Advice on retaining wall replacement (pics inside)
We're in the process of planning out the replacement of two failing retaining walls that flank our sloped driveway on the side of a townhouse. The first picture below is the current state of affairs for one side. The timber retaining walls on both sides are canted in, the only thing holding them up right now are the metal fence post sunk in concrete at the top of picture #1 and some metal braces drilled into a stone retaining wall on the other side (which is now slowly pulling the stone it is attached to from that wall).
When I first saw this was wondering why they didn't put any deadmen in at all, but now that I started digging out behind the wall face I know why: there is an existing original brick retaining wall behind that was never torn down. The timbers were essentially a facade, and probably started failing the day after they were installed.
My plan is to tear out everything and replace with an interlocking block gravity wall, but there are a couple issues to sort out first.
1) It looks like the original wall actually has a secondary wall in front of it (picture #2). The brick porch foundation was added after the fact, but the walls appear to tie into it. What is the best way to deal with this? Remove as much as possible and work around it with the new block? It is probably more important on the other side where the wall seems to tie into the actual house foundation. It doesn't look like there are remnants of a second wall on the other side, but we'll need to excavate more to be sure.
2) Roots. The root systems from the trees in the first picture do run up to the original wall and probably under it (there is one large root that cracked the driveway cement under those trash bags). How should we deal with them as we dig out and replace the wall? We don't know how bad that problem is yet. How far back should we plan on excavating down to ground level behind the new wall line?
3) Wall slope. At the far end the wall is 4', and we'll end up about there with the new wall. It slopes to ground level towards the sidewalk. Slope is about 5:1 along the entire length (~ 21'). When digging out and laying base is there a rule of thumb on how we should step this? Given the slope I'm guessing we may have 2 courses of wall blocks under ground before we step up to the next level (if the lowest level is set one block deep into the ground, then we wouldn't step up until the second course is entirely underground which would then become the first course for he second step). Would that be sufficient for the footing? I guess we would need to then set the steps back as we go higher to account for the minor slope built into the engineered blocks as we go higher.
4) Drainage. We'll lay a proper base and backfill with gravel, but for the drainage tile is a 5:1 slope too much? How do you incorporate the drain exit in the exterior of the wall through the blocks? There is a storm drain at the lowest point of the wall, so no issues there, just wondering how the wall should be built around it.
The current timber wall is set on concrete footings that will need to be demo'ed (picture #3). These are sitting on driveway concrete but it looks like that ends just behind the timber wall line; that will be where we dig out and lay base. Thanks