Behind the Wall: The Essential Guide to Retaining Wall Standards and Best Practices

If you don’t want to excavate a site, your best option would be to construct a retaining wall to contain a pile of dirt. A retaining wall’s primary task is to keep in check the movement of earth or other material through erosion, collapse or sliding, which involves holding the weight of the soil or material. 

Retaining barricades aren’t just practical; they enhance the property’s curb appeal. You can use diverse materials to make retaining barriers, including wood, stones, limestone blocks, boulders and concrete, poured concrete or concrete blocks. What’s more, you can spruce up any type of retaining wall in your favourite colours and finishes that is according to the retaining wall standards.

As with any other wall, retaining barriers can pose a danger to humans if built poorly. That is why legislative bodies enacted laws to govern the safe construction of retaining barriers. Thankfully, these are usually easy to follow and will help you erect a retaining barricade that meets quality standards and lasts longer.

This guide explores all you need to know about retaining barriers in Australia by dissecting the retaining wall’s Australian Standard AS:4678 ‘Earth Retaining Structures’ and the National Construction Code of Australia 2019.

How high can I build a retaining wall without council approval in Victoria?

In Victoria, you must acquire a building permit to construct a retaining barrier one metre or higher and/or built on/near a property boundary that could potentially damage the neighbouring property.

Under the building code, sleeper garden edging and shorter retaining walls under 1 m do not require a building permit. Neither do you need a permit to plant trees, put up ornamental ponds or garden beds, or lay a driveway or paving.

Before building a retaining barricade, it is advisable to consult with your local council and ensure the engineering design meets design regulations.

Who is responsible for a retaining wall design in Victoria?

Each landowner in the State of Victoria is liable for keeping in check the soil they or their predecessors interfered with at ground level. That may include excavations below the existing ground level or fill added to the surface.

Although your retaining wall builder neighbour is responsible for maintaining the land’s natural state, if you suffer loss or damage because of their actions, you must take reasonable measures to limit the extent of that loss or damage. If you don’t take any steps to minimize the damage, you could lose your compensation claim.

Therefore, it is more practical and cost effective for neighbours to put up a single retaining wall to hold back the earth after filling or excavating rather than having each property owner build their own barrier.

What is Australian Standards AS 4678-2002?

The Australian Standard 4678: ‘Earth Retaining Structures’ establishes requirements and recommendations for designing and building structures that retain dirt, rock and other materials. This standard only addresses walls higher than 800 mm but less than 15 m and an inclination of 70° or more from the horizontal. 

It also provides requirements and recommendations for strengthening the ground and rock materials. However, it excludes the use of revetment structures with lower tilt. Although AS4678 is the national guideline for erecting retaining barrier heights above 800 mm, local and state government regulations, say Queensland, NSW, Brisbane or Sidney, may have their own slightly different set of requirements.

Here are the main features the Australian Standards AS 4678-2002 addresses:

Design requirements

Section 3.1 to 3.9 provides the following design requirements:

General requirements: Details what you must consider while designing earth-retaining structures. Following the guidelines will mean a robust, stable and serviceable retaining barrier for its lifespan. You must consider aspects like drainage and the impact of the construction on the surrounding areas. Other issues to consider include anticipated site conditions, the project’s constructability and temporary support measures when choosing a suitable retaining structure during the initial design phase.

Ultimate limit state: The design process must consider the structure’s classification, design life, durability and maintenance.

Serviceability limit state: It must be adequate so that local damage from the base, retaining barrier, or reinforced dirt movement does not reduce the structure’s expected lifespan or necessitate excessive maintenance expenses.

Design life: All earth-retaining structures must have components that can withstand the elements and work reliably for their intended service life.

Durability: Refers to performance over time, typically the structure’s ability to withstand degradation from improper installation and environmental agents, such as weathering from time and artificial sources like chemicals and biological organisms, while maintaining its intended strength throughout its anticipated service life.

Water removal: When designing the retaining structure, consider the site’s drainage, particularly its immediate and potential subsurface hydrological conditions.

Design loads

Section 4 delves into:

Loads: Specifications for the types of loads and activities during construction and service that you must consider while designing earth-retaining structures. These could be:

• As defined by AS 1170, live and dead loads with earth pressure and hydrostatic pressure
• Wind loads according to AS 1170.2
• Tolerable earthquake loads determined in line with AS 1170.4 and Appendix I
• A variety of other potential loads (like snow and ice).

Load combination: AS 1170.1 specifies the load combinations you must meet for the local authorities to consider the retaining barrier strong, stable and serviceable. You shouldn’t add any more factors if applying a factored load causes a reaction.

Construction

Section 6 of the AS 4678 deals with:

General construction: You should describe in drawings or other appropriate documentation in sufficient detail all earth retaining structures subject to Australian Standard 4678 to allow their construction according to designs that meet the specifications of Sections 3, 4 and 5. It also states that you shall build every structure (a) in conformity with the drawings or other relevant documentation and (b) in line with Clauses 6.2 and 6.3.

Construction tolerances: The designer is responsible for establishing construction tolerances that allow for changes in site conditions without compromising the integrity of the original design.

Specific requirements: Deals with the following: 

• Site preparation/excavation
• Foundation dirt
• Footings 
• Fill material 
• Placement of fill/compaction
• Dirt reinforcement
• Facings
• Moisture removal
• Connections

Retaining Wall Standards: Australian Standard 4678 Retaining Wall Design Considerations

Factoring in the Australian Standard 4678 wall design requirements is crucial when designing a new retaining barrier. Leaving out any of these factors may lead to failure or cause it to accumulate water.

