wood retaining wall, drainage

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  #1  
Old 02-02-09, 01:59 PM
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wood retaining wall, drainage

HI,
I am building a series of terraces in my downsloping back garden and am looking for some drainage advice.

I am building 4 walls, each 3ft high and 20 ft long using pressure treated 4x4's and 2x12's running horizontal to the ground. I was wondering how much drain rock backfill I need to use behind the wall for drainage. Is 4 inch deep starting approx 4in below the bottom 2x12 board, up to approx 6in below the top of the 2x12's sufficient or too much?

Do I need to use a preforated drain pipe at the bottom of the gravel backfill to slope the water away.

Should I surround the drain rock on 3 sides with landscape fabric (ie on the soil side, the board side and the bottom)

Are 4x4's sufficient?

thanks
derek
HI,
I am building a series of terraces in my downsloping back garden and am looking for some drainage advice.

I am building 4 walls, each 3ft high and 20 ft long using pressure treated 4x4's and 2x12's running horizontal to the ground. I was wondering how much drain rock backfill I need to use behind the wall for drainage. Is 4 inch deep starting approx 4in below the bottom 2x12 board, up to approx 6in below the top of the 2x12's sufficient or too much?

Do I need to use a preforated drain pipe at the bottom of the gravel backfill to slope the water away.

Should I surround the drain rock on 3 sides with landscape fabric (ie on the soil side, the board side and the bottom)

Are 4x4's sufficient?

thanks
derek
 
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Old 02-09-09, 01:04 PM
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Derekglynn,

You are talking 4 walls each 3' tall. That's 12' that you are retaining vertically. That's a lot to ask of wood, and even PT is going to rot in 10 to 15 years. A better wall would be to use Basalite garden wall interlocking blocks. They are concrete and won't rot. They are interlocking -- no mortar involved.

How far apart are these walls?

Yes, you need drainage behind each wall. 4" is deep enough at the high end of the pipe, but you need 1/4" per ft. of slope in each pipe so that any water that gets into it will drain out. That's 5" of fall over a run of 20'.
 
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Old 02-11-09, 03:52 PM
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thanks for the response, yes I thought about interlocking blocks but the cost is in issue.

The walls are approx 8-10 ft apart.

I'm reluctant to use the drain pipe as I have no way of removing the water that flows out from it, other than placing a french drain in the ground at one end of the wall, that would allow the water to seep into the ground, but that then will potentially flow down to the bottom wall.

Should I surround the drain rock on 3 sides with landscape fabric (ie on the soil side, the board side and the bottom), or is it not required up against the wood

thanks
derek
 
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Old 02-11-09, 04:08 PM
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derekglynn,

I understand that cost is an issue, but to my thinking, pay the cost one time up front and be over it. If you do the wall in wood, you'll be back there in about 10 years and paying to to do it again, and again, and again, ....

You plan the drainage from the get-go. The drain pipes behind each wall flow into a drain pipe at the end of those pipes, and then it goes -- where?
 
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Old 02-12-09, 10:17 AM
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thanks for the response,

My problem with the drain pipes is that if I install one at the bottom of the wall, when the water flows out at the end of the wall, I have no sewers to let it flow into. I have 2 options, allow it to flow into the ground via a french drain that I'd build or pump it back up the hill(which is not an option). So I'm stuck either not using a drain pipe or letting it soak into the garden at some specific location, hence my reluctance to use the drain pipe as I would only be collecting water from in front of the upper walls and funelling it down towards the bottom wall, effectively, or into the neighbors garden with is at the end of my garden and down slope if it, (probably not a good idea), although they get free watering fr their plants.

ps I live in san francisco so we'don't get too much rain
thanks
derek
 
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Old 02-12-09, 02:19 PM
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Wink

Lefty is EXACTLY right! Pay now or pay later-and later-and later!! Go with the block and fill behind with drain rock(effectively creating a french drain). If you don't get that much rain, the upper levels will soak into the gound and lower levels at a slow enough rate that it should not be a problem. The BOTTOM tier may be somewhat different. I would put a drain pipe there to catch and move the water wherever I wanted it to go( a large hole with gravel/rocks??). Good luck.
 
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