The Ground Below
There is no mystery to what Australian clay soils do to a pipe under the ground. The clay will expand with moisture and contract on drying, and in doing so it can put enough strain on buried pipework to bend, crack or even separate it. Cracks can also allow roots entering sewer lines. You are left with stormwater that will not drain as it should, a blocked drain, water leaking from the pipes or a sewer line in need of attention.
I am Rhys McDonald. I have put in my time on maintenance jobs and around the gardens and homes of this country to know a thing or two about what goes on below the surface, even if I do not hold a plumber’s licence. The ground has a way of letting you in on a pipe problem long before the bathroom does. Be it a gurgling drain, an area of lawn that has sunk or a damp patch, they are all telling you something.
In this guide I set out the facts on reactive clay soil and the damage it inflicts on pipes, in plain English. For homeowners in Brisbane, Adelaide and elsewhere we will be looking at soil movement, the different types of pipe, what to look for by way of warning and how to go about repairs, as well as answering the questions one is apt to ask.
What Makes Reactive Clay Move
It is the minerals in the soil that are responsible; they take in and let go of water. A wet spell and the soil swells, a dry one and it shrinks back, sometimes pulling away from structures to make voids and gaps around your pipes. These cycles can go on for years, especially if the drainage has been altered around the house.
And it is seldom uniform. You might have a downpipe keeping part of the yard damp while the rest is in the full sun. The stress on underground services that puts on underground services is considerable. The surrounding ground may settle in an uneven fashion and a pipe will crack, or at least shift at a joint.
AS 2870 site classifications are the usual way of gauging reactivity. Class A means little in the way of ground movement, but then you have Class S, M, H, E and P for conditions that are more of a problem. It is a matter for building design though, not to be used as a shorthand for diagnosing a subterranean pipe.
Lateral Pressure From Expansion
A swelling clay can put pressure on a pipe from all sides. If it is well supported and of good quality it will stand up to it. Not so a brittle pipe or one that has been laid on uneven bedding.
Then there is the issue of contraction. The clay dries out and there are voids where it has lost contact with the pipe. Without support underneath, the pipe is liable to sag or move when the soil settles after rain, a vehicle has been over it or water has seeped from the line.
Risk To Your Pipes
Age, material, the way a joint is made, depth, backfill and any trees in the vicinity will determine the outcome. The soil is only part of the equation.
| Pipe Type | Typical Risk With Clay | What to expect |
|---|---|---|
| Clay/earthenware sewer | High on old sections or open joints | Root intrusion, broken joints and cracks |
| Rigid concrete or older stormwater | Moderate to high with shifting bedding | Displacement, blockages and sagging |
| PVC sewer/stormwater | Moderate if the support or joints are wanting | Leaking pipes, fittings cracked and joints separated |
| Polyethylene water | Tolerates movement for the most part | Stress on exposed parts or connections |
| Copper water supply | Bends and connections come under stress | Loss of pressure, leaks and damaged fittings |
Do not think a rigid pipe is going to fail and that a flexible one is exempt. Where a flexible line meets a rigid fitting or has no support it is just as vulnerable. Failures tend to be at the weak spot, not the middle of the pipe.
Depth Will Not Solve It
You can bury a water pipe deep enough to keep it safe from the odd bit of garden work or temperature change, but reactive clay is not going to be deterred. Poor compaction or unstable backfill in the trench and a deep pipe will still be subject to cracking or shifting.
Some protection is also called for against accidental damage from landscaping projects like a new driveway or pool. Make sure to have a service located before you put in a large tree or retaining wall. There is no substitute for a plan, however tempting a shovel may be.
Early Warning Signs
For the most part, drainage issues stemming from clay soil make very little noise to start with. There is a drain that will not clear in a hurry, a narrow strip of lawn that remains wet or the water meter turning with all the taps shut off. Such things should be heeded before one has to put in an excavation project to put right a defect.
