Retaining Walls

[Popeye]

Hi to you all, this is my first post so I hope that you may have some advice.

We are currently buliding a 1.5mtr treated pine retaining wall, and I was considering applying a membrane, Grip Set to the back of the wall to help protect the timber, and assist with drainage. I have heard though, that by using such a product, you can cause the timber to perhaps dry out on one side and therefore bow.

Has anybody heard of this happening before or do you have any advice?

Regards

[Gumby]

Better to fill the area butting up to the wall with rocks which will allow easy drainage.

[Popeye]

Thanks Gumpy, but I never intended to use soil - always aggregate

[silentC]

My experience with treated pine is that it will bow anyway. It's nearly always wet when you get it and as it drys out it will warp and twist. I think it's fundamentally unstable and you can't really expect it to stay straight. I've used slabs, wing splits and sleepers in landscaping jobs and none of it stays where you put it.

[IanA]

What type of construction were you intending to use?
Is the wall straight or curved and is it vertical or raked back?

Do you have room for a crib wall? I've seen these successfully consructed with treated pine instead of concrete members. As there are two faces of the horizontal members exposed to the air the effect of warping is different.

[Popeye]

Thanks for your thoughts. The wall will be 1.5mts high at the highest point, down to 1mtr. Intending to use 75mm sleepers with gal H beams to suppport them.

Aggie drain with appropriate backfill for drainage.

Whilst we are retaining "stable" clay/shale mix there are a million suggestions on the degree of batter. Any thoughts?

[chappo]

Suggest you think about geofab or something simular between aggregate and natural ground to allow water through the wall while holding the soil in place and not fouling the aggregate allowing it to drain. Would also suggest a couple of 300mmx300mm grated pits connected to stormwater or rubble pits at bottom of retaining wall to stop ponding of water after rains.

1500mm is a high wall might be prudent to have an engineer check over the wall design. I do this for all walls exceeding 1000mm in height.

Industry rule of thumb is 1 in 4 batter but you can go to 1 in 1, but again I would refer to an engineer, they have insurance, you probably don't. Also don't forget to allow at least half a timber size kick above the finish ground level if you intend to put a garden on the batter. Saves having to rake mulch etc from lawn back to garden after heavy rains.

Hope this helps.

[Popeye]

Chappo, thank you for your thoughts. I agree with what you have said. I have been pondering about getting an engineer to have a look, the soil is a very hard compact clay/shale mix, and has been exposed for 1mtr for a year without any movement, but I would rather do it right than have to redo it again.

Thanks for the tip about fininshing over the garden bed height. We are going to run a drain down the length of the wall and the edge of the pavers.

What do you think about using the Gripset compund to seal the back of the sleepers?

Popeye

[chappo]

Don't know much about Gripset. But if you do intend on using timber then I would use a suitable product that would protect the timber from water penetration. CCA or LOSP treatment is not enough to protect timber from moisture. Nearly all timber retaining walls at one time or another will need to be replaced as the timber will breakdown with age, moisture, contact with soil etc so the better protected the longer you will have between replacing. I would not restrict the coating to the back either but rather all round and especially where you have cut, jointed, nailed and bolted. I will have a look at Gripset and let you know.
Good move with the engineer.

[Eastie]

If you'd like to run your eye over an engineering design for what you're proposing to do drop me a line.

[Fantapantz]

1.5m is a bloody high wall for sleepers if you do it right. I was going to do a 1m wall out of hardwood sleepers and by the time you consider the posts at 1200 centres and that they had to be 1m down for a 1m wall i reconsidered. If you think 1m is pretty deep for the posts have a look around the neigborhood and check out all the wavey sleeper walls.
Look at this sight:

http://www.tradac.org.au/newsite/pub/free/8_garden_walls.pdf

About 5% dearer to have someone come in and make it out of sandstone boulders while i sat on the deck and had a few coldies nas supervised and the white-ants tend to dislike the taste of sandstone for some reason.....Sleepers life expectancy 15-25 years...stone a lot longer. Getting H beam galvanised isn't cheap either $1 per kg is about the cheapest out there, so the galvanising costs as much as the metal.