Terracing a sloped section

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boland

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#250669 21-May-2019 09:21

Part of our garden is on a slope, and I'm planning to make terraces using fence posts and retaining wall timber. Or is that overkill? Retaining wall timber is about $6 LM, so that adds up pretty quicklly.

I see blog posts where people just use random logs as the barriers.

Will add a swale at the top to improve water runoff, like this:

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#2242081 21-May-2019 09:35

You don't include a scale. Assuming your diagram has the slope angle about right (45 deg) and if the retaining walls supporting the terraces are more than about a metre, or if there's a structure or driveway above, you should probably get engineering/geotechnical advice, and find out if you need building and/or resource consent.

boland

547 posts

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#2242092 21-May-2019 09:47

Fred99:

You don't include a scale. Assuming your diagram has the slope angle about right (45 deg) and if the retaining walls supporting the terraces are more than about a metre, or if there's a structure or driveway above, you should probably get engineering/geotechnical advise, and find out if you need building and/or resource consent.

Thank you for your response!

It's not on scale. Slope varies between ~10 at the bottom to about ~45 at the top. Not planning to build terraces all the way to the top as that's too steep IMO.

There are driveways above and next to the section. So I should get advise? What type of person should I consult?

PolicyGuy

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#2242100 21-May-2019 09:54

A retaining wall over a specified height - depending on where you live, different councils have different rules from 900mm to 1200mm - requires a consent and an engineering plan.
Go to your local council office and ask to talk to the duty planner. It's free and they are usually very helpful.

Fred99

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#2242165 21-May-2019 11:16

boland:

There are driveways above and next to the section. So I should get advise? What type of person should I consult?

Absolutely you're going to need advice.

I suggest you try and get some free advice at this stage, by getting an earthworks / landscaping contractor in to take a look. It's very likely IMO that you'll need a structural engineer to do design work, council may insist that for consent, the engineer gets some geotechnical investigation work done.

Reason I'd suggest getting a contractor involved is that although they wouldn't be able to give you a firm price estimate as they won't know the exact engineering design requirements, they might be able to make some suggestions and give a ballpark estimate based on what they think may be the best solution - which may differ from your sketch.

An engineer might be inclined to look at your sketch and find a best solution based on that - as that's what you appear to want. My guess would be that the excavating marked "A" on your sketch is going to need retaining - and that's going to cost dearly when there's a driveway above. A contractor might suggest some way of achieving what you want at more reasonable cost.

IIRC, if there's a retaining wall supporting a driveway, the normal council exemptions allowing you to build a retaining wall up to a certain height no longer apply - and you'll need consent. It might seem like overkill - until a 10 tonne truck uses the driveway, rolls and demolishes everything in it's path.

boland

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#2242272 21-May-2019 12:11

Thanks @Fred99. Hadn't thought about the driveway yet. Maybe a solution is to not excavate anything, but only to fill up. But will get a contractor to come and have a look.

blackjack17

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#2242273 21-May-2019 12:11

You might not need a consent even if there are driveways above.

If the driveways aren't on land supported by the retaining wall you probably won't need consent.

Assuming you are in auckland try this link

https://onlineservices.aucklandcouncil.govt.nz/councilonline/yform/decisiontool?productCode=DECISION_RETAINING_WALL#

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kiwi_64

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#2242294 21-May-2019 12:35

Another point to consider. When I did a (sort of) similar job on my back yard, the volume of earth being moved (from A to B) meant that I required resource consent (wasn't too onerous however)

I *think* the volume limit at that time was 30m3 ... but no longer sure

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neb

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#2242473 21-May-2019 16:58

Fred99:
You don't include a scale. Assuming your diagram has the slope angle about right (45 deg) and if the retaining walls supporting the terraces are more than about a metre, or if there's a structure or driveway above, you should probably get engineering/geotechnical advice, and find out if you need building and/or resource consent.

A variation of that, if you don't need consent (e.g. multiple low walls rather than one large one) is to get an experienced contractor who can give you the necessary engineering advice as part of the job, and spend the money saved on the retaining. For example I did one level of retaining on my property with 2.4m house piles because the guy doing the work, with 30 years experience, told me I could get an engineer in and he'd charge me $800 to tell me I needed to use house piles, or he could use the money for the house piles. The whole lot hasn't moved a millimetre in several years, replacing a gradually-collapsing clay bank that was there before.

