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A guide to better Retaining Wall Design

November 25, 2020
Guides

Retaining wall design might seem simple to some, but like an iceberg, there’s usually much more going on out of sight with retaining walls than many people realise. And without knowledge, care and expertise, danger looms large.

Retaining wall failures can lead to costly damage to property and services, or even cause serious injury and death. Which is why it’s often best to have a retaining wall designed by an engineer and constructed by an appropriately licensed retaining wall builder, to ensure a safe, efficient and cost-effective retaining wall solution.

In this article, we’ll build your understanding of retaining walls from the ground up, taking you through the why, what and how of retaining wall design, before discussing how engineers (us in particular) are developing more efficient design methods that result in better outcomes for our clients - be they builders or homeowners/buyers.

What is a retaining wall?

A retaining wall is a structure that is used to maintain soil at two different elevations (created by cut and fill) by retaining the soil laterally and resisting lateral loads that are applied by earth pressure.

The Australian Standard, AS4678 – Earth Retaining Structures deals specifically with walls that are:

  • Greater than 800mm and less than 15m in height.
  • 70° or more to the horizontal (thereby excluding revetment structures with gentler slopes).

While AS4678 is a national retaining wall design standard for walls over 800mm, it’s important to note that local and state government requirements vary around Australia. While NSW requires engineering design for any retaining wall over 600mm, the Brisbane, Ipswich and Gold Coast councils generally only require this at 1m or higher. Basically, retaining a stable low-risk bank is not considered to require a ‘structural’ wall.

But even for small, 400mm (or less) do-it-yourself or home handyman walls, there are recommended guidelines for design and construction regarding factors such as depth of posts, wales, batter, drainage and backfill. These are available from industry bodies such as Timber Queensland or your hardware retailer. The cheap and quick way might cause you more trouble than it’s worth later on, so take the time to do it right, whatever the size of the job.

Why build a retaining wall?

Retaining walls are everywhere. In addition to residential construction, they have uses in a huge range of areas, such as:

  • gardens, parks and hillside farming
  • major public works such as roadways or rail overpasses
  • business, industry and mining
  • health, education, sporting and other public facilities.

This article deals with retaining walls requiring engineering approval in residential settings. For instance, you might need to build a wall on the property line to retain either your or your neighbour’s land, cut and build under a house on stumps, establish a basement, landscape your garden, or cut and fill and retain soil to create a bigger, flatter back yard.

What types of retaining walls are there?

There are many types of retaining walls, each with different surface appearance and style. Behind and under the wall is their hidden design complexity, which may include footings, drain pipes, geotextiles, waterproofing and various types/sections of backfill and surface soils. Below are the main types of retaining wall used in residential construction, each with their own pros and cons.

A concrete retaining wall designed by an engineer
A concrete sleeper retaining wall, featuring a charcoal block pattern. Image courtesy of Prime Concrete Developments.

Sleeper Walls (Timber or Concrete)

This is your “garden variety” type of retaining wall that most people see in their mind’s eye. Horizontal timber or concrete wales are attached behind near vertical posts inserted into concrete footings. 

Pros

  • Both are generally cost effective.
  • Can be constructed in a tight space.
  • Concrete sleeper walls with galvanised steel posts meet the required 60-year design life in AS4678.
  • There are many options in terms of aesthetics, whether timber species or coloured and patterned concrete sleepers.

Cons

  • If there is shallow rock, it can be difficult or impossible to install the posts/bored piers.
  • They cannot be waterproofed effectively so water will run through.
  • For timber walls over 800mm and on a boundary, obtaining the required H5 treated timber to give the required 60-year design life can be difficult and expensive.
  • Not all councils or developers will allow timber walls on boundaries.
Sawn dimensional sandstone retaining wall, with steps and custom letter box, utilising a single and double row to create a garden terrace. Image courtesy of Stonemasters.

Cut Stone (Sandstone) / Boulder / Gabion Walls

These walls use heavy, generally broad-based materials to harness gravity and give the wall strength and stability.  These gravity-type walls have been in use since ancient times (some are still standing), but there are things for you to consider.

