Wood Retaining Wall Replacement

I have a 40' wood retaining wall about 30" high made of 2x8s attached to 4x6 posts every 4' which are in concrete. It needs replacing. It holds back dirt on top of which is a concrete pad w/crushed rock as a base, normal install. The concrete pad sits about 4" away from the 2x8s (i.e. the finished edge is parallel to the 2x8's and is filled in w/dirt).

The plan is to remove the posts and concrete and reinstall posts w/concrete. They've lasted about 25 years and in another 25 I won't care what they look like.

I'm thinking of cutting all the posts flush, removing the wall so I can work on removing the concrete. To hold back the dirt where I'll be working I'll brace with 5/8 plywood supported by 2x4 braces on angle.

Any better ideas out there? My biggest concern is undermining the concrete pad causing it to crack over time. I'm hoping I'll be able to slide new 2x8s in-between the new posts and the dirt.

 

Pictures would help to understand better.

Based on my experience I do not think your plan will work. I replaced a railroad tie wall 18 months ago. Do a search on my posts to find the thread. Forum issues prevent me from adding a link here.

It is unlikely that you can keep the dirt behind from collapsing and undermining the pad.

Many years ago I did create a retaining wall similar to what you suggest. It was for a firewood bin under a deck that was built over a slope. I dug into the slope as far as I could in the post areas to dig the post holes. Then I inserted 2x8s behind the posts and drove them down as far as I could until they hit the slope. Then I excavated more below the 2x8 and drove it further down in several steps. The fill was sand and flowed easily so it was slow going. When I finally reached the bottom of the 48 inch deep wall I needed to backfill a space that was about 2 feet back from the top tier and sloped to the bottom of the wall.

Have you considered just building a new wall at the face of the existing and leaving the existing in place behind it? Offset the posts from the existing so you will not get interference from the existing concrete at the posts.

 

Do you have room to build your new wall right in front of the old wall? If so maybe you could build the new wall. Then remove the old one in small sections back filling as you go. This would also get you away from the old concrete so it's easier digging holes for your new posts.

Personally I would not do a wood wall. I would use engineered, pre-cast concrete retaining wall blocks. They sit on a bed of compacted stone and the blocks are set in place one by one so it's a job one strong person can do by theirself.

 

Sorry for the delay, was out of town. Here are a few pics. A like for like is the only reasonable replacement even though it is a maintenance item (though as mentioned I likely won't have to deal with the next time it needs replacing).

As can be seen, negative on building another wall a a bit farther out. When the wall was initially built I hired out the holes/post and the comment I received was that due to the rock/ledge in the ground he should have charged more. So I have no desire to create new holes. None of the trees/bushes were there when he installed them, in fact there really wasn't much dirt on the uphill side, I added the fill for the parking pad.

For the 2 posts I've removed, the concrete that was used was crap and crumbles easily. Probably quickened the rot due to permeability.

Might be hard to tell but the pad slopes from essentially no retaining wall to about 30" of wall.