Retaining Wall Design With Walkway On Top - March 2022

Earlier here on the blog, I posted about the idea of a 'fountain wall' right next to our patio and mentioned that I thought there was an 'order of operations' post that needed to come first to ensure that I had all of the pieces sorted out and that any project that I took on would not have to be REDONE when I moved on to other projects.  However....in order to get to that 'order of operations' post, I think I need to lay out a few more of the building blocks/underlying projects that make up the choices I have to make.  

One of the biggest ones is to finish up solving the 'entrance' to our backyard.   I've gone around-and-around on how to best solve the situation I'm facing:  a gradient that transverses the path we need to complete from our front yard (driveway) back to our patio.

Last year, I put in a gravel walkway that covers half of the area.  And, we love it.  But, I can't simply lay down a gravel walkway for the rest of this walkway because of the slope.   There are three approaches that we can utilize:

1.  Use pavers.  This requires a pair of retaining walls.  

2.  Use gravel.  This, too, requires a pair of retaining walls.

3.  Use a boardwalk.  This requires ONE retaining wall and some piers.  Or, two retaining walls.

I'm drawn to both #2 and #3.  The idea of a boardwalk is something that I've thought about for a while and would allow me to incorporate some of the ideas and trends that I've been reading about for gardens. Remember this footbridge inspiration?  I love that.   And, remember this post about 2022 garden trends?  Bridges are on there.  I also fell in love with these stairs and waterfall, but have talked myself out of it because when we're sitting on the patio, we wouldn't be able to enjoy it.   

More inspiration was in this Insta post that shows this zig-zag boardwalk that has A LOT to love about it.

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Using a boardwalk would be a nice change-of-materials and would allow me to transition BACK to gravel once I span the gradient.  

However, in order to do anything - including the boardwalk - I have to think about retaining walls.  

Here's a look at the gradient below.  On the left (where the 2x4 is located) would be a retaining wall that stacks up about 20" or so.  Along the right would be ANOTHER retaining wall that holds in the flower bed next to the house.  

Because the retaining wall on the bottom part would NOT be seen by anyone, it is utilitarian and functional - but not decorative.  That's lead me towards an exploration of the types of blocks that are available at big box stores.  I have a couple of these on hand - they are the cheapest ones ($1 a piece) - and have used a couple of tools to factor how many I would need (400 or so) to complete my 24' run with a curve.

Once these walls are built (more on that), I can then decide what material to use - gravel or boardwalk.  I think that we could go either way, but I can't escape the idea of using wood.

As it relates to building these walls, I've watched a bunch of YouTube videos and it seems that the construction of these walls is pretty straight forward:  excavate a trench about 20" wide that is 6-8" below grade.  Fill and tamp down 4-6" of paver base and create a perfectly level foundation.  From there...dry stack the blocks on top add some drain tile behind the lower course to relieve pressure.  On top of that, backfill with larger, more coarse gravel.  The blocks I'm thinking of using are 4" tall, so we're talking 5 blocks tall to get the walkway up to the right level. 

The wall on the 'top side' would be done the same way, but only be a 7" or so wall.  3 courses tall with 5" below grade.  Which, when you include the 5" or so of base, we're talking about 10" deep excavation.   

Based on a quick review of pricing, it appears that the boardwalk vs. retaining wall work are about the same price, but looking at block cost vs. lumber cost alone obscures a few things:

1.  It seems that the 'upper' retaining wall is a mandatory project.  No matter the route, that one has to be completed.

2.  The lower wall project - if completed - could be 'topped' by gravel.  Or...a boardwalk.  

3.  I'm not certain the excavation required changes based on wall vs. boardwalk. Either way, I would want the path surface to be level with the concrete pad - and thus...we'd need to excavate about 8" in order to provide the most solid base and/or to bury the joists. 

What have I solved?  Not everything.  But, it sure feels like there's one confirmation:  the upper retaining wall needs to be done first.  What is the shape of it?  That depends on other factors.  But, in terms of 'order of operations', it seems I have something to start with:

1.  Build upper retaining wall. 

That's SOMETHING that I can take on this year.  And, since it is so low and has what I believe to be VERY LITTLE pressure behind it, I think I can use something that is a nicer-looking material.  However, in order move ahead with it, I think I have to decide the material for the walkway AND the shape.  

Square corners and boardwalk?  That means, a square planter box.  Curved corners?  Curved planter box.  We have curves everywhere, so curves seems logical.  But, the squared sides - and doing something like that zig-zag boardwalk that is in the photo way up top just seems awesome.

So, material and shape of walkway.  Would allow me to take on the upper retaining wall.  From there, I have a bunch of pieces of the puzzle that need to fit in:

• Build lower retaining wall (if not going with boardwalk)
• Build stairs and landing up to patio (material, size, etc)
• Decide on wall fountain, berm and plantings
• Lay down material for walkway extension (boardwalk vs. gravel)

This feels like a big project in the making for 2022.