Pros

  • Mid-range in terms of cost effectiveness.
  • Suitable to install in shallow rock and limited or no footings are required.
  • Aesthetic appeal

Cons

  • Cannot be constructed in a tight space, as gravity walls require mass and width for stability, therefore taking up a greater footprint (depth).
  • They cannot be waterproofed effectively so water will run through.
  • Boulder walls may have unpredictable performance, particularly as there may be less contact between stones. Cut stone is generally preferred.
  • Different grades of stones are available. Ask questions to make sure you know what you are getting.
  • For gabion walls, the wire is the weakest point. It can fail prior to the 50-year design life if not done by a specialist.
A reinforced concrete block wall, with steel reinforcing and core filled with concrete.

Reinforced Concrete Block Walls

These walls use hollow concrete masonry blocks and are reinforced with steel reinforcing and core filled with concrete laid. They can be constructed without stepback, which saves space. The front wall face can have an appealing range of flat or textured rendering. 

Pros

  • They can be constructed in a tight space when utilising piers under footings (if the site is suitable).
  • Greater height can be achieved with a heel-and-toe design.
  • Can be waterproofed.
  • Aesthetic appeal can be enhanced with rendering.
  • They can be used for most walls on residential sites, including integration with the building envelope.

 Cons

  • In most cases they are a more expensive option.
  • Pier design generally does not allow for high walls.
  • Cantilever design involves a wide toe, so space is preferred in that case. Depending on the situation, the heel or toe may intrude under neighbouring property.
A charcoal link block retaining wall, featuring the block "TrendStone", courtesy of Apex Masonry.

Link Block Walls

These smaller, interlocking blocks allow for a variety of designs, including curved wall frontages popular for use in gardens.

Pros

  • Reasonably cost effective
  • Suitable to install in shallow rock. Limited or no footings are required
  • Aesthetically pleasing with many patterns and colours available
  • Smaller blocks give greater flexibility than other gravity walls.

Cons

  • Important to ensure that either geogrid or no-fines concrete is installed behind the wall as per manufacturer specifications.  Only very small walls do not need this.
  • Similar to boulder or gabion walls, these cannot be constructed in a tight space. As a gravity-type wall the required soil reinforcement or no-fines concrete results in greater wall depth and therefore takes up a greater footprint.
  • Need to ensure that an experienced contractor/engineer is involved to avoid installation issues.

There are other types like anchored/tieback walls that are very expensive and mainly used in major civil works; for example, retaining very high cuttings for freeways. They are not normally used in residential projects.

Choosing the right licenced builder or structural landscaper is an important first step in any retaining wall project. Image courtesy of Instant Greenscene.

Considerations for retaining wall design

So, you (or your client) have decided that a retaining wall is needed. You’ve probably already got a fair idea of where you want to position the wall, how high it should be, and how you want it to look. To refine your thinking and the retaining wall design a step further, here are some specific considerations to review.

Choosing a Builder and/or Engineers

While knowing about the different types of retaining walls is helpful for your initial thinking, most homeowners will benefit from expert assistance. Plenty of walls are built by DIY sweat and tears. And many are also constructed by landscapers. Before deciding on who you’ll get to build your wall, consider the structural complexity.

  • We always recommend using a builder that is suitably licenced to build retaining walls.
  • If your wall is 800mm or higher, you’ll usually need an engineered retaining wall design as per AS4678 (see our Foundation Engineering Service) to ensure council approval, maximise longevity of the wall and provide peace of mind.
  • You generally can’t engage an engineer after the fact i.e. to sign off on a wall that is already built. If in doubt, it’s best to consult an engineer at the preliminary design stage when there is still flexibility for innovative solutions.

Beyond deciding on who will engineer (design) and build the wall, there is a large range of other factors that require consideration, including:

Location Considerations

  • Size of the footprint available.
  • Slope of the ground above and below the wall, to determine surcharge loading above the wall or undermining at the base.
  • Proximity of the wall to other walls or structures.
  • Accessibility of the area for construction.
  • Presence of services, trenches or pipelines near the base of the wall.
  • Regional variation in moisture levels, salt exposure, aggressive soils.
  • Local authority and developer restrictions: sometimes these specify types of walls that cannot be used; for example, no timber sleeper retaining walls over 1m are permissible in Brisbane.

