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Fence posts in drainage gravel

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  1. #1
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    Default Fence posts in drainage gravel

    Hi all,

    Any tips would be greatly appreciated.

    The boundary with my neighbour sits on a gravel drainage channel for a 2 metre high retaining wall.

    We’d like to set some steel fence posts into this gravel 2.4m apart for a 1.8m high paling fence

    We’d like 800mm deep posts so as ensure that wind doesn’t move the posts and potentially effect the retaining wall.

    Can anybody offer opinions on whether 32mpa drymix method for fast set concrete will be good in this gravel?

    Also in regards to the weight of the concrete over time, are the fence posts and concrete blocks likely to sink?

    Many thanks for all opinions

  2. #2
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    Is this a new retaining wall?
    Block work with concrete fill and footings or segmental blocks (no concrete fill)
    It will make a difference on your approach

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    ...a pic might get you more responses to assist in your description

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    Thanks for the replies and any help going forward.

    Here is a link to a photo of the area.


    Here's the full story in short form!:


    • Neighbours complained about my sleeper retaining wall, saying it's failing and could threaten their 22 year old gravity retaining wall. Hinted they're report it as dangerous and being only 1m away from their wall.
    • I agreed that it was old and needed replacing.
    • Had a survey done- turns out that entire embankment is mine so building a new wall with steps 2m away from their gravity wall and having a 2 tiered garden.
    • I said to neighbour that if I'm replacing the wall (and fence), the fence should go in the right place ie on the boundary
    • Neighbours not happy as this means the fence would go right on the back of their blocks and they feel the fence could destabilise their wall.
    • They said "fence must be kept away"
    • I said "fix your wall so it can support a fence"
    • Have kind of agreed (verbally) to placing a fence in 150mm away from the back of the blocks so a 300mm diameter hole can be accomodated.
    • The ground seems to be muddy drainage blue metal.
    • I know somebody with an excavator + auger attachment who said that it can reach from the high side if I remove the fence, so thinking of augering 300mm x 800mm for steel posts and then using : https://media.prod.bunnings.com.au/a...c87&t=w500dpr2


    Any thoughts and advice on this project would be greatly appreciated. The neighbours are proving very hard to deal with.
    Last edited by phild01; 20th Nov 2022 at 07:33 PM. Reason: hostd site image deleted

  5. #5
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    Quote Originally Posted by asking72 View Post
    Thanks for the replies and any help going forward.

    Here is a link to a photo of the area

    Here's the full story in short form!:


    • Neighbours complained about my sleeper retaining wall, saying it's failing and could threaten their 22 year old gravity retaining wall. Hinted they're report it as dangerous and being only 1m away from their wall.
    • I agreed that it was old and needed replacing.
    • Had a survey done- turns out that entire embankment is mine so building a new wall with steps 2m away from their gravity wall and having a 2 tiered garden.
    • I said to neighbour that if I'm replacing the wall (and fence), the fence should go in the right place ie on the boundary
    • Neighbours not happy as this means the fence would go right on the back of their blocks and they feel the fence could destabilise their wall.
    • They said "fence must be kept away"
    • I said "fix your wall so it can support a fence"
    • Have kind of agreed (verbally) to placing a fence in 150mm away from the back of the blocks so a 300mm diameter hole can be accomodated.
    • The ground seems to be muddy drainage blue metal.
    • I know somebody with an excavator + auger attachment who said that it can reach from the high side if I remove the fence, so thinking of augering 300mm x 800mm for steel posts and then using : https://media.prod.bunnings.com.au/a...c87&t=w500dpr2


    Any thoughts and advice on this project would be greatly appreciated. The neighbours are proving very hard to deal with.
    The problem I see is keeping the drainage gravel from falling into the post holes and, the auger is going to spit dirt and gravel down the wall so the neighbour will have to work with you on the project.
    i.e clean up any mess on their side.
    All works should be agreed to in writing before any works start.
    Not only that but I don't see it is a good idea to have your fence so close to the block wall.
    I just cannot see it being very strong against the loose placed blocks.however, I recall something about retaining walls above one metre requiring engineers approval as well as a fence on top for safety ??
    I suggest you seek both legal and council advice and also get your auger friend to inspect and advise but keep in mind he drives the machine and you are responsible for the mess.

    As to your question even 20 mpa concrete will be ok but as I said I question how strong the post holes will be against the blocks.

    I do not envy you your position.

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    Hi cyclic,

    Many thanks for your reply. I’ll definitely be taking onboard your thoughts.

    It’s interesting that there is a fence in the photo (with their other neighbours) which is about 300mm from the back of the wall. I guess this is outside of the drainage gravel.

    I was thinking (hoping) that it is the concrete that supports the fence and not the neighbour’s block wall.

    That’s a great consideration you made about a fence on top of a retaining wall requiring approval.
    However, would this proposed fence really be on top of a retaining wall? It would be next to it but not attached. I can’t find anything which says how far a fence needs to be for it to not be on top of a retaining wall.

