Foundation on sandy soil

[Wozza]

Hi guys. Great forum and I thought I'd kick off with a basic question.

I'm redesigning the garden and a pizza oven is planned. It's going to sit in the corner where we had an old pressed metal shed. I've dismantled that although the concrete slab on which it was sitting is still in place. It's about 1.5m square (say 4 1/2 feet). Bigger than I need so my first thought was to simply use that as a base.

However...I live at Bondi and the soil in these parts is pretty much sand. We had some problems with the sewage some time back and it was no trouble digging down 2m (6 foot) to the main sewer connection. Sandy soil all the way down.

And the shed slab is just a platform of bricks laid directly onto the soil with 40mm (1 1/2 inch) of concrete topping. No reo. And the slab is not level - it slopes from one corner to the other. A drop of about 135mm (5 1/2 inch). I was wondering why the guy who built it (at least 25 years back before we bought the house) didn't lay it flat. Then it ocurred to me that maybe he did and it's gradually tilted over the years.

I'm now thinking that maybe simply levelling it off is not a good idea and I should break it up and build another. If so, then how deep should I go before I start laying a compacted base, pea gravel and reinforced concrete? And what thicknesses of each would you recommend?

I've checked on the web and read that building houses on sandy soil is better than on clay as it doesn't move or 'flow'. Short of bedrock, have a got a good base for this oven?

Once I work out the best way to post piccies I may put one up of the area in question. In the meantime, thanks in advance for any help.

[Greenman]

Re: Foundation on sandy soil

Welcome Wozza - Since there is a lot of weight involved in an oven I would be inclined to put some reinforced foundations under it. There is quite a bit of work involved in an oven project and it would be a small investment in time and energy to make sure that the foundation was right.

I would check on what is done locally but I would be inclined to place a reinforced slab on the sand. Sand is quite stable and is the foundation for the airports in Brisbane and Coolangatta.

What kind of oven are you thinking of making?

[Wozza]

Re: Foundation on sandy soil

Thanks Steve. I think a new slab is pretty much the go. Rather safe than sorry. I may commit to that and start breaking the old one up this weekend.

I'll do as much searching as I can for details of what I should put down and maybe post what I intend doing in a few days. Then anyone reading it can perhaps give it the thumbs or not as the case may be.

As regards the type, I've downloaded the plans for a Pompeii and will probably be going with that. I was a bit unsure to begin with as I didn't think I'd be able to manage that shape without any previous experience. But reading a lot of the success stories on the forum has given me some confidence. That confidence, plus all the info available on this forum plus an Indispensable Tool and I'm thinking: what could possibly go wrong.

Well, we may find out over the next few weeks...

[david s]

Re: Foundation on sandy soil

Be careful where you site it, most councils in Aus. Want anything 1.5 m from a boundary fence. You wouldn't want a complaining neighbour forcing you to pull it down. Think also about smoke issues. When you will mostly be using it there will probably be an on shore breeze.

[Greenman]

Re: Foundation on sandy soil

Good plan. I went down the Pompeii trail having never put two bricks together and happy that I did. There is a casserole in there now using up some stored heat from when it was used yesterday.

I love the whole brick dome thing and have shared my appreciation of it by helping one of my daughters and another friend with theirs. The shared brick saw and forms and IT's have made their builds easier. Both of them put their first fires in last Sunday.

If you search around a bit you can find firebricks and metal either new or in good shape to be re-purposed and make the whole build both practical, attractive and reasonably economical.

I agree with david s, the planning is important to save grief later.

The build is within the skills of most people and with some care it will be something to be proud of in the back yard.

Good luck with it.

[cobblerdave]

Re: Foundation on sandy soil

G'day
I've built on sandy soil and on a slope. Top edges are 150 mm thick downslope is 300 mm thick. Middle of slab is 75 mm thick.
After the hole was dug the middle was raised with a mound of clean sand. The outer edges had a row of steel reo and a strip of mesh ran all the way around. The all steel reo to the slab mesh.
The resulting slab was cup shaped on the bottom and rides on the bed of sand.
To deep of footings will not work because of the forces of the groundwater if they are to deep in my area. I suspect that yours could be the same.
You could well check with with the local council on the most common type in your area . Starting the conversation with " I'm building a wood fired oven " is usually enough to get them to be more open, rather than asking for advice for a 5 storey building.
Davids advice on set backs from boundaries should be checked as well. Especially in crowded Bondi as current rule may not be the same as the old ones
All the best with your build
Regards dave

[Wozza]

Re: Foundation on sandy soil

Council permission wasn't something I'd considered. Now that everyone is mentioning it I'd better check on this.

Browsing local council literature on line this morning, the nearest applicable requirement I can see is for bbqs and they are exempt if 900mm from boundaries. I can do that from the side boundary which is a wooden fence, but I really need the oven to be built off the back boundary, which is a block work wall.

Guess it's a trip up to the council offices on Monday to plead my case. Having dealt with these guys a few times over the years it's not something I'm looking forward to. If you had to describe the speed at which they process applications, the term 'glacial' would be the first one that would spring to mind.

[Wozza]

Re: Foundation on sandy soil

I've been playing around to see if I can post piccies and I think I've cracked it (using Photobucket and a reduced photo size).

This is currently the state of play in regards the back of the garden. I was thinking of using the area in the corner for the oven but centring it along the back wall is now the preferred option. With some hardwood sleepers forming a low wall either side, pretty much where I have the temporary wall up at the moment.



