Sterling Board/OSB for foundation shuttering

[Tony Strong]

I need to cast a foundation for a wall, and for the surface levels to be reasonably accurately.

It will be 300mm deep and 600mm wide, with a 68mm step in the top edge so that the rear shuttering only needs to be 232mm tall. The whole foundation will be below ground when completed, so we were looking to back-fill the outsides of the formwork prior to pouring the concrete to help prevent the formwork distorting.

(bad ASCII artwork of design) ||---------|| ||-------|| || || || || || || || ||v^v^v^v^v^v^v^v^v^|| base cast onto excavated surface

Using 18mm OSB as the side shuttering/formwork and 50x50x600mm timber pegs to locate and hold it down, what spacings would you recommend for the bracings? These shutters are a one-off and will be 22m long, so I do not want to make them over strong and waste monies that could be spent elsewhere.

thanks

Tony Strong

[BigWallop]

Why not use the soil itself as your shuttering? Why go to all the expense of building a whole timber box, when you can line and backfill to the sizes you need. Brace out the internal sizes, backfill and compress the soil to hold it all. As you reach the internal bracings, remove them as you go along.

[Rick]

If you can drive a 50x50 timber peg 300mm into the ground, then you may find thet building inspector wants some more work doing, as the soil may not be strong enough. I had to use 10mm rebar and a slege hammer to get far into my ground (an old glacial river bed). I'd get the building inspector out befor you start shuttering. In all he came out 5 times to check my foundations.

To a large extent you can put the soil back against the shuttering, to support it as you pour the concrete. If this is not possible you can use some blocks.

The method of getting the concrete into the hole is also important, if you pour a couple of tonnes from a dumper in a second or so, then you need very strong shuttering, if you pump it in, it can be less strong.

I made my first shuttering from 3x6 that I had piles off, and supported it with rebar and backfill. a good result was acheived, and it was easy to level. It took days to do the shuttering.

My second go, I poured straight into ground trences from a dumper, a much less good result was acheived.

Next time I think I will go for OSB & 3x2 scant - which is so cheep you can chuck a huge pile of it at the job.

How you are going to accuratly get a 68mm step in the way you describe I have no idea, unless you can put the extra 68mm on after the main chunk is poured.

Rick

[Tony Strong]

Thanks for the advice, I've uploaded a couple of diagrams, hopefully showing the plan more clearly than I can explain:

- an isometric view of two of the sections of shuttering

- a more detailed view showing the downstand shutter "suspended" between the front & back faces.

The 2"x2" stakes will extend further out of the ground if required, as it is very stony and they will not be likely to be driven fully home.

finally, a section of the proposed wall is:

- this shows the foundation and the interlocking blocks to be dry-stacked, the wall will be back-filled with clean 20mm gravel The reason we need the heights of the step is to ensure the wall slopes back at a slope of 20 degrees.

[BigWallop]

To obtain the higher section of the pour, you'd need to seal the top of the lower section to hold the mix at that level. Even concrete finds its own level, and this is usually the highest point, of course. So, to make the difference in height will mean building a sealed box with the height change section built on top of it.

Saying that, the log pour will need more of a base width to hold back the outward pushing force of the stacked blocks on top of it at the angle you want to obtain. So the plan of a straight sided foundation isn't the way I'd personally go with this. I'd tend to look at building a sloped log with a wider base than the top to obtain the right balance of holding forces against the build of the blocks. Like the sloping side of a pyramid, especially on the outward side of the log.

Would it be possible to drop the first row of blocks into the wet pour? If the blocks themselves could be held at their ends on the sides of the shuttering, then the pour pumped around and below them, then surely the first row of blocks would be at the angle you'd need, and they'd be supported along their full length rather than just at the two points of contact shown by your diagram.

The shuttering could then be made to taper in from a wider base to a narrower top where the blocks lie, and thus achieve the correct weight of balanced force against the rest of the blocks when they are placed. It would also mean just building two walls of shuttering, only having one side higher than the other to achieve the angle you need for the blocks.

Leaving one block out ever X metres would allow the mix to be pumped in. Then place the missing blocks in when the pour has been done. If yaw's sees what a mean.