    Looking around my street, I can see several fences next to retaining walls. I wonder what the legislation/building code on this is regarding a setback.

    Thanks

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    Quote Originally Posted by asking72 View Post
    Hi cyclic,

    Many thanks for your reply. I’ll definitely be taking onboard your thoughts.

    It’s interesting that there is a fence in the photo (with their other neighbours) which is about 300mm from the back of the wall. I guess this is outside of the drainage gravel.

    I was thinking (hoping) that it is the concrete that supports the fence and not the neighbour’s block wall.

    That’s a great consideration you made about a fence on top of a retaining wall requiring approval.
    However, would this proposed fence really be on top of a retaining wall? It would be next to it but not attached. I can’t find anything which says how far a fence needs to be for it to not be on top of a retaining wall.

    Looking around my street, I can see several fences next to retaining walls. I wonder what the legislation/building code on this is regarding a setback.

    Thanks

    If the neighbours are not on board with the proposed fence then you will probably need the advice of an engineer.
    Building a structure close to the wall means that the zone of influence comes into play, this is the area that any footing or foundation has potential to impact on the retaining wall. Building outside the zone of influence (ie the same height as the retaining wall) results in no force impacted on the retaining wall.

    In your case building within the zone of influence it is not a simple question of how close can you build to it but how much force will the fence place on the retaining wall and how how to design the footing to keep forces below the structural limits of the retaining wall.

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    Quote Originally Posted by droog View Post
    If the neighbours are not on board with the proposed fence then you will probably need the advice of an engineer.
    Building a structure close to the wall means that the zone of influence comes into play, this is the area that any footing or foundation has potential to impact on the retaining wall. Building outside the zone of influence (ie the same height as the retaining wall) results in no force impacted on the retaining wall.

    In your case building within the zone of influence it is not a simple question of how close can you build to it but how much force will the fence place on the retaining wall and how how to design the footing to keep forces below the structural limits of the retaining wall.
    Hi Droog,

    Many thanks for your reply. I can say that this whole situation is giving me a headache. I called just about every building certifier in Brisbane and nearly all of them weren't sure and those who did give advice conflicted with each other.

    The zone of influence is interesting as I looked into this when building my new sleeper wall. From my understanding the zone of influence is taken from the bottom of a retaining wall and at a 45 degree angle.

    As the neighbour’s retaining wall is 2m high, then this would mean my fence would need to be set back 2m from their wall (even through that embankment is natural ground level).

    I didn’t think that fences came into consideration in relation to zones of influence, or all boundary fences on all hills in all suburbs would be in the wrong place?

    This whole situation is really a giant rabbit hole

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    Quote Originally Posted by asking72 View Post
    Hi Droog,

    Many thanks for your reply. I can say that this whole situation is giving me a headache. I called just about every building certifier in Brisbane and nearly all of them were sure and those who did give advice conflicted with each other.

    The zone of influence is interesting as I looked into this when building my new sleeper wall. From my understanding the zone of influence is taken from the bottom of a retaining wall and at a 45 degree angle.

    As the neighbour’s retaining wall is 2m high, then this would mean my fence would need to be set back 2m from their wall (even through that embankment is natural ground level).

    I didn’t think that fences came into consideration in relation to zones of influence, or all boundary fences on all hills in all suburbs would be in the wrong place?

    This whole situation is really a giant rabbit hole
    I believe your understanding of the zone of influence is correct the other way is to have the foundations deep enough to take it out of the equation so footings roughly 2 metres deep.
    I dont know if zones of influence are generally referred to with boundary fences but it is a footing and it will impart forces on its surrounds, as you have found there does not appear to be a standard setback for your situation.

    As your neighbours are not in agreement then if / when you build the fence, and if there are issues with the retaining wall they may point to your fence impacting on it. That is why I suggest talking to an engineer.

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    Quote Originally Posted by droog View Post
    I believe your understanding of the zone of influence is correct the other way is to have the foundations deep enough to take it out of the equation so footings roughly 2 metres deep.
    I dont know if zones of influence are generally referred to with boundary fences but it is a footing and it will impart forces on its surrounds, as you have found there does not appear to be a standard setback for your situation.

    As your neighbours are not in agreement then if / when you build the fence, and if there are issues with the retaining wall they may point to your fence impacting on it. That is why I suggest talking to an engineer.
    Thanks Droog. I think you're right about consulting an engineer. More cost!

    I've half a mind to just bang in star pickets and put up some screening in between and then a row of potted lilly pillies.

    If the council ever comes around at least it can easily be dismantled.

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    Without knowing how segment wall was constructed will make it hard to construct fence.

    With these types of walls you would install sleeve's for fence posts while building the wall, wall should have geofabric install in layers (which are laid in to blocks)
    This wall height would typically have three layers, this is not something you should attempt to put an auger through. Maybe dig a test hole by hand and have an engineer take a look.
    The whole process wont be a cheap or easy fix.
    So I'm guessing your neighbor excavated originally for house, then your side added another wall to fill to level?

