An enormous number of Victorian residential soil reports have incorrectly understated minimum founding depths and related requirements, resulting in the failure of thousands of slabs.

A headline in The Age last year read “Thousands of suburban home owners facing financial ruin.” This referred to an official report on the failure of mainly waffle pod slabs to stay within permitted flexure limits in the western and northern suburbs of Melbourne, and the effects of that failure have been devastating.

Although owners and builders could have demanded a second soil report for any new home project and the building surveyors could have required more information via a more stringent soil test (that may have resulted in a stronger slab or bored pier construction based on that report), this obviously has not happened for possibly more than 10,000 Victorian new home owners.

The local councils are not required to become involved, merely acting as filing clerks for the documentation for future use by subsequent owners. It’s a pity, because local building surveyors often have ready access to vital knowledge that could have helped avoid many a slab failure.

So it’s up to those involved with the house projects, prior to commencement of construction: the soil report writers, structural engineers, building surveyors, and possibly the builders, draftspersons, architects, and even owners, to ensure that the most informed soil report possible is carried out, particularly in light of the fact that we have had a recent record 14-year drought.

For many a housing project, vital information was actually obtainable if just one of the aforementioned people had cared enough to ferret it out.

I inspected a slab in Melbourne’s outer north west where the slab was over 90 millimetres out-of-level. The builder has since been permitted to install substantial bolstering of the slab and improve drainage, rather than everyone going to VCAT. The jury’s out as to whether what has been done will be enough.

After a little delving, it was eventually discovered that a map had actually been available, and it showed where large trees (about 1.2 metres in diameter) had existed just a few years prior to the development of the estate. One of these trees had existed within the house envelope, and its absence caused considerable heave (swelling due to extra-dry soil regaining its moisture, the causes of which are often difficult to pin-point) past the limits set out in the soil report due to the prolonged 14-year drought in Melbourne just ending when the house was built.

There was also a retained tree, and it caused excessive settlement to both slab and the few supporting piers provided to the edge of the slab nearest the tree, proving that the original classification was wrong and/or that any extra requirements or recommendations were inadequate. Amongst other things, Appendix D of the Code AS2870 had been ignored.

Another excessively distorted unit in Melbourne’s outer east resulted from combined excessive (65 millimetre) settlement/heave due to inadequate allowance for the effects of large removed trees and a row of retained trees.

This demonstrates negligence on the part of one or more of the soil engineer, structural engineer and relevant building surveyor – those who should have known better and who were actually required to expect dire situations where (highly) reactive clay soils are concerned, especially when the ground had been dried by a record drought and/or significant trees, past or present.

What a fiasco! What a tragedy!

But just what were the specific details of the scenario that basically caused soil report writers to fail in their duty to classify the soil correctly and fail to provide sufficient extra requirements (as distinct from recommendations), so that the structural engineers would design adequate concrete slabs for the sites on which the houses were to be built, particularly in the NW Melbourne volcanic plains and similar problem soil areas?

There were two pertinent codes involved with soil reports:

  • Residential Slabs and Footings – Construction AS2870 – 1996 (since revised), and
  • The Building Code of Australia (BCA).

According to AS2870, the basic requirements for soil classifications are:

  1. Normal weather conditions must prevail
  2. A history of unaltered drainage must be the go
  3. Rows or groups of trees or large trees (on or in the immediate vicinity of the block of land), must not remain and must not have been recently removed

Otherwise further vital information must be sought.

According to the BCA from 1996 onward (at clause P2.1(b) (xiii)), the effects of shrinkage and swelling of soil ‘must’ be considered.

Therefore (it is implied), further vital information must be sought. Pretty simple, really!

But in possibly tens of thousands of instances it appears these codes were used without that information being obtained or Appendix D being consulted, because by doing that, those slabs would have resulted in greater measures being taken, albeit at considerably greater expense.

The obtaining of the cheapest quotes for soil reports seems to have led to many soil report writers to taking the easy path, Clause 2.2.1(b) in AS2870, whereby the characteristic surface movement estimation methodology could (they thought), serve as a shortcut to the consulting of Appendix D of AS2870.

Classifications were basically downgraded by ignoring the basics of these codes, leading to this enormous tragedy. Owners (who were kept in the dark) signed the contracts, and project after project sailed along unobstructed by bored piers, stronger slabs and re-vamped finance. Those involved with the process of ascertaining the correct soil classification all had a job.

How did this happen?

There are basically two paths soil report writers can take to assess the classification of house sites, but due to abnormal moisture conditions in reactive clay soils (M,H,E) each of these paths still required the soil report writers to seek out the aforesaid information plus consult Appendix D of code AS2870 for minimum founding depths plus personally assess whether the soil immediately footings had been excavated prior to the placement of underlays and reinforcement.

One of the paths to a soil classification, where the surface movement factor is estimated appears to be permitted to bypass all other requirements of AS2870 whilst ignoring the basics of the code, clauses 1.3.2 and 1.3.3. But for virtually all sites west of Melbourne, this was not an option.

Reactive clay sites with abnormal moisture were to be classified P (P meaning problem), and thus had to comply with clause 2.4.4 of the Code AS2870. Soil report writers were therefore required to consult Appendix D anyway; and reports required (as opposed to recommended) greater founding depths as a result. For P soils, the footings or slabs had to be structural-engineer-designed as well.

But even with all those safeguards in place, relevant building surveyors did not make a stand and question the shortcut approach of the soil report writers who were asked to report for less than half of what the cost should have been. And the structural engineers who designed the slabs, trained in the basics of civil engineering, were surely also required to know the basics of code AS2870, and that at least ‘P’ classification was required for these sites due to the long-term drought issue alone.

So why did no authority or the Institute of Engineers or the Building Surveyors Board ring the alarm bells? And when the code was revised, why was there no public forum for this enormous problem?

In 2012, when the 14-year drought was ending, the code AS2870 was finally overhauled, despite many slabs distorting excessively for years prior to that.

Extra classifications were made available to choose from: H1, H2, H3 (instead of just H). Further requirements for reinforcement in slab designs were implemented. It all sounds good!

But will these revisions be sufficient to prevent people from taking shortcuts in future? Does the revised Code AS2870 ensure that soil report writers can no longer ignore Appendix D and the rules for P class sites? Either way, I hope they have mentioned heave and (long-term) droughts this time too.

There will still be many more tragedies to play out just from the pool of houses already built, but will the saga continue unabated?

Let’s think ahead in future and not bury our heads in the relatively young volcanic loam.