Things that can go wrong when building a retaining wall
Retaining walls serve a number of purposes including holding back banks of earth, preventing erosion on a property, and creating zones as part of a landscape design.
When correctly erected, retaining walls can transform the aesthetics on your property and make your outdoor areas safe. However if they aren’t built correctly, they can fail to do the job they were designed for, pose a safety hazard, and look unattractive.
Replacing a failing retaining wall in itself is expensive, but if the wall’s failure causes damage to your or your neighbour’s property, costs go up. So it’s always best to ensure your retaining wall is well designed and constructed, and uses materials appropriate to the job it needs to do, and to the landscape.
What can go wrong when building a retaining wall
Building a retaining wall may be a simple or complex undertaking, depending on the purpose of the wall, how high it needs to be, and what the land conditions are on your property.
A lot needs to be considered, including the grade of your land, the soil you’re building the wall on, whether or not you are building a tiered retaining wall, where you will direct water to prevent waterlogging, and whether or not you’re building a retaining wall near to a neighbouring property.
If your retaining wall is low, complications are reduced. Whereas a retaining wall higher than 1 metre will come with more complexity, and may require council approval, and engineering. (See more about when you need council approval for a retaining wall here.)
If you have any doubts about your retaining wall project, the best thing to do is to engage an engineer and an experienced builder licensed to build such a wall.
Top 10 things that can go wrong when building a retaining wall
Here is a top 10 list of things that can go wrong when you are planning a retaining wall build. The list is by no means exhaustive, and no doubt a builder or engineer would talk about different issues that aren’t included below. But this list gives you an overall picture of what you need to consider to avoid things going wrong.
We suggest that it’s always best to consult with your local council if you are planning a retaining wall. It’s also well-worth reading in detail any retaining wall regulations your council can provide.
Disclaimer: The following points are general in nature. We don’t suggest that you use this information as advice on building your retaining wall. We are here to illustrate potential issues that can arise with building retaining walls, but recommend you do your own research and consult with an engineer and local council before a build.
#1 Not checking if you need council approval
One mistake many home builders make is not checking with their local council if they can build their retaining wall. Most Australian states regulate how high a retaining wall can be before approval for a build is required, and height may vary between states. If your state doesn’t nominate a height, your local council will specify.
It’s also worth noting that whilst a council approval is not required for a wall below 1 metre in some states, if that wall is within a certain distance of the boundary, council approval still may be needed. Again, check with your local council.
Remember, retaining walls are not considered ‘fences’, hence any height requirements relating to fences do not apply to retaining walls. In circumstances where you wish to build a fence on top of a retaining wall, development approval should be obtained if the total height (measured from the lowest level of the retaining wall to the highest point of the fence) exceeds the amount defined in your local planning regulations.
#2 Not using an engineer
Many home builders neglect to use an engineer when they need to.
Any retaining wall over a certain height will need to be approved by your local council, and an engineer’s report may be required as part of that process. Contact your local council for information on requirements.
Retaining walls will need to be engineered if they are retaining a:
- Parking space
- Swimming pool
- Another failing retaining wall
- If they go above certain height
- Slopes of certain grades, with large banks of earth.
All of these undertakings involve heavy banks of earth that need a sufficiently strong containment structure. Failure to contain could be hazardous.
For all such endeavours, an engineer will need to assess your property, draw up plans, and you’ll need a building consent from your local council.
A Soils Report may be required
As part of your retaining wall build proposal, an engineer may request that a soils report be provided by a geotechnical engineer, before they make their own recommendations.
The aim of a soils report is to understand the soil on your property, and assess how it may impact your retaining wall. This is most often requested if your retaining wall is intended to hold back a sizeable bank of earth, however may be required for smaller walls, as the soil you build on can determine what kind of sleeper and/or reinforcement is best.
A soils report (or geotech report) will let you know about the nature of the soil you’re building your wall on or against, and how the soil may impact the wall’s footings. It will also look for evidence of any existing soil erosion and disturbed soil, and will make an assessment of the ongoing risk of erosion, mudslides and rockfall.
