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Randell Tarin 06-18-2008 11:33 PM

Board Retaining Wall
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I've built some guest cottages on the side of a slope and during the construction was compelled to to carve out about a 30' Long by 2' deep section running in a North-South direction and parallel to the cottages.

I placed the dirt removed from the excavation above it to form a berm. At the base of the excavation I cut a temporary lateral drainage ditch. I have observed the water flow during an unusually heavy torrential rain. I can see that as the water runs diagonally down the slope from East to West and from North to South, it is diverted at the foot of the berm and removed to a nearby creek. Water running below this follows the temporary ditch into the same creek.

The soil substructure is predominently red clay with a sand mix. Even during the downpour, the "wall" above the excavation remained stable with the exception of a bit of sand that washed down to the drainage ditch. It appears that the drainage imposed by the berm alieviates much of the water that would pose any potential hydraulic pressure against a retaining wall.

I plan build a board retaining wall using 4' long 4"x6" post set 2' into the ground and backed by 2"x12" pressure treated wood.

My question is this: with my particular situation, would an 8-foot post spacing be sufficent? Do I really need to go 4-feet? It just seems like a bit of overkill in this instance.

troubleseeker 06-19-2008 08:54 PM

2 x 12's across an 8' span will result in an unacceptable bow pretty quickly. Even the 4' span will bow, a couple of inches over time. I think in the long term, you would be much better off spending the $ and laying 4 x 6's for the wall, with 6' support spacing. I also would go no less than 3' on depth, to fend off the problem of long term leaning. Even the modest pressure from only the two feet of fill you are retaining, will have dramatic effects when applied 24 hours a day, every day, long term.

downunder 06-20-2008 04:45 PM

Troubleseeker, what do you think about a deadman in there?
RT, I'm impressed with that diagram!:thumbsup:

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