Help calculating Vol to mass for diff concrete types

The box sections are the fence posts 1.2 centers. Fence panel horizontal supports are 3 horizontal 3x2 timber.
Above ground, initially it will be sheets of ply as hoarding. However for future considering old scaffold boards with 1cm spacing between.

Below ground or between heights of 0 to 0.6 (6 deg Slope).. The retaining section which will be filled with (gravel and have perf pipe coming through to avoid water weight) will be retained by bitumen emulsion coated corregated box section.

But what about the mix.

Property is sheltered on both sides. So wind load should be reduced somewhat.

Posts are primed /galvanised spray. where worn coating was worn thin.. rust treatment applied and 2- 3 coats of barn paint. Resin filled at both ends to avoid ingress.

champagnecharly said:

The box sections are the fence posts 1.2 centers. Fence panel horizontal supports are 3 horizontal 3x2 timber.
Above ground, initially it will be sheets of ply as hoarding. However for future considering old scaffold boards with 1cm spacing between.

Below ground or between heights of 0 to 0.6 (6 deg Slope).. The retaining section which will be filled with (gravel and have perf pipe coming through to avoid water weight) will be retained by bitumen emulsion coated corregated box section.

But what about the mix.

Property is sheltered on both sides. So wind load should be reduced somewhat.

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Concrete area is *probably* OK, fence posts are definitely on the small side...

Concrete area of 0.2 or 0.3 ok? ST1 ok?
I'm now a little worried about the size/height. I was under the impression It would take considerable force to bend these posts. Would you recomend changing height and reducing to around 2.4m. of which only 1.7 exposed to wind.

champagnecharly said:

Concrete area of 0.2 or 0.3 ok? ST1 ok?
I'm now a little worried about the size/height. I was under the impression It would take considerable force to bend these posts. Would you recomend changing height and reducing to around 2.4m. of which only 1.7 exposed to wind.

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ST1 will be fine.

If I was designing it I'm absolutely certain you would need more concrete than you are describing - sheet piles for example usually go twice as far into the ground as the height they retain, and a strip footing would be 450mm wide for a 600mm high retaining wall. Of course you have some base bearing and some active pressure from the low level soil.
600mm isn't a lot and you say the fence is sheltered - as long as you don't surcharge the higher level it would *probably* be OK, but to be honest I would be tempted to dump in a bit more concrete.
Don't ask me how much - it's not a simple calc...

I'm going to redesign and will post some drawings 1st step and final. Thinking also to fil the posts with a mix although I get a feeling it will take some time spoon feeding it. Although the concrete will be brittle it should stop bending.. For now am just digging a series of 900 holes. Am surprised though am still mainly in light clay with only with only bits of chalk.

Well it was only on the first one I dug that didnt get to chalk and flint.. stopped diggin the second just at the point where chalk started and third on were real swine as the auger would just bite lumps of flint and stop dead wacking me in the leg.. and whats worse was getting the poudry stuff out i felt i'd have been better with a vacuum cleaner than a post digger.

In terms of size of hole.. thoese dug are 800-850 deep and around 380-400mm sq so more in line with the the 600high retaining wall you mentioned.



In the image.. note that behind the box section there is currently nothing so its not strickly retaining.. this void will be filled with rubble bags so i dont break a limb. When the whole drive gets taken up I'm considering filling the same void upto drive level with concrete. If that is done (and a little voice insude my head me says thats OTT) the 200mm wide concrete should act as the retaining section.
Believe it or not where that void is were the footings of a single skin brick wall (to my recolection with no piers) holding a 1.6 m fence.. not sure how many years it lasted but when i saw it it was about 10deg from the verticle.


Have you any further thoughts on the 40x60 box section and final height? If I'm going to fill them with muck then am going to have to chop the top off anyway...

You say dont overload.. 'IF' i go down the scaffold boards route. Vertical load would be around 0.5KN per 1.2m section twice that of the ply. but with reduced wind load because of the gaps.

After some thought I'm a little confused why you say type of concrete doesn't matter but that the volume does. I think i'm misunderstanding the concept of something important.. St1 is a weak mix and will be more prone to cracking either shrinkage or under load. If/ when it cracks the overall structure will be prone to failure. by using a stronger mix isnt the risk of failure reduced. Or does volume outweigh strength in terms of practicality?

What sparked this thought was looking into concrete filled box sections and the recent research that was based around the use of fibres for composite steel/ concrete.. obviously fibers reducing shrinkage cracks etc.

I think an explanation is needed as to why go so far with box section.. The reason is that the drive is so narrow and on a bend that every mm counts. As it is I could barely get a mini up there let alone reverse my trailer back to the garage doors.

champagnecharly said:

After some thought I'm a little confused why you say type of concrete doesn't matter but that the volume does. I think i'm misunderstanding the concept of something important.. St1 is a weak mix and will be more prone to cracking either shrinkage or under load. If/ when it cracks the overall structure will be prone to failure. by using a stronger mix isnt the risk of failure reduced. Or does volume outweigh strength in terms of practicality?

What sparked this thought was looking into concrete filled box sections and the recent research that was based around the use of fibres for composite steel/ concrete.. obviously fibers reducing shrinkage cracks etc.

I think an explanation is needed as to why go so far with box section.. The reason is that the drive is so narrow and on a bend that every mm counts. As it is I could barely get a mini up there let alone reverse my trailer back to the garage doors.

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You are seriously over thinking this!

ST1 is indeed a weak mix, and as such is less likely to suffer from shrinkage cracking, not more, as the heat of hydration is so much lower due to the lower cement content.
Under compressive load, ST1 will fail sooner than something like RC35, but its compressive strength is more than sufficient for the purpose of a garden retaining wall.

Cracking in mass concrete starts to become an issue in much much larger structures such as dams where the heat of hydration can cause large differences between internal and external temperatures. Have you ever known anyone worry about cracking in a trench fill foundation for example - which is essentially what you have, only smaller.

Look, if you want something that will *probably* be OK, you've got it. If you want something designed by a structural engineer, you will end up with larger foundations and larger box section than you've got.
If I was designing it at work, it'd be the latter, if I was doing a 600mm retaining wall for my own driveway I'd "eye" it in and I bet it would be fine, and a lot cheaper.

I'd just want more concrete around my posts as you are relying on the load being spread over a large enough area that the soil doesn't compress and sufficient weight of concrete that the foundation doesn't start to rotate.
Alternatively, throw in a 500x250mm mass concrete footing and build a 600mm high, 215mm thick wall off that. Shove the posts into local deepenings and attach the fence above the wall.

RonnyRaygun said:

You are seriously over thinking this!

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Yes well.. I frequently do. Unfortunately I just like to understand.

Thanks for the explanations. Much appreciated.

I've now eyed it in on both drawing and in situ. ended up with 40x 50 and 700-800 depth. made sure on solid chalk on base and as cuboid like as is possible. I'd like to think it would take a hurricane to make those rotate.
add hopefully there is enough support from the chalk to stop too much settlement.

I'm left however with one concern... the composite steel concrete box. What type of mix shoudl I use on that? Obviously 10mm agreagate would be my max. But what mix? Pouring it from a jug would be ideal.. but should i go down the fibre route? thinking of maybe shoving some rebar down the middle also. Really dont wnat to have to dig these up once in.