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I have a concrete pad next to a soil portion of the yard. I'd like to make the soil level with the pad (and will be adding sod later), but to do so, I need some sort of retaining wall. There is wood below the fence creating a sloped retaining wall, but to raise the dirt, I'll need to go above the current wall onto the fence.

My current thought is I have two options

1) Build a second, more shallow retaining wall against the fence to retain the new soil.

If I do this, do I need any space between the old retaining wall the new one? If so how much? Will filling with gravel be sufficient?

2) Add new pressure treated 2x6s to the base of the existing fence (on top of the panels).

This seems like much less work, but will I have any issues with this? Drainage seems to be working fine and the wood is holding up well on the current fence. In the picture below, the bottom vertical piece of wood is 'submerged' under gravel and the entire yard drains towards that point and I've yet to have issues. . The bottom of the fence is badly rotted in the corner, I just couldn't see it under the gravel, so something else clearly needs to be done there.

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For reference, the soil on the left most part is ~8in" below the concrete and above that there is about a 6" lip

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How much vertical earth do you need to retain? The fence panels should be considered non-structural, only the posts themselves might hold back something. There could be a legal issue with adding additional loads to property line fences that might not even be yours, or maybe just half yours. We can't speak to this issue here, other than to point out it might be an issue. –  bcworkz Feb 6 at 18:01
    
@bcworkz - It'll be about 9". I'll be working with my neighbor on the fence issue if I go that route –  dfb Feb 6 at 18:19
    
Hardly any sideways pressure on 9". You should get away with attaching treated 6x2" across the posts right to the bottom. I'd then coat it with mulseal or a thick black plastic despite being treated. –  Matt Feb 8 at 15:28

2 Answers 2

Wood exposed to earth, or even gravel, as you discovered, is a compromise at best. You can hedge your bet by using treated wood. Use wood with the maximum amount of preservative. "Outdoor wood" is not enough, the preservative levels must be at least adequate for ground contact. Preferably wood treated for foundation service should be used. Doing something to ensure any water that does contact the wood can easily drain away is also important, as well as trying to prevent water contact in the first place.

9" of soil does not impart much of a lateral load, so almost any retaining structure will suffice. Still, it would be better to not involve the fence if possible IMO. How far back from the existing structure would depend on what you end up doing to retain the extra earth. You could extend the existing structure, which would obviously have 0 offset. At the other extreme, relying on gravity to retain the earth, such as placing a row of large stones, you would want to see a minimum 45 degree slope between the existing wall and the tops of the new structure.

By using an impervious retaining material like stone or concrete blocks, you can backfill with plain soil, no gravel is required for minimal height walls. Taller structures often backfill with stones to prevent hydrostatic pressure from pushing on the wall. When back filling against wood, do everything reasonably possible to prevent water from contacting the wood, even if it is treated.

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".. you would want to see a minimum 45 degree slope between the existing wall and the tops of the new structure." - does this mean packing dirt from the top of the retaining wall to the bottom of the old one before backfill? –  dfb Feb 7 at 19:23
    
No, it's an imaginary line to ensure there's enough gravity force to resist the tendency of the retaining structure to topple over. It's not an absolute rule, the ideal slope depends on the materials involved. It's adequate for most DIY gravity retaining structures. –  bcworkz Feb 7 at 19:46
    
Thanks, that helps a bunch –  dfb Feb 7 at 19:51
    
There is a neglegible sideways force at 9" high. The fence posts will carry it easily vertical. –  Matt Feb 11 at 5:06

We had a 2 1/2 foot high garden built out between a new deck and a fence by a professional team of builders. The company we used belong to the Master Builders who provide 10 year guarantees so they know what they are doing.

The fence was sufficiently sturdy enough that it was used as the posts to hold some 2x6" horizontals for the retaining wall to hold the garden. The whole garden was lined with a thick black plastic.

The top of the wall between the fence and retaining wall was covered by the same hardwood decking as the rest of the deck. That covered the gap between retaining wall and fence pailings and prevents dirt getting trapped and weeds growing up in between.

I would think you can do what you're saying without any problems. Although if soil is going to meet the wood you should protect it with either a plastic or bitumen based paint like mulseal.

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The OP's fence does not appear to be as sturdy as yours must be to retain 30" of soil. For any wood retaining wall, I would want to see a geotextile spacer structure against the wood to maintain an air gap against the wood and provide a positive drainage channel. Over that the waterproofing can then be installed. –  bcworkz Feb 6 at 17:55
    
He's only going 9" up and his fence looks like it should be able to handle it. –  Matt Feb 8 at 15:30

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