What do you use for boxing up under a concrete slab

sandman55 · Oct 26, 2016 9:52 pm

Hi all I am in the process of getting council approval for a new carport for my future caravan.
The carport will be 7m x 4.6m with 4 posts on each 7m side and will be on a slope. I will be cutting down the ground on the high side but I can’t cut down fully to give a flat surface so the four posts on one side will be on flat ground and on the other side there will be a build up to compensate for the fall of the land. The build up will be (from the ground to the top of the slab) 360mm at the front falling away to 700mm at the back.

I will be getting a builder to build it as my wife doesn’t want me taking it on. I can do it but a builder will be faster.

What my concern is what material does a person use for boxing up that will be under the slab. I am not happy to have timber left in the ground to attract white ants and I will have some surplus rough sawn permapine boards from the fence that the builder might suggest we use but I think in time that would rot and attract white ants. I’ve seen builders use Quarry fines under slabs where they dig trenches in but 600mm is a bit deep. I have wondered about old corrugated iron but I would be interested if anyone has any better suggestions as this would be something that I’m sure many of you would have encountered. I will have plenty of fill for levelling out the ground.

Thanks in advance for any ones ideas.

bashworth · Oct 27, 2016 8:15 pm

Boxing under a slab on ground is a bad idea.

You need to build the ground up in 100mm layers with several passes with a vibrating plate across each layer.

sandman55 · Oct 28, 2016 10:12 pm

Thanks for that bashworth I guess with the ground compacted then it can be dug away at the edge and then shutters put in place. From your suggestion I did a google and saw they had the edge of the dig away sloping to stop it caving in.

Rory · Nov 02, 2016 8:10 pm

It sounds like you need to build retaining walls around your slab. They do not have to be made of timber.
Once this is done crushed rock compacted every 100mm of depth added as mentioned above.

sandman55 · Nov 02, 2016 10:54 pm

Rory

It sounds like you need to build retaining walls around your slab. They do not have to be made of timber.
Once this is done crushed rock compacted every 100mm of depth added as mentioned above.

Thanks for your input Rory. I take it your suggesting I make the retaining walls first so that I can remove any internal shutters then fill and compact then do the slab.

I wont be doing it myself but I am getting good info on ways it can be done for when I get a contractor. My planning approval should be in any day now.

Rory · Nov 03, 2016 10:59 am

Its a bit hard to advise as I cant actually see the job but im sure you can get a concreter to come in and quote to do both the retaining and pour the slab. Try emailing a photo of your job to a few local concreters and uif they are up for the job get them to come and price it up.

sandman55 · Nov 03, 2016 5:47 pm

Thanks Rory, at present I am chasing up quotes for the materials for the carport and then I will seek out a builder. I mainly wanted some info for my own knowledge on how it could be done.