The ground under Mandurah
Most people in Mandurah know they live on sand. What is less obvious is that it is not the same sand everywhere, and the differences matter when you put a retaining wall into it.
Broadly, the coastal strip of the Peel is made up of several bands that run roughly parallel to the shore:
- Young coastal dunes along the ocean side, from Madora Bay down through Silver Sands, Halls Head and on to Dawesville. Pale, loose sand with shell fragments, often deep, with little binding it together.
- Older dunes with limestone slightly inland. Yellow to brown sand sitting over limestone that can be close to the surface in places and much deeper a few lots away.
- Sands and wetlands east of the estuary, where the ground is lower, flatter and often damp in winter.
- River soils towards Pinjarra, along the Murray and Serpentine, which are heavier, more clayey and slower to drain.
Add in the Peel Inlet and Harvey Estuary, the canal estates built around them, and the many lots that were filled and levelled during development, and you get a lot of variety within a small area. A wall that works on one street is not automatically right one street over.
What sand does behind a wall
Sand is a forgiving material in many ways. It drains quickly, it is easy to excavate, and it does not swell and shrink with moisture like clay. But it brings three particular problems for retaining walls.
It runs
Dry coastal sand has almost no cohesion. Cut a trench in it and the sides slump. Leave a gap between two limestone blocks or at the end of a sleeper and sand will trickle through, especially when water is moving through it after rain or reticulation.
Over months and years, that slow loss of sand undermines the ground behind the wall. Paving sinks, lawn dips, and eventually the wall itself loses support. Geotextile fabric placed between the backfill and the wall stops the sand moving while still letting water through. It is a cheap item and one of the most important.
It settles if it is not compacted
Sand placed loosely as fill will settle. If the footing of a wall is sitting on loose sand, or the backfill behind it was dumped and not compacted, you will see it later as gaps, cracks and leaning. Fill should go in thin layers, wetted and compacted with a plate or roller, especially on lots that were built up during development.
It drains, until it does not
Clean sand drains well, which keeps water pressure behind a wall low. But water still has to go somewhere. Where there is a layer of limestone, clay or a high water table below the sand, water can pond behind the wall. A slotted pipe at the base, surrounded by drainage gravel and connected to an outlet, gives that water a way out.
Limestone close to the surface
In parts of Halls Head, Falcon and other suburbs on the older dune ridges, limestone can be found within a metre or so of the surface. That is good news and bad news.
The good news is that limestone is a strong base. A footing bearing on sound rock is unlikely to settle.
The bad news is that limestone is rarely flat. A footing trench may hit rock at one end and stay in sand at the other, so the footing is sitting on two very different materials. The contractor has to either dig down to rock along the whole line, break the rock out to a consistent level, or design the footing to handle the change. Breaking out rock with a small excavator and hammer takes time, so a quote should say how rock will be handled if it is found.
Post holes for concrete sleeper walls are a particular case. Hitting rock halfway down a post hole means coring or breaking the rock, or redesigning the post embedment, rather than just stopping short.
High water tables near the estuary and canals
Low-lying lots near the Peel Inlet, the Harvey Estuary, the rivers and the canal estates can have groundwater close to the surface, particularly in late winter and spring.
That affects retaining walls in several ways:
- Excavation depth. Dig too deep and the trench fills with water, making it hard to place and compact a footing.
- Footing design. Where you cannot go deep, a wider, shallower footing may be used, or the wall type may change.
- Drainage outlets. A drainage pipe is only useful if the water has somewhere lower to go. On a lot where the groundwater is near the surface, the outlet design needs real thought.
- Timing. Work that needs deeper excavation may be better scheduled for late summer or autumn, when groundwater is usually lower.
None of this means walls cannot be built on these lots. It means the design should come from someone who has looked at your ground, not a standard drawing.
Canal-side lots and water-facing walls
Mandurah has a number of canal estates where houses back directly onto the water. These lots usually have a wall at the water's edge that holds back the lot and keeps the canal edge stable.
Before you plan any retaining work on a canal lot, find out:
- Who owns the water-facing wall. On some estates the wall is part of the private lot. On others it is managed by the City or forms part of shared waterway infrastructure, sometimes funded through special arrangements for that estate.
- What approvals apply. Work that affects a canal wall, the waterway or the foreshore may need approvals beyond a normal building permit.
- Where your lot boundary is. It may not be where you think, especially on older estates.
The City of Mandurah is the first point of contact for canal and waterway walls inside the City. Do not cut into, load up or modify a water-facing wall without confirming who is responsible for it.
Retaining walls further back on a canal lot, for example to level a garden or create a pool terrace, are generally handled like any other residential wall, but the high water table and the presence of the canal wall still need to be considered. Adding load near the top of the canal wall, such as a new pool or a heavy wall footing, should be checked.
Salt, steel and coastal exposure
Masonry, limestone and concrete generally cope well with coastal exposure. Steel is the weak point.
On lots close to the ocean or a canal:
- Concrete sleeper wall posts should have heavy galvanising, and the base of each post should not sit in standing water.
- Reinforcing steel in block walls needs adequate concrete cover, which is part of the engineer's design.
- Fixings for timber walls and fences on top of walls should be hot-dip galvanised or stainless.
Limestone itself weathers slowly in salt air, which is part of its appeal. Reconstituted limestone can be sealed if you want to slow the weathering.
Fill, levels and the neighbours
Many newer Peel estates were built up on imported fill. When a lot is higher than its neighbour, or when you change levels yourself, the boundary wall becomes a shared concern.
The Residential Design Codes include standards for site works, fill and retaining near lot boundaries, which local governments apply when assessing applications. A wall or fill that does not meet the standard settings may need a planning approval as well as a building permit. Speak to your neighbour before you start, and check with the City of Mandurah or the Shire of Murray if the wall is near a boundary.
A checklist before you build
Before you ask for quotes on a sandy or canal-side lot:
- Note roughly how far the ground falls, and where.
- Find out whether there is rock below the surface. A neighbour who has built recently may know.
- Note whether the ground gets wet or soggy in winter.
- On a canal lot, confirm who owns the water-facing wall.
- Locate septic systems, soak wells and reticulation lines.
- Lodge a Before You Dig Australia enquiry, or check that the contractor will.
- Think about what will go on top of the wall: a fence, a pool, a shed, a driveway.
- Ask each contractor how they will handle rock, water and spoil.
Then ask for quotes that spell out footing, drainage, fabric, backfill and compaction. The cost guide has a full list of what should be on a quote.
Which wall types suit these lots
- Limestone suits low and moderate walls on sand, as long as fabric and drainage are included.
- Concrete sleeper walls suit straight runs, with attention to post protection near salt water.
- Engineered block walls suit taller walls, high loads and tricky ground.
- Timber is best kept to low garden walls on well-drained sand, away from permanently damp ground.
If you already have a wall that is leaking sand or leaning, start with repairs and rebuilds. To talk through your block, call (08) 9515 3332 or use the form below.