Research gets to root of Adelaide’s clay cracking conundrum

The range of trees being monitored for water usage in the Salisbury Council area. SCIENCE AND TECHNOLOGY
The range of trees being monitored for water usage in the Salisbury Council area.

A joint UniSA and Salisbury Council research project could help curb a multimillion-dollar damage bill to repair cracked houses and roads across Australia, caused by expansive clay soils.

UniSA PhD civil engineering candidate Stacey Vorwerk is working on an empirical model to predict ground movement caused by trees which redistribute moisture, resulting in the soil both shrinking and expanding.

Stacey’s project, funded by an ARC Linkage Grant, is centred on the Adelaide suburb of Walkley Heights which suffers from widespread road cracking caused by ground movement in the clay soils, costing Salisbury Council millions of dollars each year in repair bills.

The trees – with nearby damage to road surfaces in Walkley Heights – that were chosen for the research include the golden rain tree, ornamental pear, prickly paperbark and Queensland box.

Using these trees as the focus for her research, Stacey has scientifically monitored water usage for each tree species, correlating that data with ground movements and soil moisture changes.

The model she develops may be scaled out to other tree species to help engineers design house footings and road and rail surfaces to withstand soil movement over a 50-year period.

An example of how clay soil reacts in a drought.An example of how clay soil reacts in a drought.

Her findings will help councils choose and manage tree species more suited to clay soils, to limit the damage caused by clay soil shrinkage and expansion.

Stacey says that current Australian Building Industry standards do not provide enough guidance on the impact of tree species on housing infrastructure.

“We just don’t have sufficient information to know what any tree species is doing in terms of the ground movements of our clay soils which are particularly prevalent in Adelaide,” she says.

“There is also no unified approach across the world to test and compare soils.”

Newer homes are less likely to crack, but as house footings are expected to last 50 years, the updated Australian housing guidelines (introduced in 2011) have not been put to the test, Stacey says.

While no concrete figures are available for Australia, the UK’s insurance sector has publicly stated that damage produced by expansive clay soils far outweighs the total cost of damage caused by earthquakes, floods, tornadoes and hurricanes.

In a non-drought year, 15-20 per cent of UK insurance claims are related to soil shrinkage which causes road and building cracking.

Likewise, in the US and China, clay soils are responsible for a combined annual damage bill of $30 billion in repair works, according to a 2004 study.

Stacey’s research is also expected to shed light on how trees redistribute moisture in clay soils via their roots.

“The current technology is not sophisticated enough to tell us where the roots are going, particularly in clay soils. Their distribution remains unpredictable, particularly along residential streets and we need to develop more reliable, non-invasive methods to observe the tree roots.”

Expansive clay soils cover about 20 per cent of Australia’s surface and are found on every continent except Antarctica, notably in arid and semi-arid regions.

“Councils in Adelaide face phenomenal costs due to the clay soils causing roads to crack, even the relatively new ones,” Stacey says.

“It’s an engineering problem that has plagued the industry for around 40 years, but thanks to the emerging crossover between environmental science and engineering, it is now starting to get some attention.”

Stacey’s thesis, undertaken in the School of Natural and Built Environments under the supervision of Dr Don Cameron, is expected to be completed in 2019.