Soil Considerations

  • Soil properties such as density, stiffness, reactivity, salinity, permeability, bearing capacity and shear strength.
  • Availability of solid ground for a foundation.
  • Presence of groundwater.

Load Considerations

  • Ultimate or Serviceability Limit States — the point at which sliding at the base of the wall, rotation of the structure or rupture of components or connections may happen.
  • Earth pressure.
  • Hydrostatic loads.
  • Surcharge loads above the wall such as other buildings or vehicles.
  • Wind loads.
  • Earthquake loads.

Design Considerations

  • Design life required.
  • Required height and length.
  • Surcharge loads applied above the wall (including live loads such as roads, or dead loads such as buildings).
  • Aesthetics, wall batter or setback.
  • Material properties such as strength and durability of mortar and grouts, concrete sleepers or posts and timber sleepers (pre-treatment for termite and fungus).
  • Drainage
    • What is expected/possible water flow and how will it be dealt with? Consideration is required for both retaining wall design and in landscaping above the wall.
    • Backfill, including type, quantity and placement.
  • Access for monitoring and maintenance.
  • Safety during construction.
STA Consulting Engineer's retaining wall design Risk Matrix: maximising retaining wall cost-effectiveness, safety and design efficiency.

How to achieve the “best” retaining wall design

The best retaining wall design is the one that is the most cost effective, safe and efficient. Whilst all engineers in Australia follow the same Australian Standard for retaining walls, the design solution you end up with can still vary, due to the way the standards and general principles are applied to each site - resulting in different degrees of cost efficiency and effectiveness.

For this reason, STA Consulting Engineers has developed its own proprietary Retaining Wall Design Risk Matrix and in-house design tools that:

  1. Allows us to improve the client–engineer relationship using a clear, consistent and repeatable process.
  2. Creates the most cost effective, safe and efficient design possible.
  3. Adheres to relevant Australian Standards, government regulations and sound engineering principles.

The STA Retaining Wall Design Risk Matrix asks seven key questions:

  1. What is the purpose of the retaining wall?
  2. What is the type of retaining wall proposed?
  3. What is the maximum design height?
  4. What is the wall configuration?
  5. How many degrees is the natural surface slope?
  6. How far from the proposed wall are other structures, either existing or proposed?
  7. How far from the proposed wall are existing services?

The answers classify your wall into one of four risk categories:

Those categories are Low, Moderate, High and Very High Risk. The higher the risk rating that is applied to a given wall, the greater the level of engineering analysis and geotechnical investigation required.  These levels align with STA’s product offering to ensure a suitable outcome for projects of differing complexity.

More tests or design changes are done if needed.

This “risk rating” dictates the type and extent of geotechnical investigations that will be needed. The results of those investigations combined with the matrix shapes the design approach, including the type of wall and whether custom design elements are required.

The results of more engineering involvement could also include:

  • suggestions on changing wall type/material to reduce risk (a common option)
  • a terrace vs single wall
  • changing something on the site to reduce risk, such as moving the wall, adjusting height/wall loading or moving services, or anything that allows you to safely meet your needs.

In summary…

1. The STA Retaining Wall Design Risk Matrix is completed by:

  • answering questions
  • provision of hazard rating

2. STA applies their Risk Matrix to:

  • determine if further site assessment needed
  • shape design approach

3. As a result, STA can work better with you to optimise:

  • retaining wall design and type
  • site conditions

4. STA’s process leads to a retaining wall design that is:

  • Safe
  • Efficient
  • Cost effective

What’s next?

Now you know more about retaining wall design and when you need an engineer, but what’s the bottom line?

Simple.

STA’s Retaining Wall Design Risk Matrix reduces engineering and drafting times, meaning we can turn around the design faster, saving you time and money while maintaining individual attention to your project. It also ensures that the appropriate level of investigation and attention is applied to each retaining wall project. Call us on (07) 3071 7444 (QLD) or (02) 4032 6450 (NSW) to discuss your retaining wall requirements, or just to ask some basic questions. We’re happy to help. You can also contact us online to request a quote or make a general enquiry.

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