Ground levels on a property or between properties can vary dramatically. Where this occurs, the ground level can leave a property with steep slopes of unusable land. In this situation, a retaining wall is the best way to maintain level ground and ensure that the ground/soil is adequately supported. But who designs retaining walls? There is not just one engineering discipline required for this design especially for steep sites with drainage issues. To adequately design such a retaining wall, a team of engineers will be required to manage potential land slip issues, soil pressure and site drainage. Handymen or builders can build low garden walls or walls without steep slopes or drainage issues without engineering, but anything that requires engineered drawings and council approval will need an engineer.
What are the steps for retaining wall engineering design?
The first step is to identify the need for the retaining wall. As noted in the introduction, retaining walls are required to level a sloping site to allow a step in the ground level without sloping ground.
When ground conditions are part of engineering design, a geotechnical engineer should be engaged. This is because they are experts in the subgrade materials (soils) and how they will react when excavated. Geotechnical engineers can comment on the risks of landslip where they are present.
The next step is to engage with a civil/structural engineer that has experience in retaining wall design. There are many different types of retaining walls and an engineer that has experience in these structures is able to provide guidance on the most efficient retaining wall type. The engineer is able to calculate and design a retaining wall to hold back soil, manage lateral earth pressure, and accommodate water movement behind the structure. The can consider material selection (concrete, timber, block, sheet pile), footing depth, drainage behind the wall, and the load calculations needed to stop the wall rotating, sliding, or overturning under pressure.
As part of the design process, Consulting Engineers play a vital role, particularly on sites with drainage issues, where the stormwater requirements are essential. Drainage related pressures is what causes most residential retaining walls to fail. A wall designed without adequate subsoil drainage becomes a dam. Water builds up behind it, and hydrostatic pressure climbs. A wall that could hold back dry soil may be completely inadequate once that soil becomes saturated. On steep sites, this is not a minor concern as these types of sites are more likely to collect higher levels of rainfall. Therefore, either the civil/structural engineer should be experienced in stormwater design or a stormwater engineer will need to be engaged to manage this component.
Do you need an engineer for a retaining wall?
In most Australian states and council areas, any retaining wall exceeding 1 metre in height requires council lodgement and engineered drawings. In Queensland, the threshold drops to 600mm in some situations depending on the proximity to a boundary or structure. The rules vary by council and state, but the principle is consistent: once a wall has the potential to cause harm if it fails, it requires engineering certification.
Even where council approval is not technically required, a wall on a steep site with drainage issues almost always warrants professional assessment. The drainage component alone can cause structural failure if it is not designed correctly.
Can a handyman build a retaining wall?
A handyman or builder without engineering design can build a low garden wall, typically under 600mm, using sleepers or blocks in stable, flat ground with no drainage complexity.. Anything taller, anything on a slope, or anything adjacent to a structure should not be left to a tradesperson working without engineering drawings.
If an unengineered wall fails and damages a neighbouring property, the person who built it and the person who commissioned it can both face civil claims. Some home insurance policies will not cover damage caused by unpermitted structures. The cost of getting engineers involved early is minor compared to the cost of rebuilding a wall that was done incorrectly and then caused damage.
What to expect from the engineering process
An initial site visit is standard for steep or complex sites. The geotechnical engineer will assess slope angle, soil type, existing drainage, and proximity to structures. From there, they will produce a report and recommendations that a civil/structural (and perhaps a stormwater) engineer can use to create engineering drawings suitable for council lodgement. Please note that should the wall be subject to local council planning permissions that that process will need to be undertaken prior.
Matching the engineer to the site
Not every civil/structural engineer has experience with steep residential sites and complex drainage. When engaging engineers, ask specifically whether they have handled sites with similar slope angles and drainage issues. Ask to see examples of drainage specifications they have produced. A retaining wall on a level block is a different problem to one sitting mid-slope on clay soil with no existing drainage outlet.
For steep sites in particular, the drainage design is as important as the structural design. Both need to be handled together, not as an afterthought. An engineer who treats them as separate problems is likely to produce a solution that addresses one and ignores the other.