AS2870 was introduced in 1986 claiming, ‘Adherence to this Standard will reduce litigation’. However, there is a growing body of evidence denying this claim. The Queensland Building Services Authority (QBSA) reported in May 1998 (only twelve years after the introduction of AS2870), ‘A large percentage of damage compensation claims involved subsidence problems’. A subsequent QBSA press release revealed, ‘The cost of subsidence problems was doubling each year’. The Sydney Morning Herald newspaper published an article on 11 February 2007, ‘A survey of 75,000 homes across Australia by Archicenter, the building advisory service of the Royal Australian Institute of Architects, revealed, in most states, more than 35 per cent of homes had experienced cracking’. The Victorian Building Authority (VBA) reported in May 2014, ‘Research found that 5.3 percent of dwellings built in an area of Melbourne’s western suburbs between 2003 and 2011 showed some form of distress (such as cracks in the floor and walls), attributed to slab heave‘, and ’The research found the key issues relating to slab heave were associated with deficiencies in the storm water drainage systems of the dwellings’. The Age newspaper published an article on 08 June 2014, ‘Estimates suggest up to 4,300 homes in Wyndham, Melton and Hume Local Government areas may be suffering from slab heave’.
Most, if not all VBA reported slab heave problems occurred within a few years after construction, some even within a few months, all a long time short of the 50 years design life prescribed in AS2870. Why are so many slabs experiencing structural damage prematurely? AS2870 acknowledges that buildings constructed on sites subject to so-called ‘Abnormal moisture conditions’ have a higher probability of structural damage than those on normal sites. Who is responsible for abnormal moisture conditions? AS2870 suggests that site classifiers, qualified engineers, designers and builders are responsible for abnormal moisture conditions existing prior to or resulting from building construction, and home-owners are responsible for abnormal moisture conditions developing after construction. Anecdotal evidence suggests that when AS2870 has been applied and there is a structural damage investigation, home-owners are all too often blamed for abnormal moisture conditions, and are thereby effectively prevented from proceeding to litigation. This has left many new home-owners with no avenue of recourse after their homes have suffered significant structural damage. The author contends this is a very unjust outcome for these home-owners, for the following reasons:
- Abnormal moisture conditions do not cause structural damage directly. The missing link is how much the soil moisture content changed from the time of practical completion of construction to when significant damage developed. The emphasis must be on change in moisture content, because soil moisture content changes from day to day, regardless of the conditions at any particular point in time before or during construction of houses, or after practical completion of construction.
- Whilst there is no doubt whatsoever that excessive foundation movement is the principal cause of structural damage, investigators are not in a position to determine the quantum of soil moisture changes and associated foundation movements that actually cause structural damage. Nevertheless, bearing in mind that site characteristic surface movements are defined and determined in accordance with AS 2870 as occurring between extreme moisture changes that have less than 5% chance of being exceeded in 50 years, it is very surprising that most, if not all, investigators to date have not looked for explanations of structural damage other than abnormal moisture conditions. The fact that the main symptom of structural damage is significant cracking of the slab suggests that insufficient strength is the most likely explanation of structural damage. However, damage investigators are reasonably entitled to expect that standard deemed-to-comply slab design solutions have sufficient strength to sustain foundation movements at the design levels prescribed in AS2870. Unfortunately, this is not the case. Ys capacities