Do You Need a Structural Engineer to Design a Retaining Wall?

A retaining wall is a structure designed to stop earth and rock from falling down a slope, often in order to create a patch of flat ground on otherwise sloping land. Retaining walls can help to prevent erosion, landslides and the movement of soil. The level areas created by retaining walls make land easier to develop.

Retaining walls have been used to reshape land for millennia. In Asia they were used to form rice paddies on sloped ground. In Ancient Egypt they helped to limit the damage and control the flow of the Nile.

Modern retaining walls come in a range of shapes, sizes and designs, from the small and simple timber or block structures seen in residential backyards, to the large concrete retaining walls that are often installed in and around critical infrastructure.

The apparent simplicity of a retaining wall hides a lot of engineering complexity. Retaining wall design must consider a wealth of different factors, including load-bearing capacity, soil pressure and drainage, to ensure the ground stays stable, that failures don't occur, and that the wall lasts as long as the project demands.

The larger the wall, the greater the risks, and the more important the design becomes. If your retaining wall breaks or collapses, anything built on the previously supported earth might also break or collapse. The failure of a small retaining wall in your back garden might only be an inconvenience. The failure of a large retaining wall on the side of a major highway or at the base of an apartment block could prove catastrophic.

And these examples hint at the answer to our original question: do you need a structural engineer to design a retaining wall? The answer, in many cases, is yes.

In New Zealand, under Schedule 1 of the Building Act 2004, the construction of a retaining wall does not require a building consent when it is less than 1.5m high and ‘does not support any surcharge or any load additional to the load of that ground’ (for example, the load of vehicles, a building, or sloping ground). Regardless of whether a building consent is required, retaining walls must comply with the New Zealand Building Code.

There are a number of situations where you should consult with an engineer regardless of retaining wall height:

• For civil, commercial or large residential projects
• When the retaining wall will be built on particularly steep ground
• When it will be close to site boundaries
• When houses, driveways or other structures sit above the retaining wall
• When there are complex drainage requirements
• When the ground conditions are complex

Even in situations where a structural engineer isn't expressly required, bringing in a structural engineer can give you that extra peace of mind that you may not get from a general builder or architect.

The primary benefit of hiring a structural engineer for a retaining wall is ensuring that the design will comply with the structural integrity and durability requirements of the NZ Building Code.

But beyond that (somewhat obvious) point, a structural engineer will bring value to your retaining wall project in a number of other ways:

A wall that's fit for purpose: A structural engineer will ensure the retaining wall can withstand all the pressures and conditions that it will be exposed to, ensuring it will stay strong and stable for the long run.

Material expertise: A structural engineer will know the best materials and construction methods to use in any given situation, factoring in considerations like strength, durability and aesthetics.

Regulatory compliance: When you hire an engineer you can enjoy total confidence that your retaining wall meets the structural and durability requirements of the NZ Building Code.

Cost efficiency: When you design your retaining wall well, you can minimise construction issues and the need for maintenance and repairs. Ultimately a professionally engineered retaining wall will save you money over the lifetime of the wall.

Customised and optimised: A professional engineer can develop a retaining wall design that is optimised for the site conditions and your project requirements. An experienced structural engineer will find the best solution to any challenge they are presented with.

Quality assurance: The structural engineer will carry out construction observations to monitor the quality of critical elements of the wall’s construction.

The only way to answer this question is by asking another: how long is a piece of string?

Retaining walls can vary from small structures that border your backyard lawn, to large structures that support significant infrastructure. The cost of hiring a structural engineer will reflect the size and complexity of the job.

But rather than see a structural engineer as a cost, it's wise to view them instead as an investment. Bringing in the necessary expertise at the beginning of your retaining wall project can save you money over the lifetime of the wall.

At Eliot Sinclair we're passionate about shaping a better tomorrow, by providing the structural engineering expertise you need to plan, design and deliver your development project. We provide all the consultancy services needed to complete the transformative private or commercial projects that will shape a better tomorrow.

With offices up and down Te Waiponanmu, we grant you local access to the best technical minds, who can guarantee the safety and compliance of your project while driving efficiencies within it.

Save time, money and hassle while building better. Get in touch with our friendly team today.