Surcharge load

These are vertical loads applied to the retained earth over the top of a retaining barrier. It can include dead loads such as sloping retained dirt over the height of the wall or live loads from nearby roadways, parking lot, or pavements. If there is a surcharge, the barrier could do with reinforcement.

Lateral pressure

It is one of the most crucial factors to consider when designing and constructing a retaining barricade since the retained material tends to move forwards and down a slope because of gravity. That causes sideways pressure behind the wall, using internal friction (phi) and the retained material’s cohesive strength.

In most cases, the horizontal earth pressure is lowest towards the top of the wall and greatest near the base. If the barricade isn’t well-built, the earth’s forces will push against it and even topple it.

Sub-base

To prevent the whole retaining wall and backfill from sliding down the slope, it is vital that retaining barriers not settle or lean too far into the soil below them. An effective retaining wall should have a foundation buried in the ground, built from compacted earth, and a layer of sand and gravel at least 150 millimetres thick.

The base material should extend past its front and rear edges by at least one foot. When constructing a dry-laid stone barrier, the foundation may need to be as broad as the barricade is high, and you may need to place the most stable stones at the bottom.

All these steps will keep the barrier stable, increasing the contact area between the building components and increasing friction and the overall strength.

Draining of moisture

It is essential to consider the water draining requirements, which provide pointers on allowing water to flow down and away from the barrier. That’s because, in addition to vertical and lateral loads, the barricade will be subject to horizontal hydrostatic pressure from any groundwater that collects behind it but not channeled away by the gutter system. 

If water or soil moisture accumulates behind a retaining wall, it causes hydrostatic pressure, eventually leading to failure. You can control water flow from your new retaining wall using gravel beneath a layer of compacted topsoil, artificial draining blankets, drain pipes, or cut holes.

Changes in the use of the area

It is also essential to consider the possibility of alternative design instances or modifications resulting from the building sequence or change of use of the surrounding areas.

For example, you may need extra surcharges and allowances for potential excavation of the ground in front of the retaining wall, especially if you included the passive resistance of the building material in the stability calculations.

It may also be necessary to consider the impact of excavation on your wall’s carrying capability.

Common retaining wall defects identified during inspections

Depending on a site’s shape, needs and characteristics, a structural engineer will prescribe different types of retaining walls, be it cantilever retaining walls, gravity walls, or anchored/tieback retaining walls.

For instance, the engineer will likely suggest a concrete masonry retaining wall if the site requires a rigid foundation.

Cracking

In recent years, materials like masonry blocks and concrete sleepers have replaced most traditional materials used for residential retaining walls, like timber. Cracks in masonry retaining walls are almost inevitable since you will build the retaining wall on foundation material that expands and contracts with the seasons.

However, it is essential to note that not all types of cracking in retaining walls indicate severe wall failure. You should be wary of large trees near your retaining walls as their root systems can undermine them.

Efflorescence

Efflorescence refers to the accumulation of white mineral salts that have leached out of the surface of retaining walls due to water movement. It is often a sign of improper water channeling and has an unattractive aesthetic impact. Unsealed limestone retaining walls are susceptible to efflorescence.

Rotation

There are several potential causes of rotation. The most prevalent is that the discharge from the soil exerted too substantial a pressure onto the wall and pushed the wall outwards. Since wet soil is almost twice as heavy as dry soil, poor moisture draining is a typical source of high-load discharges.

Poor design of retaining walls, such as insufficient vertical supports or defective footing, may also cause rotation. Rotation weakens the retaining wall’s structural integrity.

Termite damage in timber retaining walls

Pests like termites may significantly damage a retaining wall constructed from timber or sleepers. Hardwood is the most popular wood used for retaining wall construction and the most vulnerable to termite infestation and rot.

Preservative treatments (CCA – Copper, Chrome, and Arsenic) may not absorb into all timber parts entirely. With time, termites find these exposed areas and attack, making them degrade and decay faster owing to the absence of preservative treatment.

However, recent changes in the legislation for retaining walls may phase out the use of hardwood sleeper retaining walls, which has been a significant issue regarding the dependability and structural integrity of retaining walls.

Part of the legislation requires that any new retaining wall built higher than 800 mm must have a minimum lifespan of 60 years, which rules out timber retaining walls.

Wrapping Up

Constructing a retaining wall for your property, whether for garden landscaping or a massive concrete structure, is a great addition that significantly prevents erosion and improves the property’s aesthetic value.

Although primarily used to retain soil on a property during construction, you can use retaining walls to level terrain and enhance drainage.

For retaining walls higher than 1 metre, you may need building approval and have it inspected to comply with Victoria State retaining walls standards. You may also need to check the local council requirements if your proposed construction requires a planning permit. That’s because some councils in Victoria may require a planning permit for any building works on a site under 500 square meters.

Also, if you are building on a boundary, some councils may require you to apply for site consent, or protection work notices served to the adjoining neighbours. Some may require an overlooking or shadow assessment if you are increasing the finished floor level.

The Australian Standards AS4678-2002 for earth-retaining structures also stipulate requirements and recommendations for consideration in the design and construction of retaining walls.

If you want to construct a retaining wall in or around Victoria State, our experts in retaining walls can help you meet the legal requirements. You can rely on SQM Architects to deliver to your satisfaction while adhering to Australian Standards for retaining walls. Contact us if you’d like to find out more about retaining wall standards.