All depending on my friends at the local council of course...

[Wozza]

Re: Foundation on sandy soil

Well, rock and roll. Not a perfect result but a good one.

The local council recommended I search for the Electronic Housing Code, which turned out to be a bit confusing looking for what I needed. But there's a help line you can call and a very helpful guy gave me what I needed.

For those in NSW thinking of building, the web site you need is:

NSW Legislation

The relevant section says this:


The construction or installation of a barbecue or other outdoor cooking structure is development specified for this code.
2.14 Development standards

The standards specified for that development are that the development must:
(a) not have an area of more than 4m2, and
(b) be not higher than 1.8m above ground level (existing), and
(c) if it is not on land in Zone RU1, RU2, RU3, RU4 or RU6—be located behind the building line of any road frontage, and
(d) be located at least 450mm from each lot boundary, and
(e) not be used for commercial purposes.


The oven can now go centrally on the back wall, albeit 450mm off it. I'll run a low wall either side, build it up to just below the hearth either side of the oven and fill in the sides and 450mm on the back with some greenery.

I can smell that first pizza already...

[Wozza]

Re: Foundation on sandy soil

G'day
I've built on sandy soil and on a slope. Top edges are 150 mm thick downslope is 300 mm thick. Middle of slab is 75 mm thick.
After the hole was dug the middle was raised with a mound of clean sand. The outer edges had a row of steel reo and a strip of mesh ran all the way around. The all steel reo to the slab mesh.

So I'm thinking of a 1.5m square slab. The top will be about 60mm below the current level of the garden pavers so I can pave up to the blockwork walls (and into the wood storage under the oven).

I'll compact the soil, put the formwork in and put road base in the bottom and compact that to 100mm. Then pvc sheet over that and lay wire mesh and reo and pour concrete to 150mm depth. Sound OK?

And some questions if anyone can help:

Is the mesh and the reo meant to be at the same level and tied together (midpoint of the depth of slab)?

Or do I have two levels of mesh and reo? One at low level within the slab and one at high?

And how much reo do I need? Is it the case that more is better? I'd rather fork out the extra dollars and fill the formwork with a few rows of steel if it would improve matters.

And...due to the difficulty of getting ready mixed concrete to the back yard, I was thinking off mixing the 40 x 20 kilo bags on an existing slab alongside where the oven is going (see the piccie a couple of posts previous). Is this feasable? I appreciate it'll be hard work, but there'll be 2 of us (possible three).

And would I need to hire a concrete vibrator to settle the mix into the fomwork?

[cobblerdave]

Re: Foundation on sandy soil

G'day
Compacting and road base is sound as you can get.
Mesh needs to be in the middle of your cement and that includes the sides of your boxing it needs to clear by 50 mm. The complete covering of your cement means the steel is in an alkaline medium which stops any rust .
You can purchase what are called chairs ... plastic spacers which the mesh can be tie wired to. If not the old school way is to wire to brick pieces at the correct height.
I wouldn't build it below the level of you pavers the moisture can than enter our stand and then wick up to you oven. A secound layer of builders plastic can be used on the top of your stand directly under the insulation base layer for peace of mind.
I've had a stand flooded ( 200 mm in a day) it's not pretty having a stand full of wet wood and no secound waterproof layer under the insulation. I've now fitted a full drain and grate across the entrance just in case this should happen in the future.
You don't need a vibrator for such a small slab, just stick your shovel tip in the mud and use a chopping motion and you'll find the cement will liquify, and flow into the gapes. Tap the formwork with a block of wood to get the bubbles to move.
3 men sounds a good amount could however be a bit on the "spendy" side, if they knock over that slab real quik they'll have more time to drink your beer.
Regards dave

[Wozza]

Re: Foundation on sandy soil

Thanks Dave. I think I'm pretty much good to go with the slab and I'll take your advice on keeping the top above ground level.

I think I can get it formed up and ready to go by the weekend. Then make sure the beer fridge is stocked and check to see when help is available for the pour. But with a friend's stag party this Saturday and the wedding itself the following weekend it may be delayed for a while.

I was hoping to get the foundation and hearth up ASAP to give me a chance to get the oven finished by the new year. But there's so much other work to do in the garden I can't see the boss being too happy if we get to Xmas with a nice looking pizza oven sitting in the middle of what looks pretty much like a building site at the moment.

Thanks for your input.

[cobblerdave]

Re: Foundation on sandy soil

G'day
The one variable that would bother me if I was to build a raised garden bed at the rear of the oven is that rear block wall that separates a you and your neighbor.
How well it was built?, has it got good foundations?, is it block filled? , all this might tell in sometime in the future.
Of course you'll waterproof it but that's a lot of weight.
You might consider pot plants at the rear or even a lower garden and taller plants.
Just a thought
Regards dave

[Bren]

Re: Foundation on sandy soil

Good luck. i will be watching to see how you go. I just laid my slab two weeks ago and did the stand out of LockBlocks from Baines masonry out at Appin. You lay the first layer level, then the rest just glue on with water curing urethane ( glue in a caulking gun). Super quick.

If your still looking for fire bricks in sydney. al be it very south sydney. i have a friend who was possibly clearing theirs. The bricks are really heavy though so may take a few trips from the gong... may pay to just get the bricks from Furnace shop at wetherill park for $450.