By themselves these are no proof of damage to an underground pipe anywhere in Australia; a number of symptoms together are more telling, particularly in the wake of a hard rain or after a dry spell, or where there have been major alterations to the garden or construction work in the vicinity.
- Toilets that gurgle once they are used, fixtures running slow and drains on the block.
- Patches of lawn that are damp, sunken or an odd shade of green for no surface reason.
- A meter moving when it should not, unaccountable bills or a drop in pressure.
- Sewage backing up, wetness about the sewer line or an odour at the inspection opening.
- Stormwater that fails to make its outlet and instead ponds, overflows or is lost in a soft spot.
- Any movement around an inspection opening, soil having sunk by a trench or cracks in the paving.
The mistake is to think the break is right under the nearest wet patch. Water will find its way through the backfill in a trench and show up metres from the actual problem. Sewer smells can be most pronounced at an inspection opening even though the damage is further down the line.
Drainage And Sewer Line Problems
Not all clay is poorly drained. Some of it will hold water, other places have their sand and fill or fractured material for the water to run through. It is a matter of how the soil at your property is inclined to act and if surface water is being channeled to or from what is buried.
With intense rain you can get ponding and runoff from clay that is slow to absorb. Then as the soil dries out and shrinks it can leave a gap by the pipes. A leak will put moisture in the trench and soften the support with a localised wet spot inviting more movement.
Sewer Lines Need Special Care
For wastewater to make its way to the connection point the falls in a sewer line have to be uniform. Should the ground move and put in a low spot, a belly forms in the pipe for waste and water to congregate. A blocked drain is the result and a minor defect at a joint becomes a household annoyance.
Old earthenware in a clay soil environment can also see broken joints. The ground will shift and the section part company somewhat. Wastewater is then in the soil and fine roots and other matter will oblige themselves to come in. Roots do not have much manners about it.
When Roots Join The Trouble
Roots can compound any movement already associated with the clay. They go for the moisture of a leaking or separated joint and grow through the smallest of openings to ensnare paper, grease and the like.
While people put the whole onus on the roots, they may be the symptom of something else. A well put together sewer line is not so inviting. If a CCTV shows them, one should enquire as to whether there is a failed joint, cracking, displacement or poor fall.
A plumber might put in recurring root treatment, relining or a replacement depending on the state of the pipe. Cutting the roots is only a stopgap, it does not close the opening they came in by.
Prevent Damage Before It Starts
You start with supporting the pipe and managing the water. One cannot expect to keep reactive soil in check entirely but the idea is to limit any abrupt changes in moisture so the pipe can stand some ground movement.
Have a word with the licensed professionals over the drainage design, pipe routes and the soil report on a new build or any sizeable renovation. With an existing property, do not let the ground make an excavation of it; maintain the basics and look into early signs.
- Where you can, direct irrigation, surface runoff and roof water clear of the building and pipe trenches.
- Put right any leaks in the irrigation lines, downpipes or taps without delay to avoid making a zone of persistent wetness.
- Make sure the bedding and backfill are properly compacted when installing or replacing.
- Do not have large thirsty trees in proximity to the sewer, and consult an arborist before you take out an established tree.
- Employ flexible joints where the plumbing and design will have it.
- And leave the inspection openings where they can be got at and note down their position before you pave or plant over them.
Some may turn to Seasol for the plants but it will not mend a pipe or put a stop to the soil expanding and contracting. Garden products are no substitute for proper installation and grading. Wishful thinking is costly in this line of work.
Keep Access Clear
Time and money are both conserved with good access. See that the plumber has an unobstructed path for his vehicle, drain camera and any excavation gear by way of the driveway or side gate. Should the pipe in question run along a tight side path, put any furniture or movable pots out of the way prior to the appointment.
While one does not have a “route” for a buried pipe as such, the logistics of the site are important. Make the plumber aware of where the water meter is, the internal fixtures, stormwater pits and sewer inspection opening. In the event of restricted access, make it known at the time of booking so he can put the right crew and equipment in place.
