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