#3 Not checking property plans
One mistake some home owners make is neglecting to check the plans of their property before building their retaining wall. (If you don’t own a copy of your property plans, request a copy from your local council.)
When building a retaining wall, you will need to dig into the ground to install the retaining wall’s footing. This usually needs to be at least as deep as the height of the retaining wall itself, so if your wall is 100cm tall, the footing will usually be equally as deep. Naturally, when you dig deeply into the earth you need to be aware of anything below, such as sewerage or stormwater pipes.
In addition to checking your property’s plans, it’s a wise idea to contact your electricity provider and gas provider to ensure there are no underground wires or gas pipes on your property. Hitting an underground cable or gas pipe could be disastrous.
#4 Using an unlicensed building contractor
If you’re outsourcing the build of your retaining wall to a contractor, it’s vital they hold a current trade licence.
If a tradesperson you’re considering has an expired licence, it may be because a licence was denied to them by a relevant authority, based on prior poor workmanship, failure to abide by building regulations, and/or failure to consult with a structural engineer when required. Obviously this is not someone you want to trust with your retaining wall, as it’s likely they will choose poor methods and materials, and you’ll have little power if you need to follow up on any issues.
There are unlicensed builders out there who will offer to build your wall at a lower cost, but it’s never worth the risk. It can cost you a whole lot more money in the long run if they build a retaining wall that fails, and you have to rebuild. Also, a failing retaining wall can be hazardous, and a risk to your or your neighbour’s property.
#5 Building too close to your neighbour
If you need to build a retaining wall near a neighbour’s fence, such as parallel to the fenceline, then you need to allow sufficient distance between the fence and the wall. Remember, the entire cut, fill and drainage for the wall needs to be on your side.
The general rule is that the wall needs to be set back the same distance from the boundary as the height of the wall itself. In saying that, each local council or State has their own planning regulations about the distance, so you certainly need to check with them.
It’s worth noting that if you wish to build a retaining wall and don’t engage an engineer, your neighbour may engage one if they believe the wall itself, or the work needed to complete the wall, may pose problems for their property. For example, the build may cause excess water run-off onto their property, or excavating equipment used in the build may vibrate the earth, potentially damaging structures on their property.
You may be required to pay the reasonable costs of obtaining the engineer report, plans and specifications.
Building on boundary lines:
It’s possible to build a retaining wall on a property boundary, but both neighbours need to consent in writing, and the retaining wall is not considered a ‘fence’. If a retaining wall is built on a boundary, it’s a good idea to have it engineer-approved.
Following the build of the wall, if either neighbour does anything to undermine the structural integrity of the retaining wall, they are responsible for correcting the issue, even if this means rebuilding the wall, and/or compensating for damage that occurs to the neighbouring property.
What can undermine the structural integrity of the retaining wall on a boundary?
- Excavating next to the wall, causing the wall to sag or collapse
- Tampering with water drainage pipes, so that water is directed where it was not originally designed to go
- Raising the height of the soil bank on your side of the retaining wall, so that more water flows into the neighbouring property. (If you do need to raise the soil on your side, consult an engineer again.)
- Failing to maintain the wall on a particular side, and allowing plants to grow roots close to the retaining wall causing excess pressure against the wall, or allowing them to grow into the retaining wall, undermining its strength.
There are many more important factors to consider when building a retaining wall on or near a boundary line with your neighbour, so it’s important to thoroughly read the retaining wall building codes for your state and/or local council.
#6 Not considering vegetation
Vegetation can interfere with the integrity of your retaining wall’s structure.
Vegetation growing through retaining wall
If you are using cement blocks, bricks or stones for your retaining wall, the roots of plants can grow into the mortar and create spacing between those blocks. This will usually result in wall failure over time, as bricks, blocks or stones may be pushed out of place.
Vegetation pressing against wall
You should factor in the impact of any existing or proposed trees or shrubs when planning the position of your retaining wall. As trees and shrubs grow up, their root systems grow out and down, which places pressure on your wall.
It’s recommended you multiply the height of your retaining wall by 2, and place any trees or shrubs at least that distance away from the wall, if you want to maintain the wall’s structural integrity into the future.
