Thread: Pre-Shed Build Questions

Originally Posted by Christos

There has been quite a lot of information in regards to going thicker than 100mm for the slab. I have not fully decided but tending towards staying at 100mm as per the original plans/design. We have had the ground prepared meaning one side raised and waiting for this to settle. We are not expecting to have anything done until the end of the month.

As rustynail said, the ground will not settle on its own. It needs to be either mechanically compacted in layers or you treat it as fill and will need to either thicken the edge of the slab down to virgin material or if it is too deep install concrete piers. If the slab is not supported on the same bearing material you can pretty much guarantee that it will crack

Originally Posted by FenceFurniture

In the last couple of years I have painted *rather a lot* of sleepers and posts with water based Bitumen. It does not like being exposed to long term sunlight, and painting over it with acrylic paint will lead the paint to blister off.

Thats interesting, I have also painted a lot of posts primarily steel and not timber, I have not experienced paint peeling on my steel posts and that was at my own place and that was after 10 years. Mind you cannot remember if I used acrylic or enamel paint to be honest.

It may have been something to do with the sleepers still being moist, although they are sold as KD (supposedly). They may have been K, but they certainly weren't D!

The bitumen is supposed to be able to make a waterproof membrane, right? That's the whole idea - to prevent moisture getting to the wood or steel. So I would have thought it would work both ways through the bitumen membrane.

However, and this gets a bit weird, any surface that was just bitumen has not bubbled. Most surfaces that were paint on bitumen have bubbled. Some surfaces that were just paint on timber have bubbled. The paint is the common denominator, and these results seem to indicate that the paint is a better moisture sealant than the bitumen is. Bitumen was always applied in two fairly heavy coats with a roller.

Go figure.

Originally Posted by Christos

We have had the ground prepared meaning one side raised and waiting for this to settle. We are not expecting to have anything done until the end of the month.

a month is just not long enough, nor is "level"

for this method to work, the fill needs to have a surcharge load -- typically a pile of dirt 10 to 12 metres high, and then you need to let the surcharge load sit for a coupe of years.
A situation more typical of a bridge building site than a domestic back yard.


as others have advised, you need to compact the fill or install piers

There are piers in the plans where the columns are going to be standing. As part of this the piers will be going into natural ground then measured down from there. One side of the shed will be on natural ground and the other side needs to go down by another 800mm before natural ground is reached.

The dirt has been compacted when it was levelled and on a side note the piles of dirt have been sitting on this site for over 6 months prior to levelling. I have driven steaks into the ground to mark out the shed so that I can get an idea of size but found that I barely got down 40mm when the steak started to break.

Originally Posted by Christos

... I have driven steaks into the ground to ...

Ham or beef?

And what size steak, texan, nth qld? or was it a tomato stake.
Friction on the stake will yield what is known as "point of refusal" ie: it reaches a point where movement is minimal as a % of load/force acting on it. Hitting a stake with a sledge hammer introduces load or force which is disproportionate to its true load bearing value in relation to the surrounding ground and its bearing value.
You say it has been compacted but was it controlled and verified by geotech testing or was it a case of the drott / bobcat just running backwards and forwards to achieve "pedestrian" grade compaction?

Originally Posted by rwbuild

And what size steak, texan, nth qld? or was it a tomato stake.

Have you seen Christos? He's as skinny as a stake. Needs more steaks. I'm surprised he was able to bury that steak 400mm without a shovel, and the sledgy would have splattered it everywhere.

These stakes are 20 x 20mm with a length of 300mm and only managed 40mm depth before resistance was heavy. I was using a small stumpy to hit these stakes.

So far only a bobcat had travelled back and forth at time of levelling. I am just waiting for the building certificate before anything else happens.

What sort of ground material, hard dry clay, granulated shale or what and were the pegs pine or hardwood with or without a point?
A bobcat would not be heavy enough to achieve any acceptable geotech compaction.

LOL, poor Christco is probably wondering what have I done, I only asked a prebuild question, it's 5 pages long and I have not even levelled the site. Cannot wait till you ask a technical question Christco, lol. All T.I.C

Looking forward to some photos.

Originally Posted by rwbuild

What sort of ground material, hard dry clay, granulated shale or what and were the pegs pine or hardwood with or without a point?
A bobcat would not be heavy enough to achieve any acceptable geotech compaction.

The dirt used is a clay fill it was taken from about half a meter down after the original top soil was removed. Where the edge of the concrete slab would end there has been more fill added about 1.5 to 2 meters before it slopes away. The reason for this idea of a buffer was so that we do not loose dirt from the edge of the concrete slab. There will be a lot more dirt in the future placed around the shed pad to increase the buffer and for landscaping which will also include a driveway(gravel) and possible retaining wall. I think the pegs that I used were pine and each had a point.

If there is a possibility that I will loose dirt from under the slab after pouring then I will have to look into this further.

Christos the issue isn’t loosing soil from under the slab but that the fill material will continue to settle over time so your slab is not uniformity supported and will crack.
For consistent support under a structure you need to bear on the same material. Placing drilled or bucket piers through the filled section will resolve the situation

Originally Posted by Beardy

....For consistent support under a structure you need to bear on the same material. Placing drilled or bucket piers through the filled section will resolve the situation

It is on the design plan to have piers for the columns and the depth of the fill will be added to piers to ensure that the piers will be in original ground. I have already discussed this with the builder and it was included as part of the quote for concrete.