What is a Retaining Wall?
Retaining walls are structures designed to hold back soil and are commonly used to create usable spaces on sloping sites. If you are planning to build a retaining wall on your property, it is essential to understand the requirements of the New Zealand Building Code, which sets the standards for construction in New Zealand.
We have noticed that many of our clients are unsure when they need to get Building Consent and an engineer involved; this article will briefly discuss current guidance to help decide when you need our help. If you are unsure, feel free to give us a call to talk directly to a real engineer, and we can discuss your ideas.
The Importance of Correct Design
As previously discussed in one of our other articles: https://www.dtce.co.nz/post/retaining-walls-are-getting-bigger , New Zealand is a land of continuous earth movement. But earthquakes aren’t the only worry. Retaining walls play a crucial role in reducing the likelihood of landslips, particularly in areas prone to high rainfall levels or other extreme weather conditions.
Correctly designed retaining walls can help to prevent landslides by stabilising the soil and preventing erosion.
By redirecting water away from the slope, retaining walls can also help to reduce the amount of water that infiltrates the soil, reducing the potential for instability.
Additionally, retaining walls can be designed to provide structural support for adjacent buildings and infrastructure, further enhancing their effectiveness in mitigating landslide risks. Retaining walls are a critical element in managing the risks of landslides and related damage and can help protect public safety and the economy.
When do I need an engineer?
Generally, retaining walls over 1.5 metres in height or within 1.5 metres of a property boundary or building require a Building Consent from the local council. In addition, retaining walls over 3 metres in height, or intended to retain significant amounts of soil, require a specific engineered design. However, several other factors can affect whether building consent and engineered design are required. These can include:
type of soil
the slope of the site
the proximity of trees
the presence of underground services
the proximity to structures of value
The Ministry of Business Innovation and Employment has a detailed document outlining work that does not need Building Consent. See the link below, and note that all building work, whether Building Consent is required or not, must comply with the New Zealand Building Code. https://www.building.govt.nz/assets/Uploads/projects-and-consents/building-work-consent-not-required-guidance.pdf
No Building Consent required?
In brief, here are the plans for some common retaining wall circumstances that do not require Building Consent and engineering design:
Building Consent required!
Here are the plans for some common retaining wall circumstances that do require Building Consent and engineering design:
Engineered design involves a detailed analysis of the site conditions and the proposed retaining wall design and is necessary to ensure that the retaining wall is safe and structurally sound. The design must be carried out by a Chartered Professional Engineer (CPEng), who will produce detailed drawings and specifications for the construction of the retaining wall.
Resource Consent
In addition to Building Consent, you should be aware of whether your retaining wall also needs a Resource Consent. Resources Consents are generally required if your building project affects the environment or community. For retaining walls, this is usually if:
It is close to a boundary
The retaining wall is over 2.5m
The retaining wall requires more than 80m^2 of digging or filling
More information on resource consents and how to apply in Wellington is here: https://wellington.govt.nz/property-rates-and-building/building-and-resource-consents/resource-consents/find-out-if-you-need-a-resource-consent How can we help?
If all this seems a little too complicated, don’t worry, we are here to help! We have helped with hundreds of retaining wall assessments and designs over the years and have experienced engineers on hand to help.
Call us, and we can organise a site visit or a free initial consultation here at our office to discuss your project and give you a little more clarity on what you may need.