Piling for slab on sand

[EEL]

Hello
I am looking for advice on piling under a house slab on a sandy site.
I am planning to get a concrete slab for a house laid on deep(>10m) sand in a rural area. Part of the slab(about 3m one end) will be suspended about 2m above the ground where the sand bank drops off to the next level, so it will need piers. We don’t want to build up the ground level. A soil test of the site has found loose to medium density sand and the engineer designing the slab has recommended piling. The engineer’s first suggestion was treated timber piling about every 1.5m under the slab. We have a bore not far from the house site and don’t want the risk of arsenic(or copper/chrome) in the water so are looking at alternatives. Concrete piling is too expensive due to remote site and depth of sand. One alternative is screw piles. A little wilder suggestion was to replace the treated timber with recycled plastic posts.
Does anyone have experience with screw piling – it seems expensive but is it actually good value? Anyone venture a comment on the suitability of plastic posts – I understand you can get them in various diameters and up to 5m long(cost between $40-$50/m). Would it be sensible to use a mix of the two ie. screw piles at critical corners/under piers, and posts under the body of the slab? Thank you in advance for any help/thoughts.
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David

[journeyman Mick]

David,
you may get some good advice here, but when it comes to structural stuff and especially the footings for your house you can't go past a proper engineered design. There's a good chance that the screw piling mob will do their own engineering, or organise it. Can't say I've even heard of recycled plastic pilings. You may be better of witht the screw piles as they can be fitted with (depending on dia. and depth) a handheld hydraulic power pack, or a post hole borer mounted on either a backhoe or bobcat. This is prbably going to be less expensive than machinery required to drive piles.

Mick

[EEL]

Thanks for the advice Mick. I keep getting steered back to screw piles so that is probably the way we will go. The CCA treated timber people swear there is no leaching, but evidence from the US says that is not the case so I am still ruling that out.
We are planning strawbale walls, so once we are into that I might have something to contribute - there seem to have been a few people looking for advice in that area.
David

[TassieKiwi]

Sand isone of the most stable substrates to build a concrete slab on - unless it is wet. The columns fo rthe elevated section of the slab could be founded on spread/pad footings. Another option for piles would be small steel piles, say 150UC's. Corrosion can be allowed for by using a heavier section than required.

All, said, screw piiles are usually very competitive with driven piles.

Dennis

[thebuildingsurv]

Formatube may also be the go it is often used for bored piers on sandy sites.

[EEL]

Thank you everyone for the help . We have decided on the screw piles - we will need around 53 for the house and garage. The screw pile people were very helpful

[mickfull]

i know that this is an old post but i am having a good look at the screwpiering industry and i wondered whom you used and what they were like.

Does anyone else out there having any stories to tell

Regards

Mick