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.