The Right Repair
What kind of repair is called for will be a function of the defect, the pipe material, how much access there is and what ground movement is to be expected. A competent plumber will want to do some testing or CCTV work to verify the condition; he will not just start digging at the first sign of damp and see what happens.
| Option | Best Suited To | Main Limitation |
|---|---|---|
| Local excavation and repair | An accessible break or a joint that has given way | Inevitably upsets the garden or paving |
| Pipe relining | Sound sewer sections | Will not do for every collapse or connection |
| Pipe replacement | Pipework that is collapsed, badly out of position or a repeat offender | More costly and disruptive |
| Root cutting and cleaning | To get the flow back while a permanent fix is put in hand | A remedy for the symptom more often than the opening |
With pipe relining a new liner is put in the old pipe which spares some excavation. It is no magic carpet though: the host pipe needs to be made suitable for the method, cleaned and assessed. Where there is major displacement or a section is missing, replacement is in order.
Costs can be all over the place depending on the location, depth, traffic control, reinstatement and the like. An easy repair might be in the hundreds or low thousands of dollars; replacing a long stretch of sewer under a driveway or landscaping will set one back several thousand. Get a written scope from the plumber with diagnosis, excavation, testing and the rest itemised.
When To Put In A Call
If you are dealing with a blockage that will not stay put, an elusive water leak, a drop in pressure for no reason, sewage backing up or a stormwater line that has quit after some ground movement, engage a licensed plumber. Getting in early can spot a broken joint before the soil is saturated or a void has formed.
Sewage overflow is a hygiene matter. Heed local public-health advice for the clean up, keep pets and children at a distance and do not come into contact with the water. And if an underground electrical service could be in the way, put the digging tools down and go through the proper service-location process.
To make the most of the visit, have some idea of when the trouble started and if it is weather dependent, what fixtures are involved and if there has been any construction or change to the nearby trees and paving. A video of some gurgling or ponding will put an intermittent fault in perspective.
Reliable Building Guidance
One can look to AS 2870 for residential slabs and site classification and an understanding of why Australian homes are put through their paces for soil movement. For the National Construction Code and other building guidance the Australian Building Codes Board is the source. Any plumbing repairs must be done by the properly licensed tradesperson and in accordance with the requirements.
Frequently Asked Questions
How Does Clay Soil Affect Drainage?
Clay is slow to take in water, hence heavy rain can mean poor drainage and runoff. When it dries the clay shrinks and opens up around the pipes. The answer is to manage surface water and look into any persistent wet spots instead of blaming the soil.
How Deep Should Water Pipes Be Buried?
It is not a case of one size fits all in Australia. Climate, protection from traffic, local rules and the type of pipe will dictate the depth. A licensed plumber will know what is right for the site. Burying it deeper is no guarantee against damage from movement.
Does Seasol Improve Clay Soil?
It may be good for the plants but Seasol is not going to stabilise reactive clay or solve the plumbing issues that expansive clay brings. One can improve the structure with organic matter and better drainage, but plan your work around underground services to avoid making new dry or wet zones.
Do Clay Soils Have Good Drainage?
Not as a rule compared to a coarse sand, but it comes down to the type of clay, the slope and compaction. There can be a reasonably well drained surface and yet a saturated pocket near a trench.
Can Clay Soil Crack A Sewer Pipe?
It can. Poor bedding or an old pipe will be put under stress by the soil, as will the fittings. Sometimes a pipe is shifted to the point of a leak or a low spot without a crack to show for it. Testing and a CCTV inspection are the sensible approach.
Act Before It Spreads
Given the right route and drainage, the pipes under an Australian clay soil will stand up for decades. But add in tree roots, leaks, age and changes to the surface drainage and you have problems.
Should you find yourself in Brisbane, Adelaide or any part of Australia with subsidence, recurring blockages or a change in water use, have it professionally assessed. You will have more options for the repair and less chance of turning your garden into a roadworks site if the defect is found at an early stage.