Soils and the drainage system
To ‘build on solid ground’ is something of an assumption. It presumes the stability and durability of a surface that, in reality, is more akin to a collection of moving parts.
Soil is susceptible to varying degrees of movement, which in Australia can be attributed to two main phenomena.
Reactive soils expand and contract with exposure to or lack of moisture. Dispersive soils appear to dissolve in fresh water due to a chemical imbalance within clay and are prone to subsoil erosion resulting in the formation of underground cavities and tunnels.
Searching for soil movement
Paul Trentini of the Footings and Foundation Society of Australia says reactive soils are found all over Australia, particularly in population centres.
“Reactive soils are typically clay but not necessarily only clay,” he says. “Clays are found all over Australia, particularly in population centres.”
For example, Paul says at least 50% of metropolitan Melbourne is built on ‘m’ rated soil – indicating a medium level of soil reactivity resulting in movement of between 25mm and 40mm.
He says the problem is of a similar size in rural Victoria, Queensland, New South Wales, South Australia and Western Australia.
The clay in reactive soils reacts with water and changes volume according to its moisture content. It expands with increased moisture and contracts as it evaporates. This movement shifts foundations, cracks walls and breaks pipes.
Dispersive soils are prone to tunnel erosion. When fresh water such as rain or stormwater comes into contact with a dispersive soil, the clays expand and disperse resulting in slurry that moves downhill and forms a cavity.
Over time these cavities expand into tunnels that are prone to collapse.
Marcus Hardie is a research fellow at the Tasmanian Institute for Agricultural Research who has studied dispersive soils on behalf of the Department of Primary Industries and Water in Tasmania.
He says it is an important soil type as at least 30% of soils in Australia have the potential to disperse.
“We have seen tunnel erosion caused by the collapse of dispersive soils in every municipality in southern Tasmania and quite a few municipalities in northern Tasmania,” he says.
“It has also been reported in other regions including the Hunter Valley, the Gold Coast and as far away as Christchurch in New Zealand.”
Refusing to move
Paul says most modern drainage systems are connected with solvent and do not allow for any mechanical movement.
“When soil movement – either shrinking or swelling – exceeds even as little as 25mm to 30mm the junctions that connect the pipes simply crack due to the pressure of the movement. Fluids then seep into the sub-soil and the cycle continues.”
“A standard slab, 20-square brick house with a tile roof has a downward pressure footprint of approximately 20kPa, whereas reactive soils may have an upward pressure of approximately 400-600kPa per square metre as a result of the clay shrinking and swelling.”
This causes unwanted heave – far in excess of that allowed for by the design engineer – and floor and wall cracks appear in the building above.
“The ground moves, the pipes break, water gets into the soil and if the water is near a structure it is not good.”
The problem has been exacerbated by drought conditions in much of southern Australia throughout the last 10 years.
Efforts to save water have resulted in dry drains which are more susceptible to cracking. Consumers have become more aware of cracks appearing in their walls as the ground below contracts in response to increasingly dry conditions.
What’s a plumber to do?
Paul says a soil test should be carried out on site before construction work commences in accordance with AS2870.
“You shouldn’t do anything until you’ve completed a soil test. 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.”