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    Quote Originally Posted by asking72 View Post
    Thanks Droog. I think you're right about consulting an engineer. More cost!

    I've half a mind to just bang in star pickets and put up some screening in between and then a row of potted lilly pillies.

    If the council ever comes around at least it can easily be dismantled.
    I like your thinking.

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    Quote Originally Posted by asking72 View Post
    Thanks Droog. I think you're right about consulting an engineer. More cost!

    I've half a mind to just bang in star pickets and put up some screening in between and then a row of potted lilly pillies.

    If the council ever comes around at least it can easily be dismantled.
    It might be a simpler option.

    Here are a few links that all agree it should be at least 3 feet back:
    https://australianpaving.com/knowled...etaining-wall/
    https://diyretainingwall.com/install...etaining-wall/
    https://allanblock.com/literature/PD...iningWalls.pdf

    One link is Australian, two make note it can be closer but requires professional advice (engineer).
    Reference that with the info from the certifiers you spoke to.

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    Quote Originally Posted by cherub View Post
    Without knowing how segment wall was constructed will make it hard to construct fence.

    With these types of walls you would install sleeve's for fence posts while building the wall, wall should have geofabric install in layers (which are laid in to blocks)
    This wall height would typically have three layers, this is not something you should attempt to put an auger through. Maybe dig a test hole by hand and have an engineer take a look.
    The whole process wont be a cheap or easy fix.
    So I'm guessing your neighbor excavated originally for house, then your side added another wall to fill to level?
    I was intrigued by the mention of geogrid so have just been out to dig and got down to 700mm and not only wasn't there any sign of geogrid but also no sign of any drainage gravel which goes against my presumption of there being drainage gravel! Maybe it's a bit further down.

    Soil was heavily compacted!

    Yes their house was all cut into the hill and then my house was built a few years later with another wall.

    Quote Originally Posted by droog View Post
    It might be a simpler option.

    Here are a few links that all agree it should be at least 3 feet back:
    https://australianpaving.com/knowled...etaining-wall/
    https://diyretainingwall.com/install...etaining-wall/
    https://allanblock.com/literature/PD...iningWalls.pdf

    One link is Australian, two make note it can be closer but requires professional advice (engineer).
    Reference that with the info from the certifiers you spoke to.
    Thanks for the links. Looking around my suburb I'm guessing that most Australians fencers don't about where they put the fence. Maybe as they perceive problems as only appearing after they're long gone.

    Quote Originally Posted by cyclic View Post
    I like your thinking.
    It's my last resort!
    Last edited by phild01; 20th Nov 2022 at 07:36 PM. Reason: hosted site image deleted

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    Quote Originally Posted by asking72 View Post
    I was intrigued by the mention of geogrid so have just been out to dig and got down to 700mm and not only wasn't there any sign of geogrid but also no sign of any drainage gravel which goes against my presumption of there being drainage gravel! Maybe it's a bit further down.

    Soil was heavily compacted!
    An interesting development today. The neighbour accused my digging of this hole leading to soil in his wall drainage pipe and also the hole destabilising the wall. The hole is no more than one would dig for a plant!

    I told him that it's nonsense and then pointed out that his blocks are loose and the lack of drainage gravel. I said that if his wall fails it will be because of these reasons and not some measly hole that I dug in the soil.

    The blocks are glued together with much of the glue having failed.

    screenshot-2022-11-21-164843.jpg

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    Is that a capping piece ?

    The lip on the back indicates it is one of the interlocking blocks designed to be stacked and self supporting normally without any additional products. Often installers use liquid nails on the top capping blocks just to stop them moving about, not because it is required as part of the construction.

    Refer to my previous recommendation regarding neighbours not in agreement in post 9

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    Quote Originally Posted by droog View Post
    Is that a capping piece ?

    The lip on the back indicates it is one of the interlocking blocks designed to be stacked and self supporting normally without any additional products. Often installers use liquid nails on the top capping blocks just to stop them moving about, not because it is required as part of the construction.

    Refer to my previous recommendation regarding neighbours not in agreement in post 9
    Hi Droogs. Yes it's a hollow capping block. The block below it is hollow too.

    I called an engineer today (one who has visited the site) and he told me that there is no way he could or would design a fence to go anywhere near that wall. 2m away is what he suggested!

    I'm now thinking of driving in star pickets as they only weight 4.5kg each and then attach lattice screening between them to avoid any wind loading. This way I can also peek through the lattice to see if their wall is moving.
    With such light weight, I cannot see how though.

    I believe that this serves as a dividing fence in the eyes of QCAT.

    Then if the neighbour wish to have a proper dividing fence, they can work out how to have one installed - and then I'd be happy to remove the lattice.

    Will keep this thread updated so it helps anybody else in a similar situation.

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