Retaining walls

We now have two questions to do with retaining walls.

A new ACM or acceptable construction manual was introduced in NCC 2019, in Volume Two for earth-retaining structures.

It's AS 4678 Earth-retaining structures. It was referenced for the first time to provide a Deemed-to-Satisfy solution for a range of earth-retaining structures.

Since then, we've been asked if the old structural design manuals referenced in Part 3.0 can still be used. Yes or no? Well, Performance Requirement P2.1.1 requires that buildings and structures be structurally stable and resist the actions that are reasonably applied to them.

3.1.2.0, which we added last year, says that this is achieved if you design and then construct your retaining wall in accordance with AS 4678, the new acceptable construction manual.

But that's not the only way. Part 3.0 Structural provisions points out that you can achieve that same Performance Requirement by following 3.02 to 3.05, which are about determining and resisting actions, and how you can use the 1170 series, for instance, to do this.

This is acceptable construction practice, so it's a Deemed-to-Satisfy solution to use AS 1170 or any of the other manuals they aren't in 3.0. And in case that's not obvious, we've placed explanatory information in the new Part referencing AS 4678, that indicates that this is simply one option amongst others that can be used for determining the structural resistance for a retaining wall.

So, the answer is yes, you can still design retaining walls to the structural design manuals in Part 3.0, such as AS 1170. Still on retaining walls, what happens if you do AS 4678 for your retaining wall and you come to the design life requirements in that standard. That is, of course, how long the retaining wall has to be designed to last. Are these design life provisions, found in that standard, normative or informative?

Now, that's one question, but there's surely somebody wondering about another question, what on earth does this guy mean by informative or normative? Well, if that person's you, I'm about to help you out. Standards Australia is the nation's peak, non-government standards writing body and they draft Australian Standards to a very strict set of guidelines. And those guidelines say that a normative element is an element of the standard which describes the scope of the document or sets out provisions.

It's a provision or a requirement. Provisions in Australian Standards, that are referenced in the NCC, must be complied with, in as far as the NCC references that particular standard. When the provision says it has to be complied with, then it is a requirement to do that under the NCC. And when it has to be required with it, we use language such as shall or must within its text, to make it clear that it is a prescriptive requirement and therefore something which has to be followed. An informative element of a standard is an element that's intended to assist the understanding or the use of the document, or provide contextual information.

When there's an informative element in Australian standard, you'll often see it in an appendix. You'll see the word informative in brackets to indicate that it is indeed informative. Informative, it's guidance about a requirement. So, when AS 4678 talks about design life, is it normative, a part of the standard or is it informative, simply something which can be taken as guidance? Well, the section in question starts with very clear language. It says shall, meaning that it must be complied with. This part of the Australian standard is setting out a provision and on that basis it is normative.

In this case the design life has to be considered. The provision also refers to a table where typical design life requirements are set out. That's Table 3.1 and here are some of the items from that table. Now, Table 3.1 isn't exhaustive, it doesn't cover every scenario, but it does have an item for residential dwellings and this means, of course, a retaining wall which is directly related to the house, and for that scenario, a design life of at least 60 years applies.

A retaining wall in landscaping, like the one in my front yard, for instance, that's a different matter. It isn't directly related to the house and so that scenario is not listed in Table 3.1. It remains that Clause 3.4.1 requires that I consider the design life of that retaining wall, even though it's not in Table 3.1, but I'll have to use a different means of determining an appropriate design life for that type of retaining wall.

So, are the design life requirements of AS 4678 normative or informative? Yes, they are normative and they must be followed and in some circumstances Table 3.1 will place a design life of 60 years on that retaining wall.