It should be a comprehensive assessment that takes a number of samples from a single site, rather that multiple samples from one area, as soil quality can be extremely variable even across a small area of land.

“Plumbers need to know what sort of soil they’re digging in, apply it to the Standard and fit the drainage system accordingly.”

As industry knowledge of dispersive soils is in its infancy, less is known about its testing procedures. However, Marcus says the process is relatively simple.

“Place a dry soil sample the size of your thumb nail into distilled water. It can’t be tap water – the water has to have no salt in it at all.

“Importantly, don’t shake it up. Just put the sample into the water and let it sit for a couple of hours. If it’s dispersive you’ll see a milky halo around the soil and that means you’re at risk.”

Ideally, builders should inform plumbers of the soil quality they will be digging, but if this is not the case plumbers should carry out a soil test independently.

Paul says there are a number of clues that should alert plumbers to the presence of reactive soils before testing is carried out.

“When you’re using a back-hoe or shovel and can’t get the dirt off the shovel or out of the back hoe and you have to shake it a lot, or you walk on the soil and feel as if you’ve grown two inches, that’s reactive soil.”

Equally, Marcus says dispersive soils are easy to spot when familiar with the indicators.

“When sub-soils have been in contact with water they develop a distinctive dribble or worming pattern on the outside of the clay. It looks like the kids have been dribbling sand – it’s quite distinctive.

“Pitting, where there are holes in the surface of the soil, and spew holes, where there is clay and sand popping up during rainfall and spewing out onto the surface of the soil, are an excellent indication that soils are dispersing.”

Once plumbers are alerted to the presence of reactive or dispersive soils on a construction site they have a duty of care to alert the property owner.

Reactive soils can be counteracted by fitting expansion joints, which Paul says for an entire property generally cost $2,500 to $3,000 extra.

Repairs to existing structures, particularly when the drainage system is fitted underneath the slab, are extremely difficult and expensive to fix.

Marcus says it is pertinent that fresh water, particularly rainfall run-off and stormwater, don’t come into contact with dispersive sub-soils to avoid tunnelling.

“One of the most important things is how stormwater run-off and discharge are managed, particular in semi-urban environments which don’t have controlled stormwater.”

In most cases, sites containing reactive and dispersive soils can be built on. Marcus suggests lowering risk by ensuring that dispersive subsoils are not exposed to rainfall and taking care with the placement of stormwater run-off, septic systems and power cables.

Both Paul and Marcus agree that there is potential liability for plumbers should they neglect their duty of care.

In an increasingly litigious society it is crucial that appropriate testing procedures are carried out and clients are informed accordingly.

Ultimately, the key is avoidance, Marcus says.

“Unlike a lot of other problems, by the time you’ve found reactive and dispersive soils the damage is done. If you can stop the process early it’s a lot easier.”