Oh, and massive amounts of drainage, scoria backfill, and geofabric also helped. Main thing is to get someone with experience to look at your specific situation and give advice.

neb

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#2242477 21-May-2019 17:03

boland:
There are driveways above and next to the section. So I should get advise? What type of person should I consult?

In that case you'll definitely need engineering advice and consent. As my previous post suggests, getting advice from a contractor will be a lot cheaper than an engineer. If it's consented work you'll need an engineering report eventually, but you can get at least the preliminaries done with cheaper input. You can also ask them how to structure the work to minimise the headaches you'll have dealing with the council, as with a good accountant they should be able to save you more money than they charge in fees/time.

Also, download and read through your local council's publications on what needs to be consented and what the requirements are for this type of work before you begin, to get an idea of what's involved.

Bung

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#2242536 21-May-2019 18:07

Just a comment on "fence posts" & "house piles". Fence posts are usually only H4 treated and aren't always that great timber. AFAIK retaining wall posts need H5 treatment and that is what house piles get. For the price of a 125x125 house pile you can probably get a 175mm round H5 pole if you like the chunky look.

froob

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#2242626 21-May-2019 20:14

Just building on what others have said above, these are the specific requirements around whether building consent is required for a retaining wall:

Under the Building Act the "default" position is that all building work requires consent, but there are some specific exemptions set out in Schedule 1.

One of the exemptions (no. 20) allows you to building a retaining wall up to 1.5 metres, provided that it is not surcharged. This page has some good diagrams explaining surcharge on a retaining wall. You can find further information on this exemption here.

There is also another exemption (no. 41) which applies only to retaining walls in rural zones. Both of those exemptions apply across the country.

As others have said, you may also need resource consent if you are moving a lot of earth, or if walls are placed in particular locations, etc. Those rules are based on the local district plan, which varies region by region.

It pays to have a talk with the council before you begin work, and also to seek professional advice where you need it. I've unfortunately seen many retaining walls that have failed because they were not built to code, and also plenty of situations where people have carried out earthworks that undermined their neighbour's property and caused landslips.

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antoniosk

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#2242636 21-May-2019 20:33

boland:

Part of our garden is on a slope, and I'm planning to make terraces using fence posts and retaining wall timber. Or is that overkill? Retaining wall timber is about $6 LM, so that adds up pretty quicklly.

I see blog posts where people just use random logs as the barriers.

Will add a swale at the top to improve water runoff, like this:

Where in the country is this?

________

Antoniosk

neb

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#2242652 21-May-2019 20:49

Bung: Just a comment on "fence posts" & "house piles". Fence posts are usually only H4 treated and aren't always that great timber. AFAIK retaining wall posts need H5 treatment and that is what house piles get. For the price of a 125x125 house pile you can probably get a 175mm round H5 pole if you like the chunky look.

Ah, yeah, by "house piles" I meant H5 construction poles, somewhere around the 225 or 250mm mark. The rest is done in 125x125 house piles.

If you want that really rustic look, you can get H5 uglies which are debarked rather than peeled. Downside is it looks like you've rammed old pine tree trunks into the ground...

boland

547 posts

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#2242754 21-May-2019 21:44

antoniosk:
boland:

Part of our garden is on a slope, and I'm planning to make terraces using fence posts and retaining wall timber. Or is that overkill? Retaining wall timber is about $6 LM, so that adds up pretty quicklly.

I see blog posts where people just use random logs as the barriers.

Will add a swale at the top to improve water runoff, like this:

Where in the country is this?

Lower Hutt, thanks.

LookingUp

411 posts

Ultimate Geek

#2243645 23-May-2019 12:02

You'll also need to consider soil type. If it's clay, or something like it that "dissolves", you need to be very careful not to let water collect and pool at the retaining structures. Over time, it'll find its way down and under, and will quietly wash away until you suddenly discover you have a rather a large hole to deal with.

I live on a clay hill, and 10+ years ago put some edging around the lawn to make it easier to control. Last year, my wife was working in the garden just below and discovered there was a considerable hole under the garden (ie. 30cm or so under topsoil), which when dug out, took a trailer load of dirt. A month back we found a similar underground cavern under a path just down from that, and that one took two trailer loads of fill. We traced the source to the low point of the lawn edging - water collecting, seeping down, then washing out as an under-runner.

My advice - seriously look at drainage requirements.

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