H2 soil that we have been asked more than once:
“We have a soil test for our new house. It came back as Class H2. The builder we went to is planning on using a waffle slab. Will our house be ok?”
So let’s work out some of the terminology first and then I’ll explain what my advice was and why.
What is Class H2 Soil?
In Australia, soil conditions for residential buildings are classified according to Australian standard AS2870. The latest version of this standard is the 2011 version.
The standard classifies soil mainly based on soil reactivity – how much the clay in the soil is likely to absorb and release moisture and how much the soil will swell and shrink. The swelling and shrinking of soils result in vertical movement of the soil surface. Uneven vertical movement can damage houses.
It comes down to this: a class H2 soil is a highly reactive clayey soil that between wettish and dryish soil moisture conditions can produce very high ground movement. In numbers: 60mm to 75mm of vertical movement.
When is house movement a problem?
Soil movement under a house only becomes a major problem when it is uneven. If a whole house lifts and drops at the same rate then there is unlikely to be evidence of this movement inside the house.
House damage results if part of a house moves up or down 60mm and part of the house does not move at all.
That is partly the purpose of AS2870: to provide footing (and slab) systems that can tolerate or resist uneven movement.
The tools the standard uses to protect houses against uneven movement are stiffer, stronger footings, deeper footings, isolation from ground movement and protection of soils from extremes in soil moisture conditions (very wet and very dry soils are specifically excluded from the scope of the standard).
Stronger, stiffer footing systems can span further over areas of uneven ground. The house might still move, but the strength of the footing system is there to moderate the movement.
Waffle Slabs and Ground Movement
Waffle slabs use a grid of concrete beams under the slab to achieve the required strength and stiffness to counter ground movement. Deeper concrete beams means more strength and more stiffness.
Again, the higher the soil reactivity the stronger and stiffer the footing and slab system needs to be.
Can I Use a Waffle Slab on a Class H2 Soil?
AS2870 says waffle slabs can be used on reactive clay soils – even Class H2 soils – with some construction style limits:
- Timber frame: Yes.
- Articulated Brick Veneer: Yes
- Brick Veneer (without articulation): No
- Double Brick: No
Note that these rules can be over-ridden by a qualified and experienced structural engineer – but that is a whole different post.
Some Precautions to Take
These precautions apply to all reactive clay sites but they are much more important for highly reactive clay sites like Class H2 sites. They are even MORE important for waffle slabs:
- Plan your site works to ensure good drainage away from the building on all sides (even the upslope side).
- Manage stormwater drainage on the site from the very first day of the build. Do not allow water to pond near the footings.
- The bottom of sewer and stormwater trenches outside the building must fall away from the building.
- Keep trees at least 2 x mature height away from the building.
- Build ponds and water features away from the house.
- Set up irrigation systems so they do not water close to the footings. Do not position sprinkler control boxes near the footings.
- Do not position garden beds near the house.
- Ensure paths and walkways fall away from the building. Position paths so that they do not dam water against the footings. The ground under pathways should also fall away from the building.
- Manage site stormwater with surface drains (spoon drains) that are away from the building in preference to underground pipes – particularly if there are large trees and lots of leaf litter.
- Do not discharge stormwater pipes onto the ground next the building. Discharge to the gutter where possible or to bubblers positioned well away from the building in rural areas.
- Seal the gap between concrete pathways and ground slabs with flexible joint sealant (not just Abelflex).
- Builders fill used to raise the ground surface around waffle slabs should NOT be sandy soils. Clayey soils are better able to shed water away from the building.
- Position agricultural drains at least 1.5m away from the building.
What About Waffle Slabs on Screw Piers?
Here’s the thing. Waffle slabs sit on top of the soil and so when the soil is highly reactive, the waffle slab has to be designed for that ground reactivity.
The purpose of screw piers used with waffle slabs on reactive sites is to try and support the waffle slab at a depth where the ground moves less. The theory is if the waffle slab is supported where there is less movement the waffle slab can be designed for less movement. Right?
If a waffle slab is supported on screw piers and the house is constructed when the ground profile (full depth of soil) is dry, when the amount of moisture in the soil increases (ie when it rains or when you, the homeowner, start watering your Sir Walter turf), the ground surface lifts.
The ground surface pushes on the underside of the waffle slab and the waffle slab lifts. Well, it tries to.
If the screw pier is designed for that lifting force from the ground pushing up on the waffle, and the screw pier doesn’t dislodge from the underside of the waffle slab and the screw pier connectors don’t come undone and the blades of the waffle screw pier don’t lift up in the ground (wow) then the force of the ground pushing on the underside of the waffle pod slab becomes enormous. So hopefully your engineer designed the waffle slab for all of these forces and your waffle slab is really, really heavily reinforced. You’ll know if it is because your concretor will complain about all the steel in the waffle slab.
But if it isn’t or one of those other connectors isn’t tight or the waffle slab is embedded into the waffle slab with a slip joint like one of these:
Looking for More Information?
Check out these guides for more information:
- QBCC A Guide to preventing structural damage
- CSIRO – Foundation Maintenance and Footing Performance: A Homeowner’s Guide
- Plumbing Connection – A Slab of Confusion
- AS2870-2011 Residential slabs and footings