Concrete pier size?

[izakimak]

Just starting out on the observatory build.

For the pier foundations is it better to have depth or width to the concrete for the same amount of material?

[Freff]

Personally I'd go for width. But there is a few factors I would take into consideration.

Where I live the ground is very hard clay. So soil conditions. The height and width of your pier. Is it going to be concrete or steel. Bolted or set in.

These are a few things I would consider first.

[izakimak]

Hi Freff

Thanks for the help.

Going to be a steel post bolted to the concrete.

Was going to go for a 1.1m x 1.1m x 0.7m deep concrete section (roughly works out a bulk bag of ballast and 10 bags of cement, if I have done my calculations correct).

Ground is top soil followed by iron ore.

[Rols]

i am based on clay, i could do with digging in till i hit bedrock but i'm not going to try.

i intend to dig a 2ft square and go down about 3ft. if that wont do it then i will just have to balance a 12" SCT on my lap....

[Freff]

You should be ok with that size, but I'd go a bit further and go for a cubic mtr.

You will probably find the bulk bag of all-in wont go as far as you think, especially if the hole you dig is slightly oversized. Confirm the bag size, some are a tonne and some a lot less.

There will be nothing worse than running short of materials near the end of the pour.

[Bizibilder]

I would suggest you proposed size is OK. Remember concrete is really quite cheap - so don't skimp on it.

[libraryman]

i also have a base of clay, i'm going to use just under 1x square mtr, but whilst it's cheap enough i also think about the imagers who get great results from portable piers / tripods, they are'nt bolted to 1 tonne of concrete so i'm not going to worry too much.

i suspect it's the base thats key if it's suseptable to heave or collapse, dig deep to avoid frost.

Ray

[red dwalf]

Digging a 3 foot deep hole is a long way down if doing it with a spade, I'd say if It's virgin soil I.e. never been dug, I'd go as far down as possiable to avoid heave,

[Rols]

Digging a 3 foot deep hole is a long way down if doing it with a spade, I'd say if It's virgin soil I.e. never been dug, I'd go as far down as possible to avoid heave,

What if it's not virgin soil, my garden was leveled about 15 months ago and re laid to lawn, the side i intend to dig was the side that required height, you think i should go down further until i hit something more solid again??

[Gina]

What if it's not virgin soil, my garden was leveled about 15 months ago and re laid to lawn, the side i intend to dig was the side that required height, you think i should go down further until i hit something more solid again??

I think you want to go down to solid ground. Made up ground is liable to settle.

[red dwalf]

If It's anything like my garden you won't need to go down too far before hitting something harder,

[Freff]

Dig to your required depth. If you haven't reach clay or something hard, dig another 300mm then add hardcore and tamp it down well.

Don't keep digging, you could go on for ever, and you'll only have to fill it with concrete.

Don't worry about frost.

[izakimak]

Big thanks to everyone for the advice.

Build underway.

Dug a 1.1m x 1.1m x 0.5m hole today using a spade/fork/pick axe. Took a couple of hours. Need to do the same tomorrow.

[Freff]

Don't forget to take some photos, it's always good to look back to earlier stages of the build.

You can also post them here for those who need ideas and inspiration.

[izakimak]

Nearly there.

Used a drill with a very long drill bit to break up the ground in the hole. Drill in about 6 inches then use the drill bit (20mm diameter) as a lever to lift the earth.

[izakimak]

This is the test frame I used to make sure it will fit.

I have a cad system I use for work, but there is nothing like the real thing.

[pete_l]

Unless the scope you're planning to put on the pier is a monster, I wouldn't bother digging more than 60cm, or so deep. I'd also use some reinforcing rods in the hole to keep the block from splitting.

You might also find some useful information in this thread

Concrete barrowing rates.. - uk.d-i-y | Google Groups

[izakimak]

Dug the hole 1.1 x 1.1 x 0.8m.

Had to divert a stream as well.

[Freff]

Hoover Dam springs to mind. (pun unintended)

[laser_jock99]

If It's anything like my garden you won't need to go down too far before hitting something harder,

Me too........