If you don’t have sufficient space, you may need to lose some trees or shrubs with deep roots from your plan. (Note that removing shrubbery can disrupt the soil, causing it to loosen. This may create unique issues with your retaining wall which should be considered.)
#7 Using the wrong backfill
Backfill is an essential for your retaining wall, as it provides a barrier between the soil and your wall material, allows for water to flow through, and prevents water logging and excess pressure.
Inadequate backfill is one of the reasons many retaining walls fail. Allow a good amount of room behind your wall for backfill (approximately 300mm).
Certain materials work better as backfill than others.
Recommended backfill material:
1/2 inch and 3/4 inch crushed stone or scoria is ideal. The jagged edges work better than smooth rounded edges, as they lock into place when compacted.
Crushed stone or scoria also allow for a bit of movement if the ground shifts.
Unless it’s for a very specific purpose, sand is generally not recommended as backfill for a retaining wall, as it can absorb a lot of water, rather than allowing it to flow away. Too much water behind your wall means excess pressure, and potential wall rupture.
Rounded stones or river rock
Rounded stones/rocks aren’t ideal as backfill because they don’t form interlocking bonds like angular stone. This means they are less structurally sound than angular rocks that wedge together when compacted.
#8 Not making allowances for water drainage
For all retaining walls, you want to direct as much water away from the wall as possible. A wall that is taking too much water pressure can rupture.
Although retaining walls should allow for an expected amount of water seepage, it’s important to do the following to ensure water doesn’t build up behind the wall.
- Lay a drainage coil behind the wall. The ends of the drainage coil should discharge outside of the wall itself, and into a stormwater drain if possible.
- The drainage coil should be accessible at both ends, so that it can be unblocked if it becomes obstructed for any reason.
- Geotextile fabric may be used around the drainage coil to prevent it blocking with silt in wet weather, however the fabric should be avoided flush against the face of the wall itself, as it will capture dirt and stop water passing through the wall- adding to pressure.
#9 Not building the wall correctly
There are a number of additional ways you can go wrong when building your retaining wall. Other important ones to consider are:
- Ensuring you dig deeply enough for supporting posts. The holes should be as deep as the wall is high.
- What sort of earth are you building into? Can you easily dig in the posts deep enough? Can you secure them in this type of soil/rock?
- Allowing vertical joints in walls creates weak points. If you are building with stone or brick, you need to be especially careful to stagger the bricks.
- Are you considering the elements? Is your wall in an area prone to earthquakes or regular excessive rain? What's the history of slips on your property?
- Not correctly calculating the appropriate wall height, given the slope of the land. In especially sloped land, the retaining wall will play a much more important role than a very low retaining wall on flat land.
#10 Not monitoring your retaining wall
Whilst this one is not strictly about building, it's vital that you monitor the health of any retaining wall on your property.
It's a good idea to:
- Take photos of your retaining wall over time. Yearly should be fine, but if you notice problems with the wall, take more regular snaps. Compare photos over time, especially after weather events like heavy rainfall, flooding or earthquakes.
- Look out for bulges in the wall or unusual angles in the wall, roots growing through the wall, cracks, and water pooling where it shouldn't.
- Look for unusual changes surrounding the wall. Have trees or fences started to tilt above the wall? Are there unusual cracks on hard surfaces near the wall?
- Check if there have been changes to other structures on your property that might indicate earth movement? This might be doors or windows that you can no longer shut, steps pulling away from your building, diagonal cracks in your building, or floors becoming uneven.
There are a number of actions that can be taken to help a distressed retaining wall. If you aren't sure what to do, seek out an engineer, or a qualified retaining wall builder for advice.
Importantly, fix any issue before it worsens.
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If you are considering concrete sleepers for your retaining wall project, Concrete Sleepers Melbourne are a lead supplier.
Using concrete sleepers for retaining walls has become a popular option, due to their ease of installation compared to materials like brick, stone or poured concrete. Concrete sleepers are reinforced with steel, (usually 2 N-12 bars) and are built to last. If you prefer the look of timber sleepers, you can now get concrete sleepers in timber-like finishes, so you can enjoy the natural look, without concerns about termites and rotting.
Contact us today to find out all you need to know about concrete sleepers.