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Are Waffle Slabs Able to Protect Australian Homes From Movement

Are waffle slabs able to satisfactorily protect Australian homes from slab heave and foundation movement?

This question is for you, structural engineers and geotechnical engineers and foundation specialists of Australia.

If you are applying AS2870 to waffle slab designs please comment below. We want to hear from you.

These questions have been put to me many times and now I am asking you on behalf of others.

What is your opinion on waffle slabs?

Please comment below with your experience, your qualifications, and your recommendations.

  • Do the provisions in AS2870-2011 as they relate to waffle slabs go far enough to protect Australians from uneven foundation movement?
  • What is the major letdown in the system that is resulting in waffle slabs being overtly affected by slab heave in such large numbers?
  • What can we do, as a profession, to improve the system and improve the performance of the Australian residential foundation systems?
  • Are waffle slabs able to protect Australian homes from movement and damage?
  • How do you account for large tie-down forces in cyclonic wind areas?

Comments are Open

Comments are open. Please let’s make this an open forum for ways to improve foundation systems together.

If you are a homeowner affected by a poor performing foundation system we’d like to hear your perspective too.

By Matthew Cornell

Matt Cornell is a structural engineer with 25 years experience in residential structural engineering. He lives in Northgate on the northern side of Brisbane, Australia.

9 replies on “Are Waffle Slabs Able to Protect Australian Homes From Movement”

matt, you are a legend. You have really educated me and prepared me for an upcoming build.
thanks for this site. wishing you the greatest success in your future endevours.

Not a simple question to answer !!!

The three requirements for slab heave are :

1. Highly to extremely reactive clay.

2. Dry soil moisture conditions pre build.

3. Sources of wetting of the dry soil during and/or post build.

The first two factors need to be identified early so the slab design allows for the excessive ground movement. This can be done with soil sampling and lab tests as part of the soil test.
The third factor includes poor site drainage and plumbing leaks
This is the area that can be dramatically fixed if current building practices are improved or at least made to comply with the Australian building standards. The third factor really relies on enforcement which currently is very lacking.
The first two factors require some modifications of AS2870-2011. This can be a slow and frustrating process and needs an effective and practical acknowledgement of the changing climatic conditions and the weakness in the near static treatment of the climate in AS2870-2011.
So, in my opinion, the situation will only be resolved by the following :

1. Standardised soil reactivity testing methods to calculate ground movement(not the shrink-swell test!).
2. The Standards need to allow for a greater suction variation in drought conditions. It appears we are heading into conditions where they are more frequent and severe.
3. Enforcement of proper site drainage methods and regular plumbing inspection at various stages of the build including CCTV.

I am a homeowner and from my experience, waffle slabs should not be used on highly reactive soils. The footing system to my home has failed and experts are currently playing pass-the-parcel as the cause. Original site classification ā€œEā€. Currently slab 152 mm difference in level.

Significant tree and shed structure removal prior to testing that was not identified on the Geotechnical report. Other trees were planted in the front neighbours property during construction. All experts have blamed the trees for settlement until I contacted a plumbing investigator and a broken sanitary pipe at the back of home was found at the edge of the slab.

As Jason stated, enforcement is lacking. Underground plumbing is hidden and it is the homeowner who is the one that suffers both financially and emotionally. The regulators need to start regulating and with tougher penalties to mitigate and alleviate risk.

Hi Sue
Did they work out how much the slab had heaved from the sewer break and how much had settled from the tree drying ?

Hi Jason, they estimated 70% heaved from sewer break, 30% trees. They said sewer break was wholly or partly responsible for abnormal moisture conditions.
So many probables. Structural damage occurred 3 months after moving in.
Home insurer wants to rectify with underpinning and resin injection.

Sue
Resin injection and conventional underpinning won’t fix the heave problem. It may help the tree drying problem but it sounds like the foundations will be unstable for a long time. From my experience, the ground movement caused by the sewer break can keep going for years after the leak has been fixed. I am looking at one now that happened in 2009 and has heaved over 200mm.
The original cause can sometimes be that they didn’t use flexible plumbing and they remove a tree before the build and the ground is dry then the dry ground rehydrates during and after construction causing ground movement which then cracks the sewer pipe.
This, in turn, leads to more long term large slab heave which sounds like what you have.

Thanks Jason for your information. Yes, they didn’t use flexible joints and also their was insufficient grade towards the LPOD. Actually the grade was back towards the house.
Matt, you are right, the homeowner is the one that suffers.
Seven years is a long time to seek information, engage experts for investigations, battle the insurance companies, get the regulator to take action, write to MP’s and every other government department related to building/construction.
I have resorted to increasing my knowledge on the Building Act, Codes, Standards etc related to both building/construction and insurance so when the time comes, I can represent myself at VCAT.
Unfortunately, those same Acts, Codes etc do not protect me and tens of thousands of dollars is spent seeking answers for the significant structural damage to my home.
There are those who do the right thing when constructing and unfortunately there are others who take shortcuts, as it is more about